L Dr. J. B. Sanderson 211 Ruminantia Dr.T.SpencerCobbold 506 Ovum) J Uterus and its Ap- ~| . .. < r T-; TT7-77- oeo Dr. Arthur Farre ... 545 Respiration, Organs of Dr. Thomas Williams 258 pondages J SUPPLEMENT. OVUM. In Animal Anatomy and Physi- ology, the Egg, the product of parental sexual generation, from which the young of animals are produced. The Functions of Reproduction, as observed in the higher orders of animals and in the human species, are generally divided into two classes of processes ; the one of which com- prehends those operations by which the parents contribute to the production of the germs from which the young are formed ; the other, those processes or changes which occur more immediately in the product of genera- tion itself, and which relate to the formation or development of the new being from a germ or ovum. In the Article GENERATION of this Cyclopaedia, the functions belonging to the first of these divisions have been described ; and it is proposed in the present Article to treat of the second class of reproductive phe- nomena, or those which relate more imme- diately to the origin, formation, and growth of the new being, and which are usually described under the titles of Ovology, Embryology, and Foetal Development. In this, as in the former article, the history of the functions as they occur in the human species will receive the greatest share of our attention ; but in describing the process of development of the young, still more than in the history of the functions of the parents that are preliminary to the production of a perfect germ, it is necessary to extend our observations to the various members of the animal kingdom, and even in some degree also to plants, from which, as much as from direct observations or experiments in man, has been derived our knowledge of the individual facts and of the general laws relating to the process of embryonic development. The arrangement followed in that part of the article which treats of Development will be adapted more immediately to the consi- deration of human reproduction ; and the statements in regard to animals, or to organ- ised beings in general, will be made chiefly subordinate to, or illustrative of, the functions in the human species ; but the facts in human and comparative embryology are so intimately connected, that it will be expedient to incor- porate with the article such a description of the formative process in different animals as may present a sketch of the general nature of Supp. this interesting process in the whole animal kingdom.* In pursuing this plan, the topics to be dis- cussed may be arranged under the following heads : viz. 1st. Nature of the Ovum in general, with reference to the different forms of the repro- ductive function in various animals. 2nd. The structure, properties, mode of origin, and formation of the Ovum. 3rd. The changes which the ovum under- goes in the process of Fecundation, and (in so far as the ovum itself is concerned) the cir- cumstances which influence that process. 4th. The external circumstances which in- fluence the development of the ovum and embryo, especially Incubation and Utero- gestation. 5th. The Phenomena of Foetal Development in general, and the history of the origin and development of each system, organ, and tex- ture of the body in particular. 6th. The Functions of the Embryo or Foetus as compared with those of the adult. The wide-spread importance of embryolo- gical anatomy and physiology is now so generally acknosvledged by all who have made them a subject of study, that to them no apology is required for the length of this treatise. To those who have not made them an object of their special attention, it will be enough at this place to advert to the exten- sive range of topics which must be embraced in an attempt to trace the history of the first origin and subsequent evolution of all the parts of so complex and various a struc- ture as the body of animals ; and to remind them that this department of science pro- fesses to describe not merely the successive changes of external form and relation by which the several organs, springing from im- perceptible beginnings, arrive at their perfect condition, but also the more minute pheno- mena of histological development, or changes of the several textures, which accompany the more obvious formative processes ; that, as in many instances the complete knowledge of * It was originally intended to have treated in the same article of the embryology of plants; but the extent and importance of that subject in con- nection with general physiology makes it neci.'ssary to postpone its consideration to a separate article*, under the head of VEGETABLE OVUM. K OVUM. the structure and function of an organ is only to be obtained by the observation of its foetal conditions, the study of development is acces- sory or supplementary to many departments of anatomy and physiology ; that, in recent times, no branch of inquiry relating to organic nature has made more rapid progress, has presented a greater amount of new dis- coveries, or has influenced in a greater degree the views of scientific men on allied subjects, than the science of embryology ; that it is coextensive with, and illustrative of, the whole range of comparative anatomy ; that no system, therefore, of zoological classi- fication can be regarded as philosophical or complete which neglects the facts and princi- ples of foetal development: finally, that some departments of pathological anatomy receive considerable illustration from our science, and that more especially the scientific study and comprehension of teratology or congenital malformations is founded entirely on an accu- rate knowledge of the phenomena and laws of development. Our subject, therefore, is not only interesting by itself, but deeply important as an essential branch of philosophical ana- tomy and physiology.* Before proceeding with the particular his- tory of the ovum, and its development in man and the higher animals, which will form the greater part of the following article, some topics of a general and preliminary nature present themselves for our consideration. The investigation of the process of repro- duction in the lower animals has made so much progress during the last few years, that it becomes necessary to place before the reader a sketch of the aspect in which more modern researches enable the physiologist to view the relation of the ovum to the sexual generative function, and to the other means by which individuals are multiplied, or species are reproduced in the whole animal kingdom. In the Article GENERATION, the commonly received distinction was drawn between the sexual and the non-sexual modes of genera- tion ; and under the latter form a variety of processes of Gemmation and Division were alluded to as occasional or constant substi- * A variety of circumstances have contributed to cause delay in the appearance of the present article, some of them of a nature beyond the con- trol of the author. He is sensible, however; that an apology is due by him to the readers of this work on account of the protraction of that delay. He has only to say, that in the contemplation of the vastness and imperfectly known condition of the subject, he has ever felt more disposed to engage in the investigation of some of its details, than to appear before the public as a systematic writer in regard to it. The delay may have this advantage, however, that it will enable him to in- troduce a greater number of new discoveries, a more accurate statement of individual facts, and more correct and extended general views of the subject than might have been possible at an earlier period, and that it will afford him an opportunity of cor- recting and amplifying various statements con- tained in the previous Article GENERATION, which the progress of discovery since the time of its pub- lication has rendered necessary. tutes in a certain number of animals for the more permanent sexual form of the reproduc- tive process. At the time of the publication of that article, the sexual organs had not been discovered in a considerable number of the lower animals : but since then, the assiduous and accurate researches of embryologists have gradually diminished the number of animals so situated, by bringing to light the male and female reproductive organs, or their essential products, in nearly every species of the animal kingdom ; so that now only a very few, and those of the simplest organisation, remain, in which the bisexual condition has not been detected. These animals belong exclusively to the division of the animal king- dom recently established by Zoologists, as Protozoa, comprehending the Polygastric In- fusoria, Rhizopoda and Porifera.* In all other animals it is now ascertained that fecundated ova, formed by an act of sexual generation, are the means of securing the permanent reproduction of the species ; but in several of them, as is especially well known among the Polypine tribes, a vast multiplication of individuals, sometimes living separately, but more frequently associated in groups, or living in united colonies, takes place by a non-sexual process of reproduction, which may be compared in many instances to the growth or repetition of the parts of a tree or plant by budding. Recent investigations have made it more and more apparent, that the non- sexual multi- plication of animals ought to be distinguished into several kinds, according to the different circumstances in which it may occur. In a few, as already remarked, it is entirely with- out known sex : in others, the non-sexual pro- cess of gemmation, or division, gives rise to new individuals, which are simply the repe- titions of the perfect or complete animals ; and in a third set, the non-sexual multiplication occurs more frequently in an incomplete con - dition of the animal, and often consists in the production of one or more series of dissimilar forms of animals, the last generation of which alone becomes sexually complete, and propa- gates the species by fecundated ova. This constitutes the variety of the reproductive process recently distinguished by the name of Alternating Generation. Three forms, therefore, of non-sexual animal reproduction, or multiplication, are to be dis- tinguished from the sexual mode of generation, as in the following enumeration : I. True sexual generation, direct or indi- rect, in all animals, excepting the Protozoa. II. Non-sexual multiplication, occurring only in some of the invertebrated animals ; 1st. In Protozoa, in which sexual organs have not yet been discovered. * The first two of these divisions may be described as simple unicellular microscopic animalcules, the third rather as a compound or congeries of micros- copic animalcules : the Porifera, or Sponges, are in- cluded in this division of Protozoa, because the balance of evidence is decidedly in favour of their animal nature. OVUM. 2nd. In Animals known to be capable of sexual generation ; including two varieties, viz. a. Multiplication of similar individuals, either in a mature or immature condition. b. Multiplication of individuals, generally dissimilar from those producing them, and becoming at last mature or complete in the exercise of the true generative function. Some account of these various forms of the reproductive process, and especially of the last, as established by recent discover}', sup- plementary to that contained in the Article GENERATION, maybe introduced here, with a view to serve as a foundation for general views of the nature of the ovum, and its relation to the reproductive process in ge- neral. I. OF THE OVUM IN GENERAL, AS RELATED TO THE SEXUAL PROCESS OF GENERATION. The term ovum is in this article entirely restricted to the product of sexual genera- tion. This body is formed in the ovary of the female parent (or in the female organ of a hermaphrodite parent) by a gradual process of growth or development. When it arrives at a state of maturity, it is spontaneously dis- charged from the place of its formation, a process which in the higher animals has re- ceived the name of Ovulation. If left to its own unassisted powers, no organic change of importance follows in the ovum, and it remains incapable of producing an embryo. But if, at or near the time when the ovum, in a state of maturity, leaves the ovary, it be sub- jected to the influence of the male product or sperm by the contact of a very minute portion of that substance, it then undergoes the change of Fecundation, by which it has communicated to it the power' of having de- veloped within it a new being specifically resembling its parents. Although there are many great apparent differences in the form and structure of the ova of animals*, yet a general comparison of Ovarian Ovum of a Mammifer. n, entire; b, burst, showing the germ-cell, with yolk granules flowing out of the vitelline membrane ; c,the ovarian ovum at an early stage of its formation, consisting of the germ-cell surrounded by a few yolk granules. * The most important of these will be noticed in a later part of the article. their organisation shows that they consist in nearly all of parts that are essentially the same. These parts in the ovarian ovum are the following, beginning with that which appears most essential : 1st, The Germinal Vesicle, or Germ-cell; a nucleated orgaivc cell of microscopic size, generally situated near the surface of the ripe ovarian ovum : this is embedded superficially in, 2nd, The Vitettus, Yelk, or Yolk, a mass of oleo-albu- miuous matter, partly fluid, and partly cellular and granular, generally of propor- tionally much greater size than the germ-cell, and serving to furnish materials for the changes of that body, and for the develop- ment of the new being. Both of these parts are enclosed by, 3rd, The VileUine, or Yolk- Membrane, a vesicular, nearly structureless, membrane, which contains the rest, and gives to the whole usually more or less of a sphe- rical form. To the assemblage of these parts, constituting the ovarian ovum, and which may be looked upon as most immediately im- portant in connection with the formative pro- cess, there are generally added, after it has left the ovary, and in the progress of its descent through the female passages, some others, such as the albumen, outer membrane and shell of the bird's egg. In their simplest form these additional parts constitute an ex- ternal covering of the egg, to which the name of Chorion is often applied. If the ovum be traced back to its earliest origin in the ovary, it is found to consist at first of the germinal vesicle, germ-cell or its nucleus (Jig- 1, c.). To this cell the sub- stance of the yolk is added in the progress of its formation, generally in a gradual manner, but in some animals more suddenly. Fig. 2. Spermatic Filaments ( From I?. Wagner and Leuchhardt). a, spermatozoa of the squirrel. b, spermatozoa of the dog, in the interior of the developing cell. The Spermatic Substance, or Sperm of the male, when examined in its state of maturity, as it is applied to the ovum, and effects in it the peculiar change of fecundation, is observed to consist essentially of an immense number of minute bodies, generally consisting of a thicker particle, with a fine filament attached, OVUM. and almost always exhibiting, when recently mixed with water, vivid vibratory or umlula- tory movements, but in a few animals present- ing other forms, and without motion. These spermatic filaments or particles are developed by a peculiar process in the interior of the cells (tpeni-cellt) secreted in the male organ or testis. When the ovarian ovum has arrived at maturity, the germ cell disappears as such, and if fecundation shall have taken place, that vesicle is succeeded by another minute cell, with which the origin and development of the new animal are most intimately associated. This secondary organic cell of the fecundated ovum has therefore been called the Embri/o- ccll. The first changes, preparatory to the commencement of the development of an embryo, consist in the formation out of the embryo-cell and yolk substance of an organised cellular mass, or of a membranous covering of the whole or a part of the yolk: this is the germ-mass, Blastoderm, or germinal membrane. Fig. 3. cli'o Fecundated Ovum of a Mammifer, li'ith the Embryo Cell and its division. a, ovum with the first embryo-cell ; &, division of embryo-cell and cleavage of the yolk round it ; c, second division and cleavage ; d, farther division ; and e, germ-mass or Blastoderm funning; f, dia- gram of the embryo with its membranes, the am- nion, allantois, &c., within the chorion. The process by which this primary organised part is produced varies somewhat in different animals; bat it appears to consist in a mul- tiplication of the embryo-cell b}' changes of the nature of cytogenesis, accompanied with more or less of a cleavage or sub-division of the substance of the yolk, and its com- bination with the progeny of the embryo-cell. The general result is, that the first rudiments of the new being take their origin in organic cells, which are descended from the original embryo-cell. From this blastodermic mass or membrane, the embryo, or foetus, or new animal, and in the higher animals some accessory parts, which are temporarily united with the embryo previous to its birth, originate, and are gra- dually formed, by a various process of pro- gressive organic growth of an epigenetic character, which is termed Development, or Embryo-genesis. In by far the greater number of animals an ovum gives rise to only one embryo or indi- vidual, and this one becomes by itself, when its growth is complete, the perfect sexual animal, capable of contributing its share to the pro- duction of fecundated ova. But in a certain number of animals, to which allusion will be made more fully afterwards, the immediate product of development from the ovum is not at once, and by itself, converted into a com- plete sexual individual ; but by an intermediate non-sexual process of production, one or more new individuals are formed out of the body of that first developed, and to the last so formed is committed the office of sexual reproduction, or true generation. The essential conditions and phenomena, therefore, of the sexual process of generation, as related to the ovum, and as limited by the foregoing considerations, may be shortly stated to be the following. 1st. The formation of the ovarian ovum of the female sex, containing the germ-cell. 2nd. The formation of the sperm-cells of the male sex, and the development of their peculiar spermatic elements. 3rd. The mutual action of these two pro- ducts in the fecundation of the ovum. 4th. The disappearance of the germ-cell of the ovarian ovum, and the formation of the embryo-cell in the fecundated egg. 5th. The multiplication of the embryo-cell by cytogenesis, and the formation from that body, and from the yolk, or a part of it, of the blastodermic mass or membrane. Gth. The process of embryo-genesis, or development of the systems, organs, and textures of the new animal. It is right to state that the original germ- cell has not yet been ascertained to exist in the ovum of every animal, nor has its successor, the embryo-cell, been observed in all instances ; but they have been detected in so very large a proportion, that it appears extremely pro- bable that in all sexual animals the generative process consists in the process above described, or in some modification of it. I refrain at present from farther details as to these phe- nomena, and have stated the results only in their most general form, because I shall have occasion to return upon some of them in a subsequent part of the article. Looking back on this general statement of the com- mencement and progress of the genetic process in animals, it will be seen that the new being may be considered as taking its immediate origin from the progeny of cells descended from the embryo-cell. That OVUM. cell appears with great probability to take its origin from the germ-cell, or its nu- eleus, or from some part of it, in combination with a determinate portion of the sperm product, or descendent ot the sperm-cell ; and we are so far justified, therefore, in ascribing the genetic process by which the new being is formed to the mutual action of the products of two different kinds of cells, viz., the germ- cell and the sperm-cell. * In conclusion, the ovum may be defined to be a distinct vesicular body originally formed from a ce'l, presenting throughout its exist- ence the organic cellular structure, consisting of oleo-albuminous materials, formed by the female of an animal species, and capable, when acted on by the spermatic product of the male, of undergoing the successive changes of embryo-genesis, by which, either directly or through intervening generations, the species of animals is reproduced and continued. The structural distinctive characters of an ovum are, therefore, its enclosure within a distinct vesicular covering, and its original organic cellular constitution in the germ-cell : its most important physiological characteristic is its susceptibility of the changes of embry- onic development under the influence of the sperm-cell or its product. II. OF THE NON-SEXUAL MODE OF GENE- RATION. The necessity of distinguishing several kinds of non-sexual reproduction according to its occurrence in animals entirely without sex, or believed to be so, and in those which may also be propagated in the sexual mode, has already been adverted to. A farther distinc- tion of the non-sexual reproduction may be made according to the nature of the process itself: thus, some forms of it consist in the development of buds, so intimately united with the parent substance, that scarcely any difference can be perceived between their mode of formation and that of continuous growth, as in Hydra and various Polypes : other forms consist in the development of new individuals from germs so isolated in their form and cellular in their structure, that it might seem at first sight arbitrary to distinguish them from ova, as in Aphides ; others appear to hold an intermediate place and character between these two forms, as in ISalpa: while, in a fourth set, a more complex anil varied series of changes occurs, which may be regarded with probability as modifications of the gemmal or germinal processes, as in Medusoid Polypi, Taenia, c. But it will be apparent from what follows that we are as yet very far from that exact knowledge of the nature and first origin of buds, gemma?, or other kinds of germs, from which animals may be multiplied in the non-sexual modes, which would enable us to form satisfactory general These views have been stated with great clear- ness by Prof. Owen in his various writings, especi- ally in his Essay on Parthenogenesis, and Lectures on Generation, &c., in Medical Times, 1849, and by Dr. Carpenter in his Principles of Physiology, Ge- neral and Comparative. 1851. conclusions as to their mutual relations, and their similarity or difference, as compared on the one hand with organic growth, and on the other with oval development. As the accurate determination of these re- lations is in a great measure impossible, it will be expedient for the present to state only very briefly the general characters of the several non-sexual modes- of reproduction, before selecting for more particular consider- ation some varieties of the process, the recent investigation of which seems calculated to influence in a considerable degree future ge- neral views of the whole subject of reproduc- tion. We shall also defer for the present any minute consideration of the relation of these processes to the growth or development of cells, for we shall have occasion to treat more at length of that subject in a subsequent part of this article, and in that of vegetable ovum.* At this place it is only necessary to re- mind the reader, that all processes of develop- ment, whether in the earliest or at more ad- vanced stages of formation, appear to consist essentially in, or are more or less intimately connected with, a multiplication of organic cells in the parts that are developed. In the unicellular beings, fissiparous ami gemmiparous multiplication may easily be recognised to be processes of cell growth ; the one consisting in the division of the parent cell into a pro- geny of two by a nearly equal partition of its substance ; the other, in an extension and gradual enlargement of a small or limited por- tion of the original cell. But in many of the instances of fission and gemmation on the larger scale with which we are acquainted, observa- tion has not yet pointed out the primary cell, if it exists, from which the process of division or extension begins; and,indeed,mostinstances of fissiparous division may. as Dr. Carpenter has remarked, be referred to a peculiar modifi- cation of gemmation. The process of budding or gemmation is usually stated to occur in one of two modes. 1st, by the extension of a part of the parent body which remains in organic connection with it during the development of the new individual from the bud ; the attached bud either sprouting from the exterior, or being developed in the interior of the parent stock. '2nd, by the development of the new individual from a small detached portion of the substance of the parent, which undergoes the principal formative changes after its separation. These separate buds have been called gemmae, gem- mules, bulbils, &c., and two kinds of them may also be distinguished according as they are thrown off from the external surface of the parent body, or are formed and become loose within its interior. These gemmules have frequently attained to some degree of development by the time of their separation, and very often are provided with cilia over their surface, which cause them to move * For a very lucid and agreeable statement of these relations the reader is referred to Dr. Carpen- ter's able Treatise on General and Comparative Physiology. 1851. B 3 6 OVUM. rapidly through fluids. From the first they exhibit a minutely cellular and granular struc- ture : but it does not appear that they are originally formed from any single nucleated cell : they appear rather from the first to lie a congeries of cell progenies. They are desti- tute of an external envelope ; hut, nevertheless, it may often be difficult to distinguish between them and true ova. The tendency to the multiplication of indi- viduals by non-sexual reproduction is greatest among those animals which are of the simplest organisation, and more especially among those in which the cellular structure predominates; not that it is confined to them, nor that it occurs in all animals so constituted, but that it is much more frequent and complete in the simplest animals of each class in which it has been observed ; as if it were more liable to occur in those species in which the process of individual development had proceeded to the least extent of advancement in the formation of the living textures of their bodies. There is accordingly a remarkable similarity in the nature of the processes of non-sexual multi- plication and ordinary growth in these very simple animals ; and it is well known that the same relation subsists between a low organi- sation of animals, and their disposition or power to repair individual parts of their bodies lost by injury or accident.* 1st. Of the Process of Reproduction in Pro- tozoa, or animals in which the sexual distinction hait not yet been discovered. Among the Protozoa reproduction takes place in two modes, viz., 1st, by the process of gemmation or fission, and, 2nd, by develop- ment from separated gemmules or germs. For an account of the first of these processes, the reader is referred to the articles POLYGASTRIA and PoRiFERA.f Among the Potygastria multiplication by division is much more frequent than that by gemmation. Tt consists in the fission or di- vision of the whole unicellular body into two nearly equal parts, each of which becomes, when separate, a perfect animalcule like the original one : in some the division is trans- verse, in others longitudinal, and occasionally it occurs in either of these modes in different individuals of the same species. The nucleus of the unicellular polygastria has been fre- quently observed to undergo division previous to the formation of the fissure, by which the division of the external wall is completed. a fact which has led some physiologists, as Ehrenberg, M. Barry, and Owen, to attribute to the nucleus an important influence in this process of cleavage ; the first of these ob- servers having even conceived the nucleus to act the part of a male or fecundating organ. * See Mr. Paget's recent interesting lectures on this subject, published in Medical Gazette, 1849. f A considerable number of the polygautrie infu- soria described by Ehrenberg in his great work on that class, are now very generally regarded as be- longing to the vegetable rather than to the animal kingdom, such as the families Closterina, Volvocina, and Bacillaria. This latter view is not, however, adopted by many of those who have made a study of this class of animals. In some of the polygastria in which the process of multiplication is either of a fissi- parous or gemmiparous kind, as in Vorticella, Uvella, and Polythalamous Rhizopoda, the new individuals remain in connection, and are associated together in branched pediculated groups, in connected masses of a globular form, or in regular spiral united series.* The Porifera, or sponges, appear to be re- produced by a different kind of gemmation from that now described in polygastria, viz., by separate gemmules or small portions of the substance of the sponge, which, soon after having been detached from the main stock, are moulded into a spherical form, and, being pro- vided with cilia, move about in the water with great vivacity for a considerable period. These gemmules are thrown off in numbers propor- tional in some measure to the activity of the nutrition of the sponge, and therefore princi- pally during the early part of summer. Towards the approach of winter a different kind of re- productive bodies is observed to be formed, viz. small capsules containing globular germs, which, after development within the capsule, pass out of it and produce a new sponge for every capsule or germ. These bodies have been called ova, and certainly they bear very great, resemblance to them ; but too little is known of their nature and origin to enable us to form an opinion whether they are to be regarded as precisely of the same nature as ova or not. In the mean time the}* may be named the capsular germs.f But it appears that, among the polygastria, and rhizopoda also, there are sometimes formed, by a peculiar process not ascertained to be of a sexual kind, minute reproductive bodies of a cellular structure, which, if they are not true ova, are at least substitutes for them.f * See an interesting paper by Dr. Carpenter on the Genus Nnmmulina and other Foraminifera in Quart. Journ. of Geol. Soc. Feb. I860. Some ju- dicious and interesting remarks on this class of animals, and on the relations and characters of the Protozoa in general, are contained in a recent paper by Mr. Huxley in the Annals of Natural History (1851, vol. viii. p. 437.), in which he has described a curious monocellular genus named Thalassicolla, which occurs in masses, and forms spicula some- what like a minute sponge. t See Laurent's elegant memoir, Eecherches sur rilydre et 1'Eponge d'eau douce. 1842. J Allusion is not made here to the production of granules by the difttuence of an infusorian animal- cule erroneously taken by Ehrenberg for the depo- sition of ova, but to a very different process. Du- jardin, who pointed out this error (Hist. Nat. des Infusoires, p. 101.), is of opinion that, besides the processes of fission and gemmation, we know nothing with certainty of the reproduction of infusoria ; but he admits that it is possible that the minute bodies into which an infusorian breaks up by diiiluence might prove the germs of new individuals. Dr. Carpenter has mentioned several instances of a kind similar to those alluded to in the text, and has expressed the opinion that something of the nature of sexual pro- duction may yet be discovered to take place in these animals (Prin. of Gen. and Comp. Physiol. p. tM!>, and p. 917.). Observations of a similar kind are re- OVUM. Some recent observations appear to throw additional light on this subject, and to make it probable that in some circumstances this process is in some sort analogous, or at least equivalent, to one of sexual reproduction. The first accurate observation of the de- velopment of a progeny of young cells within the body of a polygastrian was communicated by Focke in 1844 to the meeting of natu- ralists at Bremen, and the fact of the pro- duction of internal germs or bodies resembling ova or spores within the body of these ani- malcules has recently received full confir- mation from the observations of Stein and of Cohn.* In (John's observations, which were made on a paramaecian polygastrian, the Loxodes Fig. 4,. each presenting two contractile vesicles like the parent. The escape of these bodies, by their passage through an aperture temporarily formed in the wail of the infusorian, was carefully observed ; the exit of each embryo occupied about twenty minutes. Soon after their escape they exhibited active ciliary mo- tion, and moved about with all the appearance of embryo-infusoria. Although the farther development of these bodies was not traced, the observations on this animal, and on an- other, the Urostyla grandis, afford sufficient proof that the infusoria may be propagated by minute separate germs, as well as by division of their bodies. A similar production, but more numerous, of an internal progeny, has been observed in the microscopic parasitic animalcule termed Gregarina, which infests the intestinal canal of a number of insects, earth worms and some other invertebrate animals.* The simple Gregarina consists of a single cell filled with granular substance, and con- taining a distinct nucleus. It has no intestinal canal, nor other internal organisation ; is gene- rally of an elongated shape, and creeps about by motions of slow contraction of its substance. The formation of the progeny or smaller bodies within the Gregarina is attended with a remarkable change in the parent animal, which has been carefully observed by Stein. This change, in which the animal appears double for a time, had been previously no- ticed by Kolliker and others, and had been interpreted by Kolliker as the conversion of a single animal into two, by a process analo- Fig. 5. Formation and extrusion of ova or germs in Loxodvs Lursaria (from Cohn'). a, animalcule, containing two .young ; b, contain- ing six; c, one of the embryos escaping; d, e, two ciliated embryos. bursaria, which is usually multiplied like the rest, in the fissiparous mode, sometimes by longitudinal, at others, by transverse division, it was found that at certain periods there were formed within the bodies finely granular colourless cells, in seme only one, more fre- quently several, and occasionally as many as six or seven, nearly of a uniform size, and ferredto under the head of 'sporiferous reproduction,' by Prof. Kymer Jones, in the article POLYGASTKIA. * Stein, Untersuch. iib. die Entwick. der Infuso- ricn, Wiegmann's Archiv., 1849, vol.1, p. 134. in Actinophrys, Acineta, and Chilodon uncinatus. Cohn, in Zeitsch. fur Wissensch. Zoologie, Nov. 1851, p. 257. GregnrinoR (from Kolliker. ~) ci, single; b, c, d, united ; c,f, g, the formation of the navicella-like progeny ; //, three of these na- vicellie (from Stein). * These animals were first accurately described by Leon Dufour in 1837 (Ann. des Sc. Nat. vol. vii. p! 10.). They have since been studied with great B 4 8 OVUM. gous to transverse fission. Stein, on the other hand, has been convinced by a very at- tentive observation of the different stages of this process, that it is of an opposite character, and that, previous to the development of the young progeny, two of the Gregarinae have become fused, or united into one. As the two are about to unite, they gradually change their form from that of elongated planaria- like animalcules, to that nearly of hemispheres, closely pressed together; then a complete fusion or union occurs, and the whole of the granules of both having become amalgamated in one sphere, the development of the internal progeny takes place gradually from the mass. This progeny consists in a vast multitude of minute bodies, shaped like the Navicellae (among the Diatomacea3), but different from these bodies, and very probably constituting the reproductive germs or embryos of Gre- garina?. The development of this Navi- cella-like progeny into the Gregarina does not appear as yet to have been traced ; as in this animal, like many other parasites, the progeny is required to migrate during its de- velopment from one stage to another, and the little bodies are passed out of the alimentary canal of the insect before undergoing farther changes. The views and observations of Stein, how- ever, should they be confirmed by others, would prove the very remarkable fact, that the phenomenon of conjugation, or fusion of two unicellular individuals, hitherto supposed to be confined to some of the simpler plants, as Closterium, Spirogyra and Zygnema, &c., may occur also in animals of a similar simple structure. These observations on the Gregarina are not altogether of an isolated kind. In a recent interesting notice of this subject by V. Siebold *, he has called attention to the ob- servation of Kolliker on the conjugation or fusion of two individuals of Actinophrys -|-, a spherical infusorian animalcule analogous to the Amoeba or Rhizopoda, by its slowly con- tractile, amorphous texture, and its long, ra- diating, contractile processes. Kolliker ob- served two individuals of this animalcule to approach each other, adhere, and gradually to fuse into one, which soon assumed the same globular form, with the radiated contractile processes, as each of the two that formed it, and differing only from them by the increase success by various observers, as V. Siebold (Beitrag. Z. Naturgesch. Wirbellos. Thieve, 1839, p. 63.). Henle (Mailer's Archiv, 184.5, p. 3(59.), and Stein in the same, 1848, p. 182. Kolliker (Zeitsch. f. Wiss. Zool. 1848 and 1849), and as many as eighty dif- ferent, species of them have now been discovered. * Zeitsch. f. Wissensch. Zool. March, 1851, p. 62. f Op. cit. 1849, p. 207. In this very interesting memoir Kolliker has proved the animal nature of the Actinophrys by his observations on its contrac- tility, and on the manner in which the particles of solid matters, vegetable and animal, are involved in its substance for the purpose of digestion, and their remains again rejected when that process is com- pleted. of size which it sustained. This very curious observation has been confirmed by Stein, in an allied genus Podophyra, both of the sessile and pecliculated kind ; and V. Siebold has ob- served the same phenomenon in a species of Acineta belonging to the same family of Infusoria. Cohn, also, has repeated and con- firmed Koiliker's observations in the Acti- nophrys sol, and has made a farther discovery of great interest in connection with the pro- cess of conjugation in these animals, having observed after the union, both in the Acineta and Actinophrys, the development, at certain periods, between the united individuals, of a spherical body of considerable size, vesicular form, and containing within it a nuclear forma- tion of variable magnitude. Although the farther development of this body has not yet been traced, it seems not improbable to V. Siebold that it may be analogous to the reproductive capsule or sporo-cyst of the conjugating Closterium or Zygnema*, from which bodies it seems to be certain that a number of reproductive spores are produced. Since the foregoing was written, indeed, renewed researches by Stein -j- have come under my notice which are confirmatory of the view previously stated as to the repro- ductive process in Gregarina, and explain in a great degree the apparently incomplete observations of Pineauj and others as to the varying conditions of Vorticella, and also extend our knowledge of the production of germs of the Infusoria. Stein observed the Vorticella microstoma to lose its pedicle, become free, assume the globular form, and at last to be enclosed in a cyst produced by exudation from its own body. After a time the band-like nucleus of the encysted Vorti- cella is divided into a number of small discoid bodies, not by a regular or progressive process of cell-cleavage, but at once and directly. These minute bodies gradually increase in size at the expense of the granular and fluid substance surrounding them in the cyst, and ultimately escape in the form exactly of Monas colpoda (of Ehrenberg). These very soon fix themselves ; and a fine pedicle is developed at the place of attachment. In other instances the Vorticella-cyst was observed to send forth long contractile processes from its sur- face, and then assumed very much the form and appearance of an Acineta or Actinophrys; and in this case a new Vorticella was formed in the interior in the manner of a bud. The Vorticella, therefore, it would appear, is ca- pable of reproduction in two modes, by the development of embryoes from the divided nucleus, which Stein on this account proposes to call nucleus germinativus (the testis of Ehrenberg) j and by gemmation from an intermediate Acineta form. The first form Stein would regard as the equivalent of sex- * See the Article VEGETABLE OVUM for an ac- count of this process in the lower forms of plants, t Zeitsch. fur Wissensch. Zool. Feb. 1852. Ann. dcs Scicn. Nat. 1845 and 1848. OVUM. ual production ; the second as coming under the category of alternate generation ; and the Vorticella embryo of the Acineta-fonn either repeats its geninial multiplication, or becomes encysted, and gives rise then by its nuclear division to embryonal production. Other new forms of Infusoria are described by Stein under the names Spirochona gemmipara, Dendrocometes paradoxus, and Lngeno- phrys vaginicola, ampulla, and nassa, in which the mode of reproduction is somewhat similar. These observations at once show the im- portance of the views entertained by some authors as to the share the nucleus may take in new production, and strongly indicate that much still remains to be known from ob- servation of the processes of reproduction among the Infusoria. Should these observations be confirmed, another analogy, in addition to those already observed, will be shown to exist between the organisation and functions of the Protozoa, and those of the lowest plants.* The ten- dency of various other recent researches, to which it has been impossible to refer more particularly in this place, seems to be to show that, in addition to the more common and obvious mode of multiplication by division and gemmation, by which the Infusoria, when vigorous and well nourished, are reproduced, there are other means by which, in dif- ferent circumstances, the more permanent re- production of the species may be secured ; that minute cells are formed within them for that purpose, which may at present be called reproductive cell-germs rather than ova, till a more complete knowledge shall have been obtained of their nature and of the circumstances attending their formation ; and that it is very probable that in the protozoa, as in the simplest plants, the com- bination of the contents of two cells, to all appearance similar, may, as in the process of conjugation, be the necessary preliminary step to the development of the reproductive germs. It ought at the same time to be kept in view that the Infusoria may, like many other animals, be subject, some to metamorphosis, and others to alternate generations. Already, since the publication of the great work of Ehrenberg, most important modifications of his system of these animals have been found necessary, and it seems almost certain that it is destined to undergo still farther changes, many of those forms which are now recog- nised as belonging to distinct genera and species being possibly no more than different stages of development of the same animal. 2nd. Of the possibility of primary, direct, or non-parental production of animals, or of so- called spontaneous and equivocal generation. From what has before been stated as to the very general, and almost universal, existence of the sexual mode of generation among ani- mals, and from the reasons that have been given for the belief that in those few and simple animals in which a sexual distinction * See the recent work of Alex. Braun, entitled Die Verjungung in der Xatur, Freiburg, ISil). has not yet been ascertained, there may still be propagation by means of minute germs, the reader will already have drawn a con- clusion as to the very insufficient nature of the proof that can now be adduced in favour of the view that certain animals may arise independently of pre-existing individuals of the same species. The hypothesis might, per- haps, be at once dismissed with the remark of a recent writer*, " that it is safer to trust to generally prevailing laws, than to confide in such of our observations as are contrary to them." But as in the article GENERATION f , the author was led by a careful examination of the evidence then available on the subject, to admit the probability of the non-parental mode of production as an exceptional occur- rence, at least among the lowest tribes of animals and plants, and as that hypothesis has since gradually lost more and more of its pro- bability, from the accumulated opposing proofs resulting from more recent researches, so as, in Ins opinion, to be now no longer tenable, it may be proper at this place to review briefly the bearing of the present more advanced knowledge "of the generative process upon this long" and keenly debated question. Admitting, in the meantime, that the ova, or separate germs of Infusoria, have not yet been discovered with certainty, there are not wanting direct experiments which demonstrate that in an infusion of organic matter which would, when exposed to the air, naturally furnish a rapid succession of these produc- tions, the development of living organisms is entirely suspended, if the arrangements are made such as to render it impossible for any germ or other part of a previously existing inf'usorian animalcule or plant to be communicated to the infusion. The experi- ments of Schultze and of Schwann are valuable, as appearing to have secured, in a <*reat measure, the above-mentioned con- ditions, without otherwise interfering with the validity of the result. The first of these ob- Fig.6. Apparatus employed by Schultze to prevent the access of germs by the air to an infusion. a, ilask for infusion ; b, tube, with caustic pot;isli ; c, tube, with sulphuric acid. * Eschricht, in Edinr. New Phil. Journ. vol. xxxi. 18-11. p. 355. t P. 429. 10 OVUM. servers* placed in a glass flask an infusion of organic matter, a portion of which was known from comparative trials, when left exposed to the open air, soon to have animalcules deve- loped in it in great quantity, and he connected this vessel with a tubular apparatus, by two apertures, in such a manner that the air, which was made to pass frequently through the vessel containing the infusion, should be drawn through strong sulphuric acid, or potash solu- tion, before reaching it ; and Schwann j~ ar- ranged a similar experiment, having in view to secure the like conditions, by causing the air, which had access to the infusion, to be pre- viously passed through an iron tube at a red heat. Before the commencement of these experiments, the infusion and the apparatus were carefully subjected to the temperature of boiling-water, by which it was presumed the vitality of all ova, or germs, or other or- ganic particles must have been destroyed : and the result was the same in both the series of experiments, viz., that, after a consider- able lapse of time, no animalcules nor con- fervoid plants were formed : but when the atmospheric air was afterwards allowed to pass freely over the same infusion, without being subjected to the processes before men- tioned, a rapid production of infusory ani- malcules took place in the usual manner. The results of these experiments appear to be on the whole satisfactory, and nearly to decide the question as far as relates to the probability of the introduction of the germs of Infusoria, &c., into infusions by the air. But, indeed, the failure of many experiments of this kind, when not performed with the most scrupulous accuracy, need not excite surprise, when the very indestructible nature of some kinds of infusory animalcules is con- sidered. It has long been known, and has been ascertained by the careful experiments of Spallanzani, Bauer, and Doyere, that some of the Rotifera and Tardigracla are capable of supporting a high temperature without loss of life, and of being kept for years even in the state of complete dryness, without loss of vitality : and, although it must be admitted that these animals differ greatly in their or- ganisation from the Polygastric Infusoria, and the latter appear to be very liable to destruc- tion from slight causes, yet it is possible that their germs may resist destruction in a greater degree than their adult forms: and, should only one of these animalcules, or its germ, be left in any situation favourable to its development, it is easily understood, from what is known of the production of these beings, with what rapidity a vast multitude of them may be brought into existence by their ordinary process of fissiparous increase. Most physiologists are inclined to reject as fanciful and inaccurate the alleged observa- tions of the actual conversion of particles of organised or organic matter into living in- * Poggendorff's Annalen, 1837, and Edin. New Phil. Journ. vol. xxiii. p. 165. f In paper on Fermentation, &c. in Poggen- dorlf's Annalen, 1837, p. 184. fusoria. At all events, statements of this kind are to be received with the greatest caution : such, for example, as the observa- tions stated to have been made by Pincau *, who affirms that he has seen the direct con- version of particles of disintegrating muscular fibre, isinglass, and wheat-flour, into various forms of living infusoria. The spermatic filaments also, which, so long as they were looked upon as independ- ent animals, were referred to as examples of an undoubted spontaneous generation, furnish no evidence in favour of that hypothesis in the view in which they are now regarded by physiologists : for they are to be considered rather as a peculiar product of organic growth within the spermatic cells, somewhat ana- logous to the fine moving processes of the ciliated texture, than as distinct organisms. -j- In so far, therefore, as the theory of spon- taneous generation may have been supposed to derive support from the formation of the lower forms of plants and animals in infusions of organic matter, that hypothesis must be considered as having lost the greater share of its probability, if, indeed, it has not been entirely disproved : but it must at the same time be admitted that a more precise ac- quaintance with the nature of the germs from which these organisms take their origin is still required to render the arguments derived from this source entirely conclusive. J The external and internal parasites which infest the bodies of almost all animals have in former times been held to afford a still stronger presumption in favour of sponta- neous generation than the production of in- fusoria ; but it will be found that in this instance, to a much greater extent than in the other, the probability of the view has gradu~ ally passed away before the increasing know- ledge which modern research has afforded of the various modes of propagation of these animals. The ready communication of various Epi- zoa, or external parasites, from one animal to another is now well known, and accurate ob- servations have demonstrated that in almost all instances this communication may be traced to the implantation of ova, or pregnant individuals into their parasitic abode, as in the researches on the Sarcoptes scabiei, &c. The parasitic fungi, also, of various cuta- neous diseases, as tinea, porrigo, plica po- lonica, foul ulcers, c. ; the yeast-plant, the vinegar-plant, and other minute fungi con- nected with fermentation ; the contagious algae of the batrachia and fishes ; the muscar- dine of the silkworm, are all well proved to be communicable by the deposit of their spores, or some part of their substance, upon the external surfaces of the bodies of the animals on which they grow, or by their intro- duction into cavities opening on the exterior. All the internal parasites, orEntozoa strictly * Ann. d. Sc, Nat. March, 1845, p. 182. f See Article SEMEN. j Consult, especially, on the whole of this subject, Dujardin's Hist. Nat. des Iiirusoires, 1842. OVUM. 11 so called, are now known to be capable of true sexual generation, by means of ova, in their perfect or complete condition, and the whole class is remarkable for the great de- velopment of the sexual organs, and the pro- digious numbers of ova which they bring forth. But it has been ascertained that their ova are rarely developed into new beings in the place of the abode of the adult entozoa : they are commonly subject, therefore, to migration from one organ to another in the same indi- vidual, or from one animal to another, or from the parasitic to the free-living condition ; and they have recently been discovered to present very remarkable changes of external form and internal organisation in their va- rious habitations ; so great, indeed, that many of them, previously believed to belong to species, and even to genera and families widely different, are now recognised as dif- ferent conditions of the same animal or species, and that many forms, whose mode of generation was unknown, are found to be derived by indirect production from ova, in a manner which will be more particularly de- scribed under the next section. Thus it appears that the only entozoa which are destitute of sexual organs, viz. those belonging to the division cystica, are very probably only imperfect forms of Taenia or other cestoids, which, so long as they are in the encysted or confined condition, do not reach their full development : but many of which, during their incomplete condition, are capable of being multiplied by a process analo- gous to gemmation. The greater number of the entozoa breed only when in the alimentary canal of animals, and the ova are excreted along with the foeces : it is obvious, therefore, that very many ova must be destroyed, and that a few only are liable to gain those peculiar situa- tions which are fitted to maintain them in their earlier conditions, or in their later stages, to bring them, as parasites, to their full state of development. The entozoa are usually found, therefore, in their most advanced stage, in the alimentary canal. There seems, on the whole, little dif- ficulty in accounting for the entrance of en- tozoa from without into the alimentary canal, or the pulmonary air-cells and other open cavities : and every new fact that has been observed relative to the occurrence of entozoa in man and animals, leads to the conclusion that the ova, or perhaps more frequently the earlier larval or undeveloped forms of the entozoa, gain access to these situations by introduction from without, and most fre- quently along with food and drink ; in those instances at least in which the entozoa migrate from one animal to another, or from an animal to the free state before returning to the parasitic condition. But the entozoa, which are, in general, in an incomplete state when situated in the close cavities or solid textures of the organs of animals, sometimes make their way from these situations into the alimentary canal, there to undergo their finul development. Surh appears to be the case with the Strongylus urmatus, living in an incomplete state in aneurismal sacs of the blood-vessels of the horse, and in a fully developed state in the intestine ; the Stron- gylus vagans, in cysts of the porpoise, and afterwards free in the lungs ; the Ligula or Bothriocephalus solidus, in cysts of the ab- dominal cavity of fishes, and afterwards in their perfect state in the alimentary canal of sea-birds. The Trichina and Echinorrhynchi, imbedded in the muscular flesh in great quan- tities, are no doubt imperfect forms of other worms, which must migrate from these situa- tions to attain to their complete state. With regard to the manner in which the en- tozoa inhabiting the close cavities of the body, or imbedded in the solid substance of organs, either in the free or encysted condition, gain access to these situations, which has to many appeared inexplicable, excepting on the hypo- thesis of their arising actually in the places which they inhabit, observations are no less decided in proving them to be of external in- troduction. In the first place it may be stated that, al- though the ova of a considerable number of the entozoa are of so considerable a size as to render it improbable that they have passed as such through the capillary vessels, yet few, if any, of these larger kinds are observed en- cysted, and in others the ova are extremely minute, and might, without difficulty, be car- ried through most of the capillary vessels. In the next place it may be mentioned that the embryoes, or earlier forms of various parasites, and the ova of others, have been observed in considerable numbers in the cir- culating blood of various animals *, as showing that by this means the entozoa may be carried in their small and early condition into any part of the body of an animal which is fitted to afford the conditions favourable to their farther development. But in what manner have these bodies gained an entrance into the blood-vessels, or, in other instances, how may entozoa have penetrated into cavities or the parenchjma of organs, without being conveyed through the blood-vessels ? To this question, also, recent observations seem to furnish a satisfactory answer : for it has been ascertained that, in a number of instances, smaller or larger en- tozoa, but especially the former, pierce the tissues of animals with great apparent facility, being frequently provided in the young state with an apparatus of sharp hooks for that special purpose. Some of them have been observed in the act of passing through the * I may here refer to the original observations of Schmitz, (Berlin, 182G), and the more recent ones of Valentin Gruby, Gluge, Vogt, and others. See Valentin, Kepertorium for 1842 and 18-13. The Annual Kepoit in Muller's Archiv. for the same years, and in Wiegmann's Archiv. for Naturgcsrh. Valentin's account of the < )va of Distoma in the fluid covering the medulla oblongata of a foetal sheep (Miil!er's Archiv. 1840, p. 317), and V. Siebuld'.s Article 'Parasites' in II. Wagner's Handworterbuch cler Physiologic. 12 OVUM. solid substance of organs or through mem- branes ; and from the various stages of ad- vancement of others already referred to, seen in different parts of the same animal, little doubt can prevail that they must have done the same : but the aperture through which they make their way, besides being in most instances very minute, seems to close very rapidly and completely after them. So that the occurrence of entozoa in entirely isolated cavities such as the aqueous cham- ber of the eye, or in the parenchyma of solid organs, does not now present to our minds any valid objection to the view that in all in- stances they are introduced from without ; and it will be apparent, from the same con- siderations, that even the occurrence of en- tozoa in the foetus, of which there are un- doubted instances, and to which great import- ance has been attached as an argument in favour of their spontaneous origin, may be explained on the supposition of their ova, or young, passing from the maternal parent, through the blood-vessels of the umbilical cord, as is known to happen with various poisons. The whole history, then, of this remarkable class of animals, as it is now known, tends to support the general conclusion that they are all capable in their complete state of sexual reproduction, and that they gain the various sites of their parasitic habitations by intro- duction of their ova, or embryoes, or of more advanced stages of their growth from without, either directly into the open cavities, or more indirectly, by piercing the coats of vessels, membranes, &c., into the close cavities and the parenchyma of solid organs.* A candid review of the whole evidence on this question leads to the inevitable conclusion, that, though all the difficulties or doubts which surround it are by no means completely removed, the hypothesis of primary or spontaneous generation receives little or no direct support from the accurate observation of the mode of origin of those animals which alone were supposed to afford proofs of such a kind of production ; and that this view must, therefore, on the strongest grounds of analogy, be in the meantime aban- doned, for that which attributes the origin and reproduction of all organised beings to an undeviating connection through ova or germs, seeds or spores, between new individuals and others of identical species which have pre- viously existed. And if the present sonie- * As to the bearing of a knowledge of the habits &c. of the Entozoa upon the question of their spon- taneous origin, consult the able essay by Eschricht ; " Inquiries concerning the Origin of Intestinal Worms &c." in Eclin. New Phil. Journ. vol. xxxi. p. 314. 1841, the article on Parasites by V. Siebold, in E. Wagner's Handwork der Physiol. ; E. Blanchard's Ju'M'urches on the Structure &c. of Intestinal Worms, in Ann. d. Sc. Nat. 1848 and 1849, parti- cularly vol. vii. p. 121. Dujardin's systematic work, Hist. Nat. des Ilelminthes, 1845. And in connec- tion with this and the whole subject of spontaneous generation, the Systematic Treatises on Physiology of .Burdach, J. Miiller, Valentin, and Longet. what imperfect state of knowledge does not permit us to affirm this absolutely, as the result of direct observation, the exceptions are so few and unimportant, that they may be disregarded in the overwhelming evidence of a positive character in favour of the opinion, derived from analogy, that every organic being, if not produced in actual union with another, derives its origin from a germ or some such connecting part that has proceeded from a being of the same kind. If this be the present state of the argument in respect to the hypothesis of the first origin of organic beings, it need scarcely be added that the opinion which has attributed the pro- duction of various animals to conversion or gradual transmutation out of other species or genera, has still less of real to be adduced in its support. In the long series of ages in which authentic observations have been made on animals, no such examples have been ascertained, and there are no established facts which give any substantial grounds for believing that in the natural or wild state of animals there is any departure from that un- deviating succession of specific resemblance between parent and offspring, which seems to form one of the most constant of the laws of organic nature with which we are ac- quainted. 3rd. Production of dissimilar individuals among sexual animals by a non-sexual process : so-called Alternate Generations. From the foregoing general views it ap- pears that in all Vertebrated Animals, and in by far the greater number of Invertebrated animals, the process of permanent reproduc- tion consists in the development of the new being from the blastodermic mass formed by a peculiar process of cytocenesis in the fecundated ovum. But, as has already been shortly stated, there are some varieties among them in regard to the degree of directness with which the product of development from the ovum arrives at that state of maturity, or sexual completeness, in which it is capable of renewing the act of sexual generation. These varieties may be classed as follows: 1st. The product of the ovum, being single, attains by a gradual process of development, when it leaves the ovum at birth, to nearly the same form and structure as its parents : this is generally called Embruohgical Development. 2nd. The product of the ovum, being single, is born or leaves the egg at an early period, and while comparatively imperfect, or, as it is called, in a larva state, and by one or more successive changes of development of a marked kind, afterwards reaches the specific or ty- pical form : these changes are usually called Metamorphoses. 3rd. The product of deve- lopment from the ovum does not itself become a complete animal, but gives rise, by a peculiar mode of generation of a non-sexual character, and therefore different from that by which fecundated ova are formed, to a new body, or to successive progenies of new bodies, one or more of which ultimately attains to the specific resemblance of the sexual parents by which OVUM. 13 the ova were produced. This is the " Alter- nating Generation" of Steenstrup, or what we might with Mr. Owen, in contrast to Me- tamorphosis, call a process of Metagenesis*; and of which the single and multiple varieties might be distinguished according as the inter- mediate progeny consists of one or of suc- cessive new productions. In the two first and best known forms of sexual generation, the term Development has been usually given to a gradual process of changing and advancing growth by which the new animal is formed out of the ovum, till the period when it leaves it, or is said to be born ; and the term Metamorphosis has been generally applied to certain more marked and sudden changes of growth, apparently depending on the circumstance of the embryo or young animal having left the ovum, or having been born, at an early period in a com- paratively incomplete state of growth. But in establishing such a distinction between these terms, it is not meant to be affirmed that the changes which a young animal sub- ject to metamorphosis undergoes are indi- vidually or on the whole greater than those which occur in an animal which attains to its full growth by a process of development ; but merely that the one series of changes is less gradual than the other; and that the more marked changes which accompany metamor- phoses are related to certain conditions neces- sary to enable the animal which is born at an early period immediately to perform those acts which belong to its independent existence. It would indeed not be difficult to show that the changes which a mammal or a bird under- goes during its viviparous or oviparous de- velopment, are quite as remarkable and com- plete as those which occur in the change of a Batrachian reptile from its aquatic to that of its air-breathing condition, or of an insect from its larva to its complete form. In both of these instances one individual only is developed from the ovum, and that individual itself at last reaches sexual com- pleteness, and as being well understood they need not be longer dwelt upon here. But in the varieties of the reproductive process which are now to be more particularly noticed, the individual that proceeds directly from the ovum does not itself pass through the whole series of changes which are necessary to bring it to the form of the fully developed animal; but before it possesses any sexual organs, or has attained to sexual maturity, it produces from a minute germ formed in its body by a non-sexual process, a new indi- vidual, or a succession of individuals, the last of which only attains to the specific resem- blance of the parents, and acquiring sexual organs propagates the species by means of ova. This is the modification of the repro- ductive process already termed Metagenesis, and which has received so much attention under the name of " Alternate Generations" since the publication of Steenstrup's cele- * Adopting a term which has been used by Mr. Owen iu his Lectures. Med. Times, vol. xx. brated treatise under the title of " Generations- Wechsel" in 184-2.* No examples of this peculiar modification of the reproductive process have been known to occur in the Vertebrata, and with one ex- ception they are confined to the lower and simpler of the classes of Invertebrated animals. They are not, however, entirely confined to the very lowest classes of these animals as dis- tinguished by the Zoologist, but rather to the simpler and less developed members of each of the several classes in which instances of them have been hitherto observed. The essential nature of this form of repro- duction consists, then, in the development from the ovum of an individual which is dis- similar from the parent or parents producing the ovum, and in the succeeding production from that individual, by a non-sexual process, of a progeny of one or more, or a succession of individuals, of which the last of the series resumes the parental form. While in animals, therefore, reproduced by the ordinary form of generation, the species is composed of entirely similar individuals, or of individuals differing only in sex ; in those animals which are sub- ject to the alternate or intermediate genera- tion, the species includes a variety of indi- viduals usually of dissimilar form; of which some are without sex, and others are com- plete as regards the development of sexual organs, It has appeared to some authors that the phenomena in question are to be regarded as no more than peculiar modifications of the processes of development or metamorphosis, of such a nature that the product of the ovum becomes multiple instead of, as is more usual, remaining in its single individuality. But to admit the correctness of this view, it would be necessary to employ these terms in a sense widely different from that commonly given to them ; and, indeed, to modify the ideas of these processes of embryological development in a greater degree than seems warranted by what is at present known of their nature. The name of larva is usually given to the imperfectly developed animal that is born or leaves the egg at a comparatively early period, and fitted for independent existence in that state; and in the changes of metamorphosis by which that larva attains to the complete specific form, great as these changes may in some instances be, we recognise that it is the individual produced from the ovum which itself undergoes these changes ; whereas in the various kinds of alternate generation, it is always by the formation of an entirely new individual, arising from a minute germ con- nected with the first, but to be distinguished from its parts, and without a sexual process, that the species is at last completed. The new individual may be single or there may be a multitude of them ; they may remain con- nected with the one producing them or they * A work which appeared originally in the Danish language, ami in (icnnan in 1812, and of which an excellent translation into English lias beeu^ pub- lished by the Kay Society in lyio. OVUM. may be detached and live independently, but they nevertheless constitute different animals, and cannot be regarded in any other light than as so many individuals distinct from the one producing them, although all are de- scended from one ovum, or all are necessary to make up the entire species. And it is further to be observed that each of these several animals may be subject to in- dividual metamorphosis, and that in some classes there is so gradual a transition from individual change to new production that it may be difficult to determine to which of these forms of reproductive development their phenomena ought to be referred. In that part of the article which treats specially of development our attention may again be called to some of the more remark- able examples of individual metamorphosis that are known : at present it is intended rather to bring prominently forward those instances of alternate generation which have been discovered since the publication of the Article GENERATION, or which, if previously known, may now be viewed in a different light, in consequence of being brought into comparison with other observations of a similar kind and of more recent discovery. We may first consider some examples of this process, or of one very analogous to it, in which the new animal is single. Ecliinodermata. In several orders of this class a variety of the reproductive process has of late years been pointed out, in regard to which it may be doubted whether it is most of the nature of a metamorphosis or a meta- genesis, but which, as it has been considered by J. Miiller, the discoverer of the most in- teresting and remarkable of its phenomena, as in some measure analogous to the alternating O ~ generation, I will mention in this place ; the more so, that it might almost be looked upon as forming the connecting link between the direct aud the alternating processes of repro- duction. In some of the Echinodermata it appears from the earlier observations of Sars that the young produced from the ova are developed directly into the parental form, passing how- ever through several marked modifications in the early stages of development. Thus, some of the star-fishes (AsteracantMon g/aciatis, Sars) leave the egg as a ciliated free moving animalcule, then they become pediculated and attach themselves, have four club-shaped pro- cesses developed on them, and, lastly, they pass by the development of the rays and the internal organs into the complete form ; but here the whole, or nearly the whole, germinal mass of the ovum is converted into the embryo or larva, and the whole, or nearly the whole, of this undergoes the farther changes of con- version into the complete and sexual animal.* From the researches of J. Miiller it ap- * Sars, Fauna Littor. Norvegia?, 1846 ; and Ann. des Sciences Nat. ; Agassiz, Lectures on Comparative Embryology, New York, 184i) ; and a Letter from Desor to J. Miiller, in Archiv. fur Hiysiol. 1849, p. 79. pears that the mode of development now described is exceptional among the Echino- dermata, and that in other families of the order Asteriadae, and in the Ophiura and EchinidiC, an embryo or larva of a peculiar kind, is formed by direct development from the fe- cundated ovum, which is not itself converted into the complete animal, but rather serves as a temporary stock from which the perfect animal is subsequently formed in a manner that may be compared to gemmation. But it does not appear that more than one individual is developed from each primary larva stock, and this gradually dies away, so soon as its attached offspring has made some advance in its formation. This body, described under the name of Bipinnaria asterigera, as con- nected with an Asterias, is a comparatively large animal, with a long pediculated body, twelve or fourteen tentacles, an alimentary canal, consisting of mouth, gullet, stomach, in Fig. 7. Bipinnaria asterigera (from Midler}. A, the yonng larva before the Echinoderm is formed. B, a more advanced larva, with the Asterias on its summit. c, the Asterias torn up to show its stomach, a continuation of the alimentary canal of the larva. OVUM. 15 testine, and anus, and moves actively through the water. Sars who had observed this body in 1835, was the first to suggest in 1844 that it might be the early condition of a star-fish*, and this view was confirmed by the admirable researches of J. Miiller f, and by observations of Koren and DanielsonJ, who have shown that the Asterias is gradually formed out of a small granular mass which surrounds the stomach of the Bipinnaria, and becomes se- parated from the stock when in a compara- tively early state of advancement. The larva stock moves about afterwards for a few days, Fig. 8. Pluteus puradoxus {from Midler). A, Fluteus before the commencement of the formation of the Ophiura. u, Ophiura formed on the side of the gullet. * Wiegmann's Archiv. 1844, part i. p. 176. f Mem. of the Berlin Acad. 1846 and 1848. 1 Ann. des Sc. Nat. 1847, p. 348. and then appears to die without giving rise to any farther progeny. The gemmiparous larva of some other kinds of the Echinodermata was first described by J. Miiller as a distinct animal, under the name of Pluteus, before he was acquainted with the phenomena of its subsequent de- velopment : in 1846 he traced the relation between one kind of this body which he had called Pluteus paradoxus, and the Ophiura, and between another kind of Pluteus and Echinus, ascertaining it to be the same that has just been stated to exist between the Bipinnaria and the Asterias. The Plntens presents the form of a quadrangular pyramidal frame, with four large ciliated limbs at the angles, and four smaller ones suspended from the middle below, while the upper part is surmounted by ;i sort of dome. It bears some resemblance to a Beroe, and might be de- scribed as the ciliograde larva of an Echino- clerm. The form differs, however, somewhat for various species of Ophiura and Echinus. In the centre of the dome and round the mouth of the Pluteus a granular mass is de- scribed, and from the side of this, non-sym- metrically, the gemmation of the new indi- vidual proceeds. The Pluteus moves at first with great activity through the water, pro- pelled by its ciliated limbs and cirrhi; but as the new Ophiura or Echinus buds from it and spreads more and more over its dome, the Pluteus shrinks, becomes less active, and at last disappears.* Various other forms of the Pluteus-like animal have been described by Midler, and the process of gemmation has been traced by which the new Echinoderm takes its rise within them. The result of these discoveries is already to throw an entirely new light on the nature and organisation of this class of ani- mals ; but the species of all of those observed is not yet determined, and something still remains to be learned of the exact mode of origin of the new animal. By some-j- the process has been looked upon merely as a secondary development from the remains of the yolk attached to the parts first formed ; but the researches of Miiller do not appear to give support to such a view ; and would rather appear to show (as in Auricularia, fig. 9.), that the new animal is formed from a minute germ in a determinate part of the parent animal without that germ being traced to the yolk of the egg. In a farther series of researches on the larvae and metamorphoses of the Echino- dermata J, J. Miiller has pointed out that the Holothuridae are formed from a larva body somewhat analogous to the Pluteus, but that, instead of a process of new formation, the whole of the larva is converted by a very remarkable metamorphosis into the Holo- thuria ; and he has been enabled, from his * J. Miiller, in Mem. of Berlin Acad. 1846 and 1848 ; Derbes, in Ann. des Sc. Nat. 1847. t As Carpenter, loc. cit. p. ( J39. J Memoirs of the Acad. of Scicn. of Berlin, Nov. 184D, and April 1850, published in 1851, p. 35. 16 OVUM. own researches and the comparison of some others, to bring the whole of the Echino- dermata under a general view, the result of which is the determination of the three fol- Fig. 9. B Auricularia, or larva of Echinoderm (from Miiller). A, young larva of Auricularia. I, alimentary canal ; a, Echinoderm beginning to be formed. B, larger larva [of the same kind, a Echiuodenn farther advanced. " lowing varieties of metamorphoses and pro- duction among them. In all of them the embryo, immediately developed from the ovum, has a bilateral symmetrical form, and passes by the subsequent metamorphosis into the radiated type. This change is, however, more or less direct, or by intermediate forms. 1. In the first variety the change of the bilateral larva, or embryo, into an Echino- derm takes place at its earliest period, when the embryo has a general covering of cilia, but not the special ciliated borders or limbs of the Pluteus. A part of the body of the embryo takes the form of the Echinoderm ; the rest of it is absorbed into the body of the new animal. This occurs in a part of the Asteriadse, as in Echinaster, Asteracan- thion, and others, described by Sars, Agassiz, Desor, and Miiller. 2. In the second variety the change occurs when the larva is fully organised, that is, when it possesses digestive organs and a spe- cial motor apparatus of ciliated borders or limbs. The Echinoderm is placed upon the Pluteus somewhat in the manner of a picture on an easel, or a piece of embroidery in its frame and stand, and incorporates a part of the digestive cavity with itself. The remains of the larva gradually disappear, as in Ophiura and Echinus ; or are broken off and die, as in Bipinnaria. 3. In the third variety the change of the larva takes place twice. First, it passes from the bilateral type with ciliated borders into the radiate type, and having taken some- thing of the shape of a barrel, it acquires a larval locomotive apparatus consisting of ci- liated hoops ; and then from this state the Echinoderm is developed without any part of the larva being separated. Either the Echinoderm is formed of a part of the Vermi- form larva, and the rest of the larva is ab- sorbed into the Echinoderm, as in Tornaria ; or the whole larva is simultaneously trans- formed into the Echinoderm, as in Holothuria. From Busch's observations it appears that the Comatula passes very rapidly through the stage of the bilateral form into that which Miiller has called pupa with ciliated crowns. It is also an interesting fact in connection with the history of animal metamorphoses, that the early condition of the Comatula is that of a pedunculated Crinoid. Miiller has remarked that these pheno- mena partake in part of the nature of meta- morphosis, and in part of that of the non-sexual gemmation of the alternate generations. As the Echinoderm arises like a bud in the larva, there is alternate generation ; but as the es- sential internal organs (that is, the alimentary canal from the stomach to the anus, but not the mouth and gullet) are taken into the new animal, there is also true metamorphosis. " I understand," says he, " by alternate ge- nerations nothing more than the succession of two forms of organism, of which the one arises in or upon the other as a minimum, or as a bud ; the second, that is, the deve- loped bud, is destined for sexual generation, producing from its ova the non-sexual larva, which again is destined for gemmation."* Adopting the view that the Echinodermata present an example of alternate generation, it is to be observed that the product is single in all the instances known : but in all the other forms of intermediate or alternate generation hereafter to be noticed, the product of non- sexual gemmation is multiple. Polypina. The animals usually compre- hended in the general denomination of Polypes or Polypina present very various kinds of structure and degrees of complication in their organisation ; and recent researches, as to their mode of development, which point out that some of them are subject to a process of alternate or dissimilar generation, would ap- pear to indicate a very different distribution * Loc. cit. p. 106. The researches of J. Miiller on this subject have been published in a separate form, as well as in the Mem. of the Berlin Acad. These Memoirs, and others ou the same subject, will be found also in Miillers Archiv. 184G,' p. 108 ; 1X47, p.l GO; 1848, p. 113; 1849, pp. 79. 84. oG4. 400 ; and 1850, p 452. OVUM. 17 of these animals in the zoological system than that which has hitherto been followed. Most Naturalists are now disposed to separate from the true Polypina the Bryozoa, or so-called Ciliobrachiatc Polypes, which, though pre- senting a considerable resemblance to the Polypes in their external anthoid appearance, yet approach much more nearly to the Tu- nicated Acephalous Mollusca by their internal organisation ; and remarkable affinities have been pointed out between some of the Poly- pina and Acalephre, which show that these classes, though very dissimilar in their external forms and mode of life, are in reality very closely allied in structure. The greater number of the Polypina are ag- gregate or compound animals, that is, consist naturally of groups of individuals united or associated together on a common stem ami branches, or on a more solid stock. But the common fresh water Polype, or Hydra, and the various Actinias of the sea coast, are, to a certain extent, exceptions to this general rule, and, as we shall see, differ also in regard to their mode of reproduction from most of the other families of this division of animals. The Actinia is usually a single animal : no doubt it is multiplied occasionally by buds, but these are thrown off and become developed usually in an isolated position. The Hydra sometimes occurs as a single animal, but more frequently during summer, and when well nourished, as a compound one ; the multiple individuals being developed by gemmation from the first or principal stock, and also themselves forming younger progenies by budding ; but the indi- viduals so formed on the Hydra generally Fie. 10. Hydra ivridis in different stages of extension and contraction, reproducing tjemmipurouslt/, attached to the roots of duck-weed. (From Roesd.) separate from the parent stock when they have attained to maturity, migrate, and esta- blish themselves as independent animals, to form new buds. Both of these animals are capable of pro- pagation by ova formed in the sexual way : in Actinia this seems to be the more common mode of its multiplication, the ova being fecundated and developed within the body of Supp. the hermaphrodite parent ; but in Hydra it would appear that it is principally in the au- tumn, on the approach of cold weather, that the sexual mode of propagation is substituted Fig. II. Hydra viridis. A, Hydra of autumn, bearing an ovum, o,and two spermatic capsules, s, s. B, spermatic capsule burst artificially, showing spermatozoa. c (from Laurent), ova with young Hydra in various stages of development hanging out of them. i>, D' (from Laurent), portions of the body of summer Hydra, with a bud sprouting, n, the ear- liest ; i>', more advanced, showing the texture to be the same as the rest of the body. C IS OVUM. for that of gemmation which takes place throughout the whole of the summer.* The ova of Hydra are simple vesicular cap- sules of a brownish colour formed in the sub- stance of the wall of the animal's body, and separated from it previous to the development of the young ; while the spermatic filaments are formed in smaller conical capsules placed nearer to the base of the tentacula either in the same or in different individuals.-]- The for- mation of the young Polype has been observed by Laurent J to take place directly from the internal substance of the ovum, in which, how- ever, he has not traced in a sufficiently complete manner the individual steps of the changes of development (.tee Jig. 1 I.e.). The origin of the ovum in this animal is shown to be quite dif- ferent from that of a bud : the former having the shape of a distinct vesicle from an early period, the latter not being perceptibly more than an extension of some part of the sub- stance of the wall of the body, and precisely of the same colour and structure (see Jig. 11. I),D'.). The Hydra, therefore, while propagating very frequently by gemmation, is capable of reproduction also by fecundated ova, which are directly developed into the parental form. But many of the true Compound Polypes pre- sent examples, in their multiplication by gem- mation, of the production of intermediate forms of animals between the ova and the perfect sexual individual, a mode of repro- duction, therefore, which may be referred to Steenstrup's general law of Alternate Gene- rations. Thus, to begin with the simplest form of these animals bearing the nearest resemblance to the Hydra, in the Coryne and Syncoryne, at certain seasons of the year, multiplication takes place from the stem or root by gemma- tion, the buds being developed in the form of attached Polypes ; but at other times there are developed from the buds, without the con- currence of sexual organs, a set of delicate Medusa-like animals, similar to the Oceania, or those of the naked-eyed kind : these soon be- come detached, swim about freely in the water, acquire some of them male and others female sexual organs, and produce fecundated ova. * This effect of the cold season in changing the mode of production from gemmation to oviparous formation, thus checking growth, but providing for the preservation of the species through the winter, is, as remarked by Dr. Carpenter, an interesting ana- logy with the change that is known to occur hi the mode of production of the Aphis insect ; see Prin- ciples of Physiology. f The co-existence of ovigerous and spermigerous capsules on the body of the Hydra has been observed by many, as, first by B. de Jussieu, in 1743 ; (Ab- hand. der Swed. Acad. 1746, vol. viii. p. 211): by Trembley, in 1744 (Mem. sur les Polypes d'Eau douce) ; by Rosel (Insecten-Bclustigung) ; Pallas, in 1776 (Karakteristik der Thier-pflanzen, p. 53) ; and more recently by Ehrenberg, in 1836 and 1838 ( Verhand. der Naturforsch. Freunde! in Berlin, 1838, p. 14) ; V. Siebold (Lehrbuch der Vergleich. Anat.) ; and by myself (Edin. New Phil. Journ. 1847). J Nouv. Rech. sur les Hydres d'Eau douce, 1814, Voyage de la Bonite. These ova give rise, by their development, to a ciliated moving embryo : this soon becomes fixed to a spot, and is gradually converted into a Polype, similar to that from which the Medusa-like animals were formed.* Fig. 12. Syncoryne, developing a flfednsoid progeny. Oceania (From Desor.} A, natural size. B, a portion enlarged, showing the budding of Medusoids in different stages. c, one of the Medusoids, naturally detached. r>, another, farther advanced ; o t, ovary, or tes- ticle, placed on the alimentary canal ; o', ova. R. Wagner appears to have been the first to observe Medusoid bodies produced from the Polype animals, as in Coryne aculeata, in 1833-h, but the more full observation of the remarkable phenomenon of their formation is due to the researches of Sars, Lowen, Steen- strup, and Van Beneden, who have ascertained the relations of the Polype larva and Medusoid progeny, and the production of ova from the latter. DujardinJ has also carefully traced the production of the free Medusoid bodies from a Syncoryne, which he has called Stauridia, and has farther ascertained the sexual condition of these Medusoids, observed the formation of their ova, and the subsequent development of these ova into Polypes. * See fig. of Syncoryna Sarsii, from Sars, Fauna Litt. Norveg. 1846 ; and Steenstrup's figures of Coryne fritillaria, tab. 1. figs. 41.43, and Desor, in Ann. des Sc. Mat. 1840, pi. 2. figs. 13. to 16. t Isis for 1833, p. 256. Also in Coryne vulgaris, in Icones Zootom. Tab. xxi. 1841. t Annal. des Sc. Nat, 1845. OVUM. 19 In a certain number of the Campanulariae, Sertulariae, and Tubulariae, of which the in- ternal structure is more complex than in the Coryne, and in which the Polype always na- turally presents a branched form, or groups of distinct Polype heads formed upon a common stem by gemmation, it is now well ascertained that the Polype state is not the only nor the complete condition of the animal, but that by Fis. 13. But the interesting observations of Lovcn*, and also some previous observations of Lis- ter f, would show that in the Campanularia Branch of Sertularia geniculatn, magnified, shewing polypes, and oviyerous capsules. a process, in some instances similar to that above described, in others, somewhat different from it, a set of bodies, charged with the office of the sexual production of the ova, are deve- loped in place of the more ordinary Polype heads or individuals. In the Campanularia ge- latinosa, according to Van Beneden, the gene- rative heads are close bell-shaped capsules, within which small Medusoid bodies are deve- loped by a process apparently analogous to gemmation, or, at all events, without sexual generation, and each of these Medusoid s be- coming free, move about in the adjacent fluid as independent animals. The farther destina- tion or changes of these Medusoid bodies have not yet been observed, but from parallel observations in other similar animals, it is believed that they afterwards attain to sexual completeness, and form ova which are de- veloped into the Polype form.* * See the View of Campanularia geniculata, \>y Van Beneden, in Mem. de 1'Acad. de Bruxelles, 1844, vol. xvii. ; and Ann. des Sc. Nat. torn. xx. p. 350, 1843. See also the very interesting account of Tu- Campanularia. ( From Desor.) A, po rtion of a branched stem, magnified, c, non-sexual head or individual ; , part ii.) See also Dujardin, Mem. sur le Developpement des Merluses et des Polypes Hydraires, Ann. des Sc. Nat. 1845. The reader is also referred to Dana's great work on Zoophytes in the United States Explor- ing Expedition. " Philad. 1848. OVUM. 21 undergoing a slight change of form, fixes itself by the narrowest end, and acquires tentacles like a Polype at the other, amounting for some time to eight. In this condition it appears to Fig. 16. Development of Rfedvsce. ( From Sars, Steenstrup, and Dalyell.) a, b, ciliated free moving embryo from the ovum ; c, embryo attached bj T its pedicle ; d, its tentaciila beginning to be formed; e, with four, /; with eight tentaciila ; g, the fully developed polype, producing other polypes by gemmation ; h, i, k,~ transverse di- vision and development of Medusae from the polype stock or strobila ; /, a pile consisting of four Me- dusoids just about to separate ; m, , and lower lateral view of Medusae separated from the polype stock; o, more advanced, natural size: p, r, (from Dali/ell), p, a pile of medusa discs separating, and new tentacula formed on the polype at the base ; r, the same, with more of the discs separated ; the strobila returning to its polype state arid budding at the side. be capable of multiplying itself, or producing other similar attached Polypes by gemmation from its side or base, or from a running stolon below it. The subsequent change of each of these polypoids is remarkable. It has been described by Sars and Dalyell as follows : The body undergoing some elongation be- comes partially divided by transverse grooves, into a range or column of imperfect Medusae, attached still to each other by their adjacent surfaces, but presenting at their borders, in various degrees of advancement, the division into rays or lobes which belong to the Me- dusa ; the upper or terminal one having deve- loped upon it a set of radiated processes dis- tinct from the tentacles of the Polype and much longer than those of the rest. These young Medusas are successively separated from the stock by the deepening of the transverse clefts between them. They then move about as independent animals, and proceed in their farther growth and development to sexual and other completeness. These bodies, therefore, are subject to two kinds of multiplication, which are very different : by simple gemma- tion a number or a colony of Strobilae may be produced, and by transverse fission and deve- lopment a number of Medusse may be thrown off from each Strobila. A considerable number of the Medusa pro- geny having been separated, the Strobila stock generally returns to its polypoid condition, Fig. 17. Production of Medusa; (Amelia aurita) from Po- lype stock. (From Desor.) A, Medusa-form larvae on the stock above the polype, which remains at the base, a. r,, lower surface of a detached Medusa. c, D, natural size. Young Medusae forming from the polype above its disc. c 3 22 OVUM. and may remain for a long time in that state ; continuing to multiply by budding into others of the same kind, and occasionally giving rise by the process of fission to its Medusa progeny. The observations of J. Reid * have shown that the Polype or polypoid stock may remain for a very long time in this condition without forming any Medusa progeny; and these obser- vations, as well as those of Steenstrup and of Desor, appear to show that these Polypes bear a considerable resemblance in their internal structure to the Medusae which they produce by gemmation. The latter author, indeed, is inclined to believe that the new Medusa ani- mals are produced not by a mere transverse fission of the Polype, but by successive gem- mation on its summit, that is, round its mouth and within the tentacula; and he states that he has observed the Polype remaining with its tentacles at the base of the Strobila of Medusa?. The observations of Dalzell and Fig. 18. Medusa larva. (From J. Held.) A, Polype before it has undergone any gemma- tion of Medusa;, showing the mouth and four canal openings. B, the strobila or larva forming Medusae. c, lower surface of one of the young Medusa;, after separation. J. Reid appear, however, inconsistent with this view ; but it is possible that there may be varieties in respect to the mode of formation of the Medusa progeny, so that in one set the tentacles of the Polype may be included in the upper Medusa, and when all the progeny is separated, new tentacles may be formed on the Polype stock at the base, while in others the budding Medusa may be within the circle of the tentacula of the Polype. It appears from recent investigations that * Ann. and Mag. of Nat. Hist. 1848. others of the Acalephaj also undergo remark- able processes of non-sexual multiplication. According to Huxley's recent most interesting researches *, the Physsophoridse, Diphydae, and Physalia, are to be regarded as compound organisms in which the floating processes of most various form are analogous to Polype or attached Medusa individuals, which are the bearers of sexual organs, in some of one kind, in others of both, and others of which are neuter, on the same compound stock. ] These are probably a progeny developed by budding from a single individual, which is the parent stem. By these discoveries a remarkable relation is shown to exist between the medusoid and polypoid animals. Some we have been ac- customed to see principally in their largest and most developed condition as Medusae, others are best known in that polypoid condi- tion in which they remain for the longest time; but we must regard that condition in which sexual reproduction takes place as the complete one, and this we have seen is in both the Acaleph or Medusa form, while the Polvpe or polypoid state, however permanent it may appear, is to be looked upon as a pre- paratory stage, in which, it is true, multiplica- tion of its own kind may occur by gemmation, but which can only effect the true reproduc- tion of the species by forming its progene of Medusans to which is committed the offic of producing the fecundated ova. This, there- fore, is another example of multiple metage- nesis, or alternating generation. J Mollusca. Among the Mollusca the only examples of alternate generation that are yet known have been observed in the Tunicated Acephala : and among these, three modifica- tions of the reproductive process are known in the Bryozoa, Ascidia, and Salpidas. The Bryozoa, or so-called Ciliobrachiate Pohpes, long ranked with the Polypes on account of their union in branched groups, their radiated arms, and retractile body, but now regarded as more nearly allied by their internal organisation to the Tunicated Mol- lusca, present a very marked example of the multiplication by budding of the progeny of a single ovum. These animals never continue for any considerable time as single or distinct individuals, but, multiplying by gemmation, form numerous colonies, in which the new individuals remain connected with the pri- mitive one from which they have proceeded and with each other. They thus always con- stitute compound groups spreading from the first individual as from a centre. All the in- dividuals of the group may acquire sexual completeness, and the male and female organs are united in each individual : the ova are fecundated within the cavity of the mantle; * Phil. Trans. 1840, Pt. ii. f Professor Goodsir has informed me that his ob- servations on Stqihanoinaia are quite confirmatory of this view. J See also on this subject the interesting treatise hy Prof. E. Forbes, on the Naked-eyed Medusae, in Itay Soc. Pub. 1848. OVUM. 23 Fig. 19. on leaving the parent body they become deve- their early state present some very striking loped into a ciliated embryo, which, for a time, phenomena of metamorphosis, yet there is moves freely about, then becomes fixed, un. nothing in either which fully deserves the name dergoes farther changes in being developed, of alternate generation, for all the individuals of which these compound animal structures consist are alike sexually perfect, and there does not appear to subsist any necessary con- nection between the nonsexual process of multiplication, and the subsequent exercise of the sexual function. There are, in fact, scarcely any intermediate stages of non-sexual exist- ence such as are described in the true in- stances of alternate generation. It is deserving of notice, however, that Lowig and Kiilliker are of opinion that in some of the Botryllidie numerous embryoes are at once developed from a single ovum by its division, these indi- viduals subsequently multiplying by gemma- tion into the perfect sexual animals. A series illustrating the development bi/ ova of Pedi- cellaria. {After Van Beneden.') and now from its own body in some, and in others only from the spreading part of the stem or base which supports it, proceeds the gemmation of other individuals of the colony, all of which apparently are capable of sexual generation when they arrive at maturity. j The Ascidian Tunicata present another mo- dification of the reproductive process now under consideration. Two forms of these animals exist, both perfect, viz. the simple and the compound; but these are not related to each other in the same manner as the two kinds of Salpians ; for each kind is capable of propagating its like by generation. The soli- tary ones rarely multiply by gemmation, and when they do so the individuals separate from the stock; but the compound animals always undergo this mode of multiplication, and the multitude of individual Ascidians are in this form collected together in a mass of various shape, in which the circulation of fluids is for a time common among the different indivi- duals. The individual animals produced from the stock by gemmation attain to sexual com- pleteness, and propagate by means of ova, in the same manner as the solitary or distinct Ascidiae do. The young of these animals undergo a re- markable metamorphosis : they are first ex- cluded from the egg in the form of a moving tailed body, somewhat like a minute tadpole"; and this caudal organ of motion is lost pre- vious to their becoming fixed, and the deve- lopment of the more complex organisation.-f- Although the changes to which both the Bryozoa and Ascidian Tunicata are subject in * Van Beneden, in Mem. de Bruxelles, torn, xviii. See the Article Polypifera, for au account of these researches. f See Mr. Rupert Jones's excellent Article TUNI- CATA for an account of these phenomena, and the .special Memoirs of Milne-Edwards, sur les Ascidies Composees, &c., Paris, 1832; Lowig and Kolliker, in Ann. des Sc. Nat. April, 1846; Van Beneden, sur les Ascidies Simples, Brussels, 1847. Fig. 20. Bowerbanhia densa. {After Farre.) a, one of the animals fully expanded. b, a similar animal completely retracted. c, an immature animal. d, one of the gemmse in its earliest state. SalpidcE. The most marked example of alternating generation among the Tunicata is that which, since its first discovery by Chamisso, in 1819, has been known to occur in the Salpidse. This process has been so well and fully described in the article TUNICATA, that it is not necessary to give more than a short outline of it in this place. These animals are known in two states, viz. solitary and ag- c 4 OVUM. gregated ; the latter being not organically united like the compound polypes, but merely adher- ing more or less strongly to one another so as to form a chain. The aggregated, but not the solitary kind, possess sexual organs; and it would appear, though this is not yet deter- mined with certainty, that all the individuals of one chain are of a similar sex either male or female. Fig. 21. Solitary and aggregated Salpce. (From Sars.) A, solitary Sal pa, with chain of aggregated ones, g, budding from it. B, this chain magnified, shewing the successive sets ill different stages. c, one of the more advanced aggregated Salpre from a ch;iin, /, the place of a foetus formed by sex- ual generation. D, foetus from another more advanced, magni- fied ; h, the yolk, by which it adhered to the pa- rent ; g, the place of the germ for the aggregated chain. All the individuals of a chain of aggregated Salpae are produced from a solitary one by a process of internal gemination, or gradual deve- lopment from an internal stolon, or germ-stock, from which they are detached gradually and in successive groups : all the individuals of the chain are contained within a tube, and become united to each other after their development, presenting a series of groups of different de- grees of advancement ; but the individuals in each group being nearly at the same stage of development. The distinct or single Salpae, which, with the exception of the want of the sexual organs, do not differ materially from the in- dividuals of the aggregated chain, are produced from fecundated ova which are developed within the body of the parent. These ova differ from the germs from which the aggregated in- dividuals take their origin in the possession of a yolk, and external envelope. Their de- velopment proceeds to its termination within the parent body, and the young Salpa is already provided with the internal stolon for the gem- mation of its chain progeny, before it passes into its separate state ol existence. The solitary Salpae may be looked upon, therefore, probably as incomplete or larva forms, and the aggregated are the fully deve- loped sexual individuals. The generation of this animal, therefore, is precisely an example of that succession of two different kinds of individuals which has been distinguished as alternation of generations ; each fecundated ovum of the sexual individuals being developed into an animal which never acquires sexual organs, and which produces by a process ap- parently of the nature of gemmation, a nu- merous brood of individuals associated in a chain ; all of which are sexually perfect, one set developing only spermatozoa, and the females among them being the producers of the ova, which are the source of the new generation.* Although no other instances of alternate generation have yet been observed in the class of Mollusca, yet it is possible that modi- fications of this process may hereafter be dis- covered. An observation related by Agassizf, in regard to the development of the ovum in one of the Eolidae, deserves to be recorded, as it may be found to constitute an approach to the metagenetic process. After having de- scribed the usual process of division of the yolk in which the first stages of development consist, and the farther progress of formation in the Eolis, he says, " But the most curious phenomenon which takes place is this ; that the whole yolk does not constantly go to form one single individual. But there may be instances when the mass of yolk, which has been subdivided into cells, is itself di- vided into two or three or more masses, which grow independently, several individual animals arising from one mass of yolk, which thus divides." Entozoa. Among the Entozoa the process of reproduction is effected by very various means. All the Nematoidea, or round worms, are of distinct sexes ; and their fecundated ova are developed into the parental form without any metamorphosis of a marked kind, (ex- cepting perhaps in the Echinorrhynchi, the * Sec Savigny, Mem. sur les Anim. sans Verteb. 1816 ; Chamisso, De Salpis, 1819 ; Meyen, Ueber die Snlpfii; Eschricht, in the Isis, 18-12; Sars, Fauna Littor. Norvegiae, 18-10 ; Krohn, Ann. des Scien. Nat. July, 1846; who first pointed out the existence of spermatozoa in certain individuals of the aggre- gated chain. f Lect. on Comparative Embryology, Boston, 1849, p. 81. OVUM. process of generation in which is not fully understood,) nor any intermediate process of gemmation. A few of them, however, ap- pear to become encysted in the parenchyma of organs in their young or undeveloped condition, and some in a form different from the parent, as in the Trichina of the muscles, the so called Filaria of the peritoneal cavity of fishes, and the Vibrio tritici. These en- cysted Nematoidea have not been observed to be possessed of sexual organs *, and they are not known to be multiplied by gemma- tion ; it is probable, therefore that, to attain the place of their full development, they must be subject to migrations from one animal to another, either directly or in other ways, as through water and vegetables. The ova of these animals appear to possess a remarkable tenacity of life, as exhibited by their long and obstinate resistance to the noxious effects of external agents, -j- The Cystic, Cestoid, and Trematocle orders of the Entozoa present a more varied process of generation, the investigation of which has of late years attracted considerable attention, and which has led to most interesting results as to the nature and relations of several forms of these animals, which were previously re- garded as of a most anomalous kind. The Cestoid and Trematode Entozoa have long been known to possess the sexual organs in the hermaphrodite arrangement, and to pro- duce fecundated ova ; while the Cystic En- tozoa have been observed to multiply only without sexual organs, and by a process analogous to gemmation, and their first origin has been till lately involved in the deepest obscurity. We shall presently see that many, if not the whole of them, may be either un- developed or metamorphosed aberrant forms of cestoid or trematode animals. J This view appears first to have been sug- gested by Steenstrup, in connection with his researches on alternate generations ; and it * See a Memoir by V. Siebold, on the Nonsexual Nematoidea, iu Wiegmann's Arcliiv, 1838. t Dr. Henry Nelson and I have observed the de- velopment of the ova in Ascaris mystax to proceed for several days, while the parent bodies containing them were immersed in oil of turpentine. J For a notice of the generation of the minute parasitic animalcule called Gregarina, see the pre- vious account of the reproduction of Infusoria. j See Eay Society's Translation, 1845, p. 100. "It is not unlikely," says Steenstrup, "that in course of time, it may happen with them (Cystic Entozoa), as it has with the whole division of the asexual Trematoda of Siebold, viz. Cercaria, &c., that they must be rejected from the system as being earlier forms of development, or earlier generations of other animals." V. Siebold remarks in a note at p. 157, of his Lehrbuch der Vergleich. Anat. part i. published in 1845, " Here the doubt arises whether the asexual Cystica really deserve to be considered as independent animals. It is very probable that the vesicular worms are undeveloped Cestoids," &c. See also note at p. 111. Von Siebold has developed these views more fully in a recent Mem. in the Zeitsch. fiir Wissensrh. Zool. July, 1850, translated in the Ann. des Scien. Nat. vol. xv. 1851, p. 177 ; and in the article Parasites, in Wagner's Handwor- terbuch der Physiologic. E. Blauchard in his Kech. has since been adopted, in somewhat dif- ferent forms, by V. Siebold, Blanchard, Du- jardin, and Van Beneden, and rendered extremely probable by the researches of these and some other observers. Previous to the adoption of this view, helminthologists, looking upon the Cystic Entozoa as dis- tinct and independent animals, were at a loss whether to regard them as ascertained excep- tions to the sexual mode of propagation, or to continue to prosecute their inquiries in the hope of being able to discover a process of generation in them analogous to that prevail- ing in the greater majority of the animal kingdom ; and many were thus misled into the error of searching for ova where none existed or were required. Thus Gulliver erroneously de- scribed certain calcareous particles in the mem- brane of Cysticercus as the ova of the animal*, and H. D. Goodsir, in his instructive paper on the production of the young in that animal, and in the other forms of Cystic Entozoa f , failed to distinguish between that which might be merely a process of gemmation and the origin of the embryoes from true ova.J Cystic Entozoa. The Cystic Entozoa pre- sent themselves in three principal forms, viz. Acephalocyst, Caenurus, and Cysticercus. The two first are usually found as compound or aggregated animals ; the last is more fre- quently seen in the single or isolated condition. Some of the vesicular hydatid tumours, constituting the various kinds of so called acephalocysts, have long been known to con- tain small Echinococci floating in the fluid of their interior. Repeated observations have demonstrated the existence of these animals in the acephalocysts ; and it seems very pro- bable that, in the end, it will be necessary to withdraw the distinctions between the various kinds of these cysts, as they will all, by suffi- ciently accurate observation, be found, at some period of their growth, to contain in a more or less complete condition, the small animals of Echinococci, or their remains. The Echinococci are produced by non- sexual generation, or by gemmation from the membrane of the vesicle, probably from the middle or germinal membrane, as it has been sur 1'Organis. des Vers, in Ann. des Scien. Nat. 1847 vol. vii. p. 120. excludes entirely the Cystica from a separate place in the systematic arrangement, bring- ing them under Cestoidea, and affirms decidedly that the distinction between them ought now to cease, as they are shown to be different states of the same animals. He refers to De Blainville as having previously entertained the same view. See also Dujardin, in Anual. des Scien. Nat. for 18 13, and Hist. Nat. des Helminthes, 1845; Miescher, Be- richt lib. die Vcrhand. der Naturforsch. Gesellsch. in Basel, 1840; and Van Beneden, Ann. des Scien. Nat. 1851, p. 309 ; and a work on the Entozoa, pub- lished at Brussels, in 1850, which I have not seen. * Med. Chir. Trans, of Lond. vol. xxiv. 1841. f Trans. Roy. Soc. Edin. vol. xv. 1844, and in Anat. and Path. Observations, 1845. J See also Rose, in Med. Chir. Trans, vol. xxxi. 1848. See V. Siebold's Report on Zoology, in Ray Society's publications, for 1845 and 1847 ; also Bur- daeh's Physiol. B. ii. 26 OVUM. called by H. D. Goodsir ; and they have been observed, in some instances, attached in pedi- culuted vesicles, singly or in groups, to the inner Fig. 22. cells, from which it is supposed other young animals or heads may be formed.* Fig. 23. Kchinococcus hominis. (From Wilson.') A and B, grouped and single Echinocoeci, at- tached by peduncles to the inner membrane of the cyst, c, a contracted, and D, an expanded Echino- coccus ; a, the peduncle. E, animal, shrivelled. Canurus cerebralis, magnified. (After Bremser.') a a, part of the general vesicle ; b, an expanded head; r, a shorter head, showing the double circle of booklets. The Cysticercus has been described in two forms ; 1st, in its simply vesicular state, and 2nd, in its fasciolated condition, or in its transition, as it may beheld, to the cestoid, or a more advanced tape form. The vesicular Cysticercus has surface of the cyst* While enclosed in the pediculated vesicles, the head of each echino- coccus is retracted within the short vesicular body in a manner which seems to be general among the young of encysted Entozoa. They are afterwards set free, and in this state are found floating as minute whitish particles in the fluid of the cyst. They then present the appear- ance of minute heads of Taeniae, with a short body scarcely larger than the head ; the latter part being furnished with a terminal double circle of booklets, and four suckers.f The mode of gemmation may probably vary in different circumstances, more particularly in regard to the extent to which the progeny of gemmation may or may not repeat the for- mation of others of the same kind ; but every thing that is known of the acephaloc* stic productions seems to point to the view that they are all nearly allied, and that they are abnormal or aberrant conditions of Taenia- Iarva3, which, when they become encysted, are incapable of development into the cestoid form which belongs to those that have reached the free intestinal habitation. The Ccenurus, which has been met with principally in the brain and some other parts of the sheep and some other Ruminating ani- mals, consists of a large cyst or vesicle with a number of small heads projecting on its ex- ternal surface : each head resembles closely that of an echinococcus animalcule, presenting the same circle of booklets and four suckers. According to H. D. Goodsir, they are at- tached to the middle membrane of the cyst, from which they sprout at first, carrying the outer one along with them : the neck contains 3 * E. Wilson's paper in Med. Chir. Trans, xxviii. 1845 ; and H. D. Goodsir, Anat, and Path. Obs. f See Curling, in Med. Chir. Trans, vol. xxiii. ; and Mtiller, iu Jahrsbericht of Archiv, 183G. p. 100. Fig. 24. Cysticerci. A, Cysticercus longicollis (from Bremsev), en- larged. B, Cysticercus from the human eye (ex- tracted by l)r. Mackenzie), magnified five dia- meters. only one head ; but the structure of that part is precisely the same as in the Casnurns and Echinococcus, and, we may add, not far different from that of the Taenia itself. They are usually developed singly, that is each vesicle with one head : but some ob- servers f allege that they have seen internal vesicles near the neck, which they look upon as young, or as a progeny of gemmation in that situation. The Cysticercus fasciolaris, as it has been observed in the rat and mouse, presents the remarkable fact of a Taenia in various states of development, from the vesicular condition of * H. D. Goodsir, loc. cit. f As Hose and II. D. Goodsir, loc. cit. OVUM. 27 the true Cysticercus, to a form in which the caudal vesicle has diminished to such an extent as almost to have disappeared, while at the same time the body has been divided into segments by transverse grooves, as in the Taenia ; and in some instances these seg- ments have even acquired sexual organs while the animal was still encysted, a circumstance which has never been observed in any true Cysticercus. Fig. 25. Cysticercus fasclolaris of tlie Mouse, and Ticnla crassicollis of the Cat. A, Cysticercus fasciolaris from the liver of the mouse, natural size. B, the head of the same, magnified. (From Dujardin.) c, head and first segments of the body of Tamia crassicollis of the cat, showing the double circle of hooks ; a few of the smaller under circle being seen where one or two of the larger ones have fallen off. A close comparison of the structure of the Cysticercus fasciolaris of the rat and mouse in its various stages of development with the Taenia crassicollis of the domestic cat, has shown an almost complete similarity between these animals, and has suggested the view that the encysted Taenia (which the Cysti- cercus fasciolaris in truth is) may attain its full development as a TiEnia in the intestinal canal of those animals which prey upon the smaller Rodentia, in whose liver it begins to be developed in its first simple vesicular form, and gives the greatest probability to the sup- position that there may be a similar general relation between the Cystic and Cestoid En- tozoa, not of the same animals, but between the tapeworms of different tribes of predaceous animals and the vesicular worms of others serving them as food.* * Dujardin, Hist. Nat. des Helminthes, 1845. E. lilanchard (who does not appear to have fully appre- ciated the necessity of change of habitation for the entire development *of the trenia), Stir 1'Organisation des Vers, Ann. dcs Scien. Nat. 1848, torn. x. p. o48. V. Siebold, in Zeitsrh. f. Wiss. Zool. 1850, and Ann. des Sciences Nat. 1851. I am indebted to Dr. Henry Nelson, for an account of some interesting researches on this subject which formed a part of his Inaugural J >isscrtation " On the Development of the Entozoa," on obtaining the degree of M. D. at the University of Edinburgh, in 1S6U. The limits of this article The different phases of development, there- fore, in which the so-called Cysticercus fascio- laris has been seen in the same and in dif- ferent animals which they inhabit, leave little doubt that they are encysted Taenise, which proceed to a much more advanced stage of development than is usual with the vesicular and encysted form of these Entozoa ; and we are warranted, from the great similarity of structure, in adopting the view that the true vesicular Cysticerci, the Caenuri and Echino- cocci, are morbid or metamorphosed and aberrant conditions of the embryoes of various Tajniae, which may be capable, to a greater or less degree, in different kinds of animals, of multiplying their own incomplete forms by a process of non-sexual gemmation, but which never, in the encysted condition (except in the instances already referred to of the fascio- lated kind), attain to sexual completeness ; but which either undergo a retrograde change, and thus form tumours and various pathological deposits in the seat of their cysts, or become developed to such an extent as to be injurious or destructive to the animal in which they reside.* Free Tapeworms. Three principal forms of cestoid worms are now distinguished from one another, viz. Taenise, Bothriocephali, and Tetrarhynchi ; the two first have long been known and sufficiently well characterised in their fully grown condition, though little under- stood in their early or incomplete states ; the history of the third, until recently, has been involved in great obscurity, as it has been most variously described by different ob- servers both in the earlier and more advanced stages of its growth. It appears now to be ascertained that all of these cestoids are com- plete animals, with a single head, a body composed of a multitude of segments, each of which contains male and female sexual organs, which are developed only when the entozoon is living free in the alimentary canal of animals belonging principally to the Verte- brata. The Taeniae inhabit chiefly the alimen- tary canal of mammals and birds ; the Bothrio- cephali and Tetrarhynchi more frequently that of fishes and reptiles, and the latter a few mollusca. The Tetrarhynchi have been more frequently described in the encysted and im- perfect condition than in the full-grown form, and in such varieties, that V. Siebold has mentioned about sixty different kinds of worms described by various authors under distinct ap- pellations, which might, according to him, be prevent me from entering into the details of Dr. Nelson's observations, which have not yet been pub- lished. It is enough to mention that a very careful comparison of the Cysticercus fasciolaris of the mouse and rat, in various stages of its development, with the Ta'iiia crassicollis of the cat, enabled him to confirm, in a most satisfactory manner, the view which, unknown to Dr. Nelson, had previously been taken by V. Siebold, that these cystic and cestoid forms are different stages of one and the same animal. See also Leuchnrt on Cysticerci, in Wieg- imnn's and Erichson's Archiv for 1848. * See Gulliver, in Mod. Chir. Trans. 1841. 28 OVUM. brought under the genus Tetrarhynchus. In fact, this kind of animal undergoes such re- markable changes in its transition from its first simple Echinococcus-like encysted form to its free segmented sexual Ttenia-like shape, that it is not wonderful that its history should have been obscure, and that great doubts should still prevail with someHelminthologists as to its origin, development, and zoological relations.* It has already been observed, that none of these three kinds of Cestoid Entozoa attain to sexual completeness while they are en- cysted ; and it seems probable that they are all subject, more or less, to migration, in order to gain their free habitation in the alimentary canal of animals, where their segments ac- quire the male and female generative organs. The fecundated ova, produced in enormous numbers from each segment, do not in general, so far as is known, become developed into embryoes in the intestine of the animal in- habited by the Cestoid, but are evacuated along with the faeces, either separately after being discharged from the oviducts of the Cestoid, or before their discharge by the disjunction of the more ripe terminal segments from the rest of the animal. The migrations to which the ova and young of the Taenioid animals are thus made subject have hitherto opposed so great an obstacle to the observa- tion of their development, that we are as yet in possession of very few continued series of observations in which the whole progress of development from the ovum to the complete segmented animal has been traced. Some important contributions of this kind have, however, recently been made, and the great modifications which the views of comparative embryologists have undergone, from the novel and various aspects in which many of the phenomena of development are to be regarded in instances of alternate generations, have already indicated paths of inquiry by which this very curious and intricate history may ere long be completely unravelled. The ac- companying figures from Dujardin's work show the progress of formation of a small Taenia inhabiting the Shrew, and give a suf- ficiently good idea of the nature of this pro- cess in a Taenia, which consists of compara- tively few segments (fig. 96. a to ?'.). Von Siebold has traced with care a part of the process of development of a minute Cestoid inhabiting the pulmonary sac of the red snail (Arion einpiricorum) in the encysted condition. Into this situation the minute Taenias are introduced from the exterior : they consist of the head with its double circlet of ten hooks each, and four suckers, and a body which is at first entirely destitute of segments, not longer than the head, and form- ing a soft vesicle, within which (as in other * Von Siebold proposes to substitute the genus Tetrarhynehus for the following five genera distin- guished by Dujardin, viz., Ithynchobothrius, Antho- cephalus, Tetrarhynehus, Gymnorhynchus, and Di- bothriorhynchus. Zeitsch. f. Wiss. Zool. 1850, and Arm. des Sc. Nat. 1851. Cystic Entozoa previously mentioned) the head is retracted, so as to give the whole a globular shape. V. Siebold regards it as nearly certain that these minute Taeniae only attain to their segmented and complete sexual condition when they have been located in the alimentary canal of Vertebrata (Birds and others) preying upon the snails in which the younger forms of the Taeniae reside. Fig. 26. Development of Tamia pistillum of the Shrew. (From Dujardin.*) a, embryo within the ovum, just about to quit it, with three pairs of hooks ; b, embryo that has left the ovum, the hooks capable of rapid and exten- sive movements ; c, embryo moving freely (of the Trenia serpentulum of the magpie) ; d, e, very young embryoes of Taniia pistillum ; f, y, h, i, different stages of growth of this Tajnia ; the separation of the segments gradually increasing, and the develop- ment of the reproductive organs in the posterior ones ; k (more magnified), the proglottis, or free moving separated segment of this Tasnia. OVUM. The instance already referred to, of the iden- tity of the Cysticercus of the liver of the mouse and rat with the Taenia crassicollis of the cat, and a variety of detached observations which prove that the Bothriocephalus and Tctra- rhynchus pass through similar changes from a small Echinococcus-like animalcule to the developed cestoid form, lead to the corro- boration of the same general view that the encysted condition of these Entozoa is an incomplete non-sexual embryo or larva, from which, when it passes into the free state, there is formed by a process of transverse Fig. 27. B Tamia solium. (From Blanchard.) A, one of the longer mature posterior segments with the sexual organs fully developed ; o, o, rami- fied ovary full of ova ; o', the oviduct ; t, the tubu- lar testis ; t', the penis, &c. B, head, neck and anterior recently formed seg- ments. fission a segmented individual or compound animal, in which each segment, as it arrives at maturity, attains to sexual completeness. In this process the new segments are always developed between the head and those already formed. If the character of sexual complete- ness is to be taken as the distinguishing mark of individuality, each segment of the Cestoid may be looked upon as a distinct animal, and the separation of them by transverse fission may be compared to the separation of Medusa individuals from the Strobila polype stock. The Cestoid Entozoa might in the same manner be considered as subject to a peculiar process of alternate generation. In the preceding sketch of the nature of the reproductive process in the Cestoid Ento- zoa, I have followed chiefly the views of V. Siebold as explained in the interesting Me- moir already referred to. It is right to state, however, that the phenomena have been viewed in a different light by several observers of high authority. Thus, Blanchard and Van Beneden consider the first stage of the Tetra- rhynchus-embryo to be a Scolex, in which, after it has been encysted, the Tetrarhynchus is formed : this, according to Blanchard, is its complete condition ; but, according to Van Beneden, the so-called Tetrarhynchus is con- verted into a Rhynchobothrius, and this is in the last place changed into a separate Tre- matode animal.* Dujardin had previously taken the same view as applied to the separate and independent nature of the joints of the Tccnia, which he regarded as individual Trc- matode animals, and described under the name of Proglottis(seejtfg. 2G.X.-.)f; but though there may be some points of analogy between the single segments of Ta3iiia and a Trematode, yet the absence of head, differences in the alimentary canals, and other circumstances, render the correctness of this view, at all events, still doubtful. Trematoda. These animals, the most common of which are known as Flukes (ex- cluding the Planariae), comprehend a set of internal parasites of a structure bearing some resemblance to the Cestoidea, but single, that is, not jointed or segmented. The nervous and vascular systems attain to a considerable degree of development : the alimentary canal, which has a mouth but no anus, is in some bifurcated, and in others more or less ramified. The male and female generative organs are united in one individual, and pervade a large portion of the body of the adult animal. The facts which have been ascertained in recent times concerning the generation of some of the Trematoda constitute one of the most remarkable parts of the history of this process among the Invertebrata. Their ge- neral result may be shortly stated thus: the fully grown and sexual Trematode animal, as observed chiefly in the Distomata, produces ova, which may pass through the earlier stages of their development either in the viviparous or oviparous mode, more fre- quently the latter. Each of these ova has formed from it an embryo in which no re- semblance to the Trematode parent is to be recognised, but presenting the simple struc- ture of a ciliated animalcule like a polygastric infusorian or a Gregarina. This embryo is * Bull, de 1'Acad. Roy. de Belgique, 1849, No. ]., and Ann. des Scien. Nat. vol. xi. 18-49, p. 13. ; also a work by the same author on the Entozoa, Brussels, 1850, of which I have only seen an extract in a letter addressed to Milne-Edwards, in the Ann. dcs Scien. Nat, 1851. torn xv. p. 309. t Hist. Nat. des Helminthes, 1845. j See also Leblond, in Ann. des Scien. Nat. 1836, and Mieschcr, Bericht Naturforsch. Gesellsch. Basle, 1840 ; the Works of Kudolphi on Entozoa; the Article ENTOZOA in this Cyclopasdia, by Owen ; Kolliker's Memoir on the Development of Inverte- brate Animals, in Miiller's Archiv, 1843; Eschricht on Bothriocephali, 1810, &c. &c. 30 OVUM. not itself converted by any direct process of development or metamorphosis into a perfect Distoma, but lias gradually formed from germ- cells within it a progeny, sometimes of one, more frequently a number of bodies, which, when they arrive at maturity, present each one an external form and internal structure and locomotive powers, entitling them to be con- sidered as independent animals. Nor are these directly converted into Distomata ; but again there is formed within the body of each, and in the same gradual manner from germ-cells, a new progeny of animals nearly similar to those producing them and equally differing from the complete Distomata. Each of this new progeny, as it increases in size, has formed within it by development from germ-cells the third progeny of the series, and the last of the cycle ; but these are different from their immediate parents, and in their internal or- ganisation scon manifest the type of the true Trematode. These animals are endowed for a time with very active locomotive powers, to which a long caudal appendage con- tributes ; their two progenitors have been confined in the parasitic condition, but these Fig. 28. Series of changes in the development and generations oj Distoma. (From Steejistrnp.~) o, Ovum with embryo or larva developed in it. e, this embryo in a free moving state ; e', another embryo in its interior. (These are of Monostomum mutabile, from V. Siebold.) E, this last embryo farther advanced. 1, first stage, soon after it becomes free ; 2 and 3, farther on, with g, the second generation, within them in various stages. o, 1, one of this second generation at an early period of its advancement; 2 and 3, farther on, with c, c, Cercariae or Distoma-larvse, within them ; g', one of the granular globules from which the Distoma larv and previous generations arise near the posterior part of the body. c, one of the Cercarise or Distoma larva with its caudal appendage, p, the same, passed into its en- cysted or pupa state, having previously lost its tail. D, Distomata. 1, young Distoma immediately after it has quitted the cyst, and has penetrated a short distance into the body of the snail ; 2, Distoma found deep in the viscera. are in general freed from confinement, and move about with great vivacity for a time in the water surrounding the animals which their progenitors have infested. In this state they have long been known as Cercariae, and as they have been supposed to be the young of Distomata, have attracted peculiar notice among Helminthologists.* The free Cercaria? are not, however, directly converted into Distomata; but appear always to undergo a previous metamorphosis in a chrysalis state, or enclosed in a pupa cyst. * Nitsch, Beitrag zur Infusorienkunde, &c., Halle, 1817; Bojanus, in Isis, 1818; A remarkable and interesting series of papers by V. Baer, in Nov. Act. Nat. Curios. 1826, vol. xiii. ; Riul. Wagner, in Isis, 1834 ; V. Siebold, in Burdach's Physiol. vol. ii. of German edit. p. 187., or vol. iii. of French transl., p. 32., &e. Previous to the formation of this cyst the Cercariae adhere to, and bore into, the sub- stance of the animal infested by the Disto- mata; the tail is cast off, an exudation from their own bodies forms the cyst, which en- closes them : within this they remain for many weeks, and even months, moving all the while, and undergoing changes of develop- ment, by which they are at last converted into the complete Distoma. The greater number of the observations from which this remarkable process of gene- ration has been ascertained to occur are due to V. Siebold and Steenstrup ; but the whole succession of changes has not yet been ob- served in any one species, and it is to the latter observer especially that the scientific world is indebted for the ingenious com- bination and interpretation of the scattered OVUM. n observations of previous inquirers, as well as the addition of new facts, from which an almost entire certainty is acquired that the various phenomena do actually succeed each other in the order above stated, and that the occurrence of alternate or intermediate gene- rations in these animals is established. Von Siebold had in 1835 described in the Monostomum mutabile the development of the first embryo from the ovum in the Gregarina- like or animalcular form, and had shown the next change to consist in the formation within the first embryo of a second body endowed with locomotive power, and independent vita- lity, and differing both from its immediate parent and from the adult.* V. Siebold, as well as others, had ascertained the Cercariae to be themselves incomplete animals, and to proceed from others by a process of internal production of a non-sexual kind. Steenstrup therefore di- rected his attention particularly to trace these Cercariae on the one band, in their development into complete Distomata, and on the other, backwards through their progenitors towards the first origin from an ovum. His observa- tions were made principally in three kinds of Cercaria, which, along with their antecedent and succeeding conditions, are found in great numbers in the fresh water snails, Lymneus stagnalis, Paludina vivipara, Planorbis, &c., and which had been previously named Cer- caria echinata, C. armata, and C. ephemera. In these, especially in the first, the conversion of an encysted Cercaria by metamorphosis into a Distoma, and the descent of the Cercaria (by metagenesis) through two progenitors, not themselves Distomata, was ascertained, but he did not succeed in tracing these bodies back to their origin from ova. By a com- parison, however, of the body formed within the animalcular embryo of the ovum of the Monostomum mutabile, as observed by V. Siebold, with the first progenitor of the Cerca- ria, to which it was found to present a remark- able similarity, the chain of evidence seemed to be complete, and Steenstrup found himself in a position to announce the general views of alternate generation, which have ever since their first publication attracted the greatest attention, and contributed in a powerful de- gree to modify and direct the investigation of the generative processes in the lower animals. To the immediate progenitor of the Cercaria Steenstrup gave the name of nurse (altrix, Amme), in allusion to its nursing or nourishing function, and to the immediate progenitor of this one he gave the appellation of " parent or grand-nurse." These terms may be objection- able, but an unnecessary amount of criticism seems to have been bestowed on them by some writers. They are adopted hypotheti- cally by Steenstrup ; they do not appear to withdraw him from the matter-of-fact state- ment of his observations; and they seem to be, in many respects, short and convenient terms in the description of the phenomena. These bodies have in the Cercaria echinata all the appearance of distinct animals, that is, a * See Wiegmann's Archiv, 1835. body with a head separated by a neck or col- lar, a tail or caudal projection, and two pro- cesses of the integument similar to limbs, a mouth and alimentary cavity, and they move with all the appearance of spontaneity; but it ought to be remarked that the form ami powers of these nursing or formative cases differ considerably in various other species, and in some present so little of the external form or endowments of an independent ani- mal, that the more general appellations of germ-cases, or germ-sacs, or sporo-cysts, may be more appropriate to them.* It is chiefly among the aquatic Gastero- pod Mollusca, and a few land ones, that these observations have been made ; but V. Siebold has extended them to some of the Trematoda inhabiting the air-sacs and other parts of water fowls, which no doubt come from the same Mollusca, and obtain access to the seat of their final parasitic habitation from the water or along with food, into which they have come as Cercariae, after having previously been parasitic in the Mollusca. It is easy to understand how the ova of the Distomata discharged from the bodies of the water fowl may gain their place in the Mollusca. V. Siebold has observed in a very interesting manner also the passage of the Cercarije into the bodies of water insects (larva; of Ephemera and Perlida), which he placed together with a quantity of Lymneus stag- nalis, from the various parts of whose bodies the Cercariee were discharged in numbers oat of their nursing capsules : the penetration of the integument of the insect by the Cercaria and the mode of casting its tail being precisely the same as that observed by Steenstrup in the Mollusca.f Both these observers agree that the first and second germ-cases (or nurses), and the Cercarise, or Distoma-larvae, arise by a process of gradual development from extremely minute granular spherules, which are at first situated in the posterior region of the body, or between the alimentary cavity and the integument. These are certainly not ova : but we are at a loss to state to what cLiss of reproductive germs they may be referred with greatest accuracy . It is known that the bodies which inhabit the aqueous chamber of the eyes of many fishes are imperfect Distomata. Steenstrup has frequently observed these larva? in the pupa state adhering to the inside, and some- times to the outside, of the cornea, and he liau occasionally noticed a delicate streak through the cornea, indicating the track through which the animal has penetrated ; and he considers it as extremely probable that all the Trema- toda of the eyes of fishes, of which a vast variety has been described by Nordmann$, are * See Victor Cams, iibcr don Generations -wechsel, for a figure of these more simple forms of sporo- cysts. t See the Article PARASITES, in R. Wagner's HandwSrterbuch der Physiologic. J See Fig. 28. , with four dilatations from contained ova, d, with two dilatations, one of which is opened to show the ovum ; e, the body of the uterus ; /, the vagina. B. A diagrammatic transverse section of the human uterus, at twelve or fourteen days after conception, somewhat less than the natural size; e, the uterine cavity, near which the ovum with its villous chorion is involved in the substance of the decidua indicated by the dotted shading ; c' c, the Fallo- pian tubes cut short, by one of which the ovum had previously descended while still of small size. c. Enlarged view of the exterior of the human ovum, of twelve or fourteen days after conception, showing the villi of the chorion projecting from its surface. E 3 54. OVUM. formative process of the ovum, including the addition of the external coverings, is completed within the ovary; and, on the other hand, there are a few instances in which, as in the trematoda and cestoid en- tozoa, the germinal vesicle and yolk sub- stance of the ovule are formed in separate organs, instead of in the usual manner entirely in the ovary. The varieties of the ovaries in different animals may be considered under two heads viz., 1st. Their relation to the passages or outlets as influencing the mode of discharge of the ova from them ; and 2nd, their internal structure as related to the form of the ovum produced. Fig. 36 Ovary and oviduct of a laying Fowl, killed twelve hours after laying the last egg. a. Left ovary ; b, opening of the infundibiilum of the oviduct ; c, d, glandular portion of the oviduct ; at d, the isthmus ; e, an egg in the uterine portion of the oviduct, in which the shell is begun to be deposited ; /, the rectum, ending in the cloaca ; g, the undeveloped right oviduct occasionally met with. a. Relations of the form of the ovaries to the discharge of ova. In the majority of verte- brated animals the ovary or ovaries are quite detached from the conducting tube or ovi- duct ; the ovules are formed in close capsules of the ovary, by the bursting or fissure of the wall of which they escape ; the oviduct opens at its upper end into the abdominal cavity, and there receives the ovum which has been discharged from the ovary. This is the general arrangement in mammalia, birds, reptiles, am- phibia, and cartilaginous fishes. There is some difference in the form of the ovary in the higher and lower of these animals. In mam- malia and birds, in chelonia and the crocodiles among the reptiles, and in cartilaginous fishes the ovary is more or less solid, and the ovules are developed in capsules which project towards the external surface ; but in the lizards Fig 37. (From Cams and Otto.) Female of the Falco buteo opened, showing the left larger oviduct and ovary, and the smaller right oviduct and ovary. a) a, the right and left ovaries ; b, the left infiindibuhiin ; c, d, the left oviduct; ,/", the rectum, ending in the cloaca, -which has been opened, showing at /<' h the openings of the right and left oviducts, and at i' i those of the ureters; g, the vestige of the right oviduct. OVUM. 55 and serpents, and in the batrachia, this organ is hollow, and the capsules in which the ovules are formed burst in dehiscence into an internal cavity, whence the ovules escape into the abdomen by the rupture or open- ing of the sack of the oviduct, generally at one, but sometimes, as in the frog, at a greater number of places. In the higher animals, in which the ovules escape from the external surface of the ovary, their en- trance into the oviduct is in general secured by the temporary apposition of the dilated upper end or infundibulum of the oviduct to the ovary, or the capsule containing a ripe ovule ; in the other animals, in which the ova come from the interior of the hollow ovary, the apposition of the oviduct does not ap- Fig. 38. Common adder, in which the ova have descended to occupy both oviducts, five in the right, and three in the left : the infundibulum is shown in each ovi- duct; a' a, the right and left ovaries, each forming a sac, opening anteriorly near the infundibulum for the discharge of the ova, which, when ripe, fall into the interior of the sac, and thence pass into the oviduct. pear to be so direct, and there are various other means by which the ova, when they have escaped into the abdominal cavity, reach the open extremity of the oviduct. It is in the class of fishes that the transi- tion occurs from the higher to the lower type of organisation of the ovaries and oviducts. In all of them the ovules are formed in ova- rian follicles, and escape by dehiscence from these follicles ; but there are several modifi- cations of the relation between the oviduct and ovary among them. 1st. In the sharks and rays, as already stated, the arrangement is nearly similar to that existing in higher ani- mals. The ova, which are of large size, come to maturity singly, or in small numbers at once : on being discharged externally from the ovarian capsules, they pass into the oviduct, and there receive a considerable addition from this organ. The majority of them, as previously stated, are oviparous, and in them a hard covering is formed by a peculiar glandular organ connected with the oviduct ; in a few which are ovoviviparous, as the common dog- fish, torpedo, &c., the external covering of the ovum is membranous and soft. 2nd. In the sturgeon and in the lamprey the oviduct is very short ; still, as it opens superiorly into the abdominal cavity, the relation may be considered the same as in the previous ex- amples. 3rd. In the genus salmo and in Fig. 39. B Ovaries and oviduct of an osseous Fish. A. Sketch of the two largely developed saccu- lated ovaries of an osseous fish, with the short ovi- ducts proceeding from near their posterior ex- tremities. B. Diagrammatic section of a portion of the ova- rian sac, showing two of the ovarian plates, from which the developed ova hang in small pediculated vesicles or ovisacs. E 4 50 OVUM. the eel among the osseous fishes, the oviduct is entirely wanting, and the numerous ova which are dischargee by external dehiscence from the ovary into the cavity of the abdomen, escape from that cavity by an orifice (porus abdominalis) situated on each side close to the anus. 4th. In other osseous fishes, the ovary and oviduct are united, or the ovary forms a saccular organ, in the interior of the wall of which the ovi- capsules are situated, occupying a variable extent of it in different genera ; and the wall of the oviduct, usually very short, is continued from that of the ovary to the outlet from the animal's body. The ova, therefore, which drop by internal dehiscence into the cavity of the ovary, pass directly out by the short ovi- duct in the laying of the spawn. Most osseous fishes are oviparous ; but in a few, as the viviparous blenny, the anableps, paecilia, and some siluroids, the ova, on escaping from their capsules into the cavity of the ovary, remain there during the development of the embryo. In the invertebrate animals there are very many varieties in the form and relations of the productive and conducting parts of the organs. Three principal female generative Fig. 40. Oviduct and ovary in a continuous tube in Insects and Entozod. A. (From R. Wagner}. Upper part of the ovi- duct or ovary of the Acheta campestris. B. (From H. Nelson.} Upper part of the oviduct or ovary of the Ascaris mystax. In both of these figures the germ -cells and germinal vesicles, with their nuclei, are seen surrounded by the granular matter which afterwards collects round them as vitelline or yolk substance. varieties may be distinguished among them 1 st. A form similar to that just now described as generally prevalent among osseous fishes, in which the ovary and oviduct are con- tinuous, but in which the ova, being formed in ovarian capsules, are dropped by dehiscence into the upper part of the oviducts. Such is still the structure in cephalopoda and some other mollusca. 2nd. A form in which the oviduct may be said to be, as in the last, con- tinuous with the ovary, but in which there is no true dehiscence of the ovules from ovarian capsules, as they are formed at once in the internal cavities of the ovary, which directly open into, or are mere prolongations of, the oviducal tubes. In this form the oviducts may be considered to stand in the relation of excretory ducts to the ovarian glands. In many of this class the ovaries present very various forms ; in some the continuity of the ovarian and oviducal tubes is very obvious and simple, as in the ne- matoid entozoa, insects, &c. ; while in others, the ovary is more complex and race- mose, and the oviducal tubes comparatively simple. 3rd. That form in which the ovaries are variously disseminated over the body of the animals, and in which there are no true oviducts, but the ova escape on various parts of the internal or external surface of the body.* b. Structure of the ovaries themselves, as related to the production of the ovitla. In mammalia these organs consist of a pair of solid oval flattened bodies, attached by inter- vening fibrous tissue to the posterior surface of the broad ligaments of the uterus, and are covered completely, excepting at this attached part, by peritoneum. Below this serous co- vering there is also a layer of firm fibrous tissue, or tunica albuginca. The internal sub- stance, or parenchyma, or stroma, as it has been called, consists of a firm basis of fibro- cellular texture, of considerable vascularity. The fibres, as well as the blood-vessels of this substance, radiate principally from the at- tached border of the organ towards the oppo- site, or free side, and the rest of the surface. The ovicapsules, or so-called vesicles or fol- licles of De Graaf, in the human ovary, are situated in this stroma; and at or after the period of puberty are found of some size ; a variable number, from twelve to thirty, or more, being of from J^ to | of an inch, and a few even a little larger. These mem- branous vesicles, filled with fluid, are situated chiefly towards the surface of the free side of the ovary. A larger number of undeveloped capsules, of minute size, also exist in the * See Von Baer's Entwickelungsgesch. der Thiere ; Owen's Lectures on Invertebrate Animals, 1843, anil on Fishes, 1846 ; Eathke (on Development of Fishes, &c.), in Geschichte der Thierwelt, Th. 3. ; J. Miiller (on Sharks), in Mem. of Berlin Acad. 1842 ; John Davy (on the Torpedo), in Philos. Trans, for 1834; and the works of Von Siebold and Stannius, R. Wagner, Cams, and others on Compar. Anat. See also in this Cyclopaedia, the articles Monotremata, Pisces, Reptilia, and Organs of Ge- neration. OVUM. 57 Fig. 41. Relation of the ova and ovaries in Mammalia. A. (From Coste, as reduced by Longet.) Human ovary, enlarged four diameters, partially dissected at ooo, to show the Graafian follicles in the ovarian stroma: one of these, more advanced", has had its double tunic, o v, cut into and reflected ; the granular membrane, m g, has also been partially opened, showing the thickened portion or granular disc, dg, in which the ovum is imbedded near the most projecting part. At o V, another Graafian follicle has been burst, and the ovum in its granular disc is seen expelled from it. u. Transverse section of human ovary, to show the general arrangement of the developed Graafian follicles towards the surface ; twice the natural size. c. Diagrammatic representation, in section, of two^Graafian follicles, in different stages of advance- ment in the ovary of a mammifer, enlarged about ten diameters, p. Peritoneal covering of the ovary, st, ovarian stroma; ov, the two layers of the ovisac; mg, membrana granulosa, near which is the discus granulosus, with the ovum imbedded. stroma ; and it has been observed, that these are present from a very early period in the ovaries, as first noticed by Carus, and since by myself and others in the child at birth. The more developed of these ovi-capsules are enclosed by a strong theca or membrane, consisting of two layers ; the external thinner and firmer, of a fibrous and vascular struc- ture, the internal thicker and softer, of a fibro-cellular structure and also of consider- able vascularity. The capsules are filled with a fluid nearly transparent, which coagulates under the action of heat ; and inside the theca, or lining it and covering the fluid, there is a layer of nucleated cells united together in the form of a soft, easily-lacerated membrane, somewhat like an epithelial lining of the cap- sule. It is in this cellular layer (tunica gra- nu/osa of Von Baer) that the ovum is placed, being situated in the thicker portion of it (cumulus proligerits), directed towards the surface of the ovary. When one of the ovicapsules and its con- tained ovule has reached maturity, which takes place in one or more of them at regu- larly recurrent periods, besides the swelling of the ovicapsule itself from the increase of its fluid and other causes, the stroma of the ovary between the capsule and the surface undergoes considerable thinning, and the ovi- capsule comes thus to project more imme- diately from the surface of the ovary. An increased vascularity is also apparent in the same situation ; and finally a small circum- scribed fissure near the middle of the most projecting part occurs, allowing the escape of the ovule and the granular layer and fluid from the ovicapsule. The ovule, surrounded by a portion of the cellular layer in which it was embedded, is 58 OVUM. received by the open fimbriated extremity of the Fallopian tube. The empty ovicapsule now undergoes a remarkable change by the deposit in its inte- rior of the substance termed corpus luteum, the quantity and nature of which vary greatly according as the escape of the ovule is fol- lowed or not by pregnancy. Of this change more will be said hereafter. The result in both cases is the ultimate closure and obliteration of the ovicapsules. In birds, scaly reptiles, and cartilaginous fishes, the greater size of the ovules when in a state of maturity is connected with a modi- Fig. 42. cr Relation of the ova to the ovary in Birds. A. Ovary of a fowl, showing at aaa the most developed ova hanging from the ovary in their pedi- culated capsules ; the non-vascular bands are seen on their most projecting sides; at bb, the empty capsules or calyces of ova that have been previously discharged ; at c c, the more compact part near the root of the ovary, where the ova are less developed. B. Diagrammatic section of one of the most ad- vanced of the capsules ; o, the extended ovarian substance forming the capsule; p, its pedicle; c, indicates in this and the preceding figure the most common position of the cicatricula or germ-disc and vesicle ; o v, the two layers of the ovicapsule or ovisac, into which the blood-vessels penetrate : the dotted line v m, marks the vitelline membrane. fication of the structure of the ovary and the ovicapsules. Previous to the age for breeding, the ovary of birds in which animals only one ovary and oviduct is usually developed or attains to functional activity is a solid organ of a less firm texture than that of mammalia, and is adherent to the vertebral column in the mid- dle of the dorsal region. It contains at an earlier period a much greater number of ovi- capsules of a considerable size than are per- ceptible in mammalia. The stroma or ova- rian substance is in less proportion, therefore, to the ovicapsules and ova. As the ovules become more developed, they increase rapidly in size ; and we then perceive that they bear a different relation to the ovicapsules from that which has previously been described in mammalia, as they fill com- pletely the ovicapsules, and there is no fluid or loose cellular layer between the outer sur- face of the vitelline membrane and the lining membrane of the theca.* As some of the yolks approach maturity their increase in size is proportionally much greater, and the theca or ovicapsule, and along with it the ovarian substance, is distended in like proportion ; and in this manner the ovary of the fowl at the breeding season has lost its appearance of compactness or solidity, and seems to be composed almost entirely of a larger num- ber of pediculated ovarian capsules of the most various sizes, filled with the yolks or ovules in all stages of development. Never- theless, a little attention shows the solid part of the ovary still remaining at the part at- tached to the vertebral column, forming the ground, as it were, from which the pedicles of the enlarged capsules spring, and contain- ing still a very large number of minute undeveloped ovules in their correspondingly small ovicapsules. The large ovicapsules of the developed ovary of the bird consist of two layers, which are loosely united together by blood-vessels and binding tissue towards the pedicle and over the greater part of the capsule, but are more firmly knit together at the free border. At the latter place the blood-vessels of the theca, which are on all the other parts dis- tributed in wide or comparatively large chan- nels very thickly set, suddenly become so small and delicate as to give, at first sight, the appearance of an absence of vascularity in the course of a band of about th of an inch in breadth, and extending across a large portion of the free circumference of the cap- sule.f This is the so-called stigma, at which, when the ovule is to escape from the ovary and to be transferred into the oviduct, the rupture of the theca occurs. * There may probably be an epithelial lining of this membrane. See H. MeckePs paper, afterwards referred to, Zeitsch. fur Wissensch. Zool. vol. iii., 1852, p. 420. f The length of this band or stigma is about equal to the long diameter, or a third of the circum- ference of the capsule at its widest part. It is some- times crossed by a second band of the same kind. OVUM. 59 Each developed ovule, therefore, in these oviparous animals, comes to be contained in a pediculated capsule, which is formed by the extension of the substance of the ovary ; but from the great extent to which the dilatation of the capsule occurs, the true ovarian stroma is reduced to a very small amount, and scarcely more remaining than the theca of the capsule itself and the ovarian coverings. In the other animals possessing the large yolked ova, nearly the same structure of the ovary prevails. In chelonia and crocodiles, it is indeed almost identical with that in birds. In lizards and serpents, the hollow state of the ovary produces some difference in the general form ; and in cartilaginous fishes (sharks and rays) other differences in the structure of the ovary may exist ; but in all these animals the essential points of rela- tion between the ovarian substance and cap- sules, and the large ovules, are the same as that now described in birds. The lining membrane of the ovicapsule of birds is thick and tough, and on its inner sur- Fig. 43. Structure of the ovisac in the fowl's ovary. A. Inner surface of a portion of the ovisac of a fully-developed ovarian capsule, magnified about six diameters, showing an appearance which might be mistaken for glandular depressions produced by the peculiar disposition of the veins. B. The same, from a calyx from which the ovum has been discharged some days before ; a -whitish flaky membrane is deposited on portions of the surface. c. The same, from an undeveloped capsule of a quarter of an inch in diameter, across the stigmatic band. D. The disposition of the blood-vessels near the stigmatic band, which seems at first sight non-vascu- lar, but is in reality traversed by ramifying small vessels proceeding from the neighbouring larger veins, and crossing the stigmatic band. E. Two of the large mouths of the veins, which give the semblance of follicular pits represented in A and c, but which are quite closed, with the smaller vessels ending in them, as' seen from the inner surface of the ovisac. face presents a soft appearance somewhat si- milar to that of a mucous membrane. In ex- amining the inner surface of this membrane, Dr. Sharpey and the author had their atten- tion arrested by an appearance such as might, on first sight, be attributed to a number of follicular or glandular pits. This appearance, as we first observed it, is represented \nfig. 43. (A, B, c) as it was seen in a fully-developed capsule in one a third of an inch in diameter and in a calyx from which the yolk had been discharged some days previously. We supposed, indeed, at first that the appearance depended on the presence of the orifices of follicular depressions or glands on the inner surface of the membrane. A more attentive examination of this membrane by Dr. Sharpey has shown that the appearance is not due to depressions of the inner surface of the membrane or to the mouths of follicles opening upon it, but is caused by a peculiar form of the blood-vessels seen through the entire and smooth inner surface of the mem- brane. The apparent depressions are in fact the sudden terminations or beginnings of veins of considerable size seen through a delicate and transparent -portion of the membrane which closes them towards the inner surface. They may be made very obvious by merely coarsely brushing the smooth blunt edge of any instrument over the membrane, and thus causing the blood to flow from the vessels in other parts in these sinuses or dilated veins. It would appear that the smaller capillary vessels in which the arteries terminate, in ap- proaching the inner surface of the capsule, ramify with considerable minuteness, and at each of the marks or apparent depressions referred to suddenly fall into or end in the 60 OVUM. comparatively large veins which constitute the hollow spaces. The ends of these veins, then, look towards the inner surface of the membrane ; and the appearance of a divided cavity in some of the supposed follicles is merely caused by two or more veins meet- ing in a common dilatation at this place. The capillary vessels, in passing into these large commencements of the veins seem to converge from its circumference to its centre. In the enlarged ovarian capsules of the turtle, a somewhat similar arrangement may be observed ; but I have not had an oppor- tunity of tracing its relation to the blood- vessels ; nor have I had the means of ascer- taining whether anything of the same kind exists in other reptiles with large yolks. In the skate 1 have not been able to perceive any similar arrangement ; and in the Graafian vesicle of mammalia the lining membrane presents internally a smooth surface destitute of any appearance of depressions or of pecu- liar venous sinuses. The appearance which I have just now described had not escaped the notice of Von Baer ; for at p. 23 of his work on develop- ment, he mentions the existence of clearer points in the inner membrane of the theca, and states his opinion that they may be open mouths of blood-vessels, by means of which the yolk may be nourished by the direct access of blood to it. In the naked amphibia and osseous fishes, the ovaries (of which the general form has been previously noticed) present a still greater decrease in the proportion of the stroma to the ovicapsules and ova. These capsules are themselves also of much more delicate struc- ture than in the higher animals ; but the rela- tion of the ovules to the ovicapsules in their formation, and the mode of their escape by the rupture of the theca, are essentially analogous to those of birds and reptiles. In the earliest condition, it is true, the ovary may present a greater amount of solidity in some of these animals : but from the prodigious number of the germs of the ovules and the small quan- tity of the ovarian stroma, as soon as the ovary has made some progress in develop- ment, it acquires the appearance rather of a mere mass of ova connected together by a membrane and fine thread-like pedicles, than of a solid or consistent organ containing them. The delicate ovicapsules containing the ovules embrace them closely as in the large-yolked group of animals, there being little or no fluid between the capsules and the vitelline membrane. The structure of the ovaries in the inver- tebrate animals presents so many varieties that it would occupy too much space to allude to them here. I refer the reader for information regarding them to the article ORGANS OF GENERATION, and others on par- ticular classes and orders of animals in dif- ferent parts of this work. For our present purpose the structure of these organs has been sufficiently indicated in the previous section. In conclusion, it may be right to recapitulate the general nature of the ovary or formative organ in its relation to the production of ova. A comparison of the forms previously indicated leads to the general view that the ovary is to be regarded as analogous to the glandular organs. _ In the great majority of animals highest in the scale, the ovisacs are close fol- licles from which the product of formation (or secretion) escapes by the bursting of the wall of the follicle in the highest animals, on the external surface of the organ, in those coming next in the series, towards an internal cavity. In other instances, principally among the lower animals, the structure is more ana- logous to that we are accustomed to consider as characteristic of the true glands, in which the secreted cellular product is formed within the same or a continuation of the tubular ducts themselves by which they make their escape. The more complex structure of the capsules in which the large-yolked ovules are produced in birds constitutes a special appa- ratus, which, though without follicular com- plication, may be looked upon as a modifica- tion or higher degree of development of the glandular structure of the ovary, provided for the rapid formation of the larger mass of nutritive substance which is present in these ova. 4. More detailed description of the ovum of birds as the type of the 1st group. Having in the previous section given a sketch of the general resemblances and dif- ferences observed among the ova of various animals, I now proceed to describe more in detail an example from each of the three groups previously distinguished, and more particularly those of Birds and Mammalia, which demand the greatest share of our atten- tion in the study of development ; and first as to the ovum of the common fowl. Quantity of matter, composition, $c. The average dimensions of the fowl's egg in this country are the following: The long diameter 2^ inches, short diameter If inch. The aver- age weight of eggs of this size is a little more than 2 oz. avoird., or 920 grains.* The extremes in weight which I have ob- served among eggs of the fowl naturally formed are 750 and 1060 grains. Double-yolked eggs are, as might be expected, much larger, reaching often a weight of 1400 grains, or S^oz. The yolk weighs about a third of the whole ; the albumen, membrane, and shell forming the remaining two thirds. These parts of the egg are in the following propor- tions to each other in 100 parts ; the albu- * The following is a comparative view of the average size and weight of the eggs of the com- mon fowl, duck, turkey, and goose. Fowl - Duck - Turkey Goose - Length Breadth (in inches). 2-25 1-7 2-5 1-75 2-7 1-9 3-3 2-4 Weight (in grains). 920 nearly 2 oz. 1100 2i oz. 1.300 3 oz. 2GOO 6 oz. OVUM. 61 men 58, the yolk 31}, and the shell with its lining membrane, 10. When eggs are kept exposed, they gradually sustain a small loss, due chiefly to the eva- poration of water, and amounting to about one grain per day. When putrefaction ensues, an additional loss from chemical changes occurs. During incubation, the loss of weight is more considerable, amounting in twenty-one days to 16 or 17 per cent., or nearly one sixth of their entire substance.* The loss by an egg during incubation, therefore, is eight times as great as that which occurs in an egg kept at the usual atmospheric tem- perature for the same period a circumstance which depends partly on the higher tem- perature, but principally on the evolution of carbon from the oily matters of the incubated egg, in combination with the oxygen of the air, or as carbonic acid, &c. Of the 17 parts per cent, lost during incu- bation, not more than 5 or 6 consist of water, and the remaining two thirds, that is 10 or 1 1 parts, are derived from the oily and other substances of the egg which undergo chemical changes attendant upon the process of orga- nisation and respiration of the embryo. By evaporation to dryness of the whole egg without the shell and membrane, about 27 per cent, of the substance are left; the oily ingredients of this residue, amounting to about lOf, are almost all contained in the yolk, and the remaining 16J parts of solid matter are nearly equally divided between the yolk and the white. The yolk, therefore, is much richer in the fixed and solid parts than the white ; but its specific gravity, as will afterwards be seen, is considerably reduced by the larger quantity of oily matter it con- tains : the per-centage of solid matter (inde- pendently of the oleaginous substance) con- tained in the yolk and albumen, is in the pro- portion of 32 in the first to 14- in the second.f The solid residue obtained by evaporation of the white at a low temperature, amounting to nearly one seventh of the whole, consists chiefly of albumen ; along with which there is also some animal matter which has hitherto been named by chemists as extractive, and a small amount of salts, which are principally alkaline sulphates, muriates and phosphates, with phosphate of lime, some free soda and sulphur. The yolk contains little more than half its weight of water, or 54 per cent. The remain- ing 46 parts consist of about 17 of albumen, or analogous principles, 28 of oily matter, and 1 of salts. These last are chiefly alkaline mu- riates and sulphates, phosphate of lime and magnesia, and traces of iron, sulphur, and * See Prout, On the fixed Principles of the Egg, Philos. Trans, and Annals of Pliilos. for 1822. Also, by the same author, On the Changes of the Egg in Incubation, in the same Journal, for 1823. ; and, Paris, On the Physiology of the Egg, in Linnean Soc. Trans, vol. x. p. 304, and Annals of Philos. 1821. f .See Prevost and Morin, in Journ. de Pharmacie for 1846, and Sacc, in the Eggs of the Bantam Fowl, in Annal. cles Scien. Nat. for 1847, p. 69. phosphorus. The albumen has an alkaline, the yolk a neutral, reaction.* The membrane lining the shell consists apparently of a protein compound, analogous somewhat to that of the elastic yellow tissue. The shell consists of earthy salts deposited in a delicate matrix of animal matter, which last constitutes not more than 3 per cent, of the whole. The earthy ingredients are in great part carbonate of lime, together with a little carbonate of magnesia, and phosphate of lime and magnesia. Of the ingredients of the egg before men- tioned, the albumen and other animal prin- ciples, together with the sulphur and salts, are no doubt more immediately employed in the growth of the embryo; while the oily matter, besides contributing, as it appears, in some part, to the same purpose, serves more directly and in greater quantity for the re- spiratory process, in which it is consumed largely during incubation. The alkalinity of the white of egg appears to depend on the presence of caustic soda, which albumen has the property of separating from its carbonate. The following tabular view exhibits in a general way the change in the relative pro- portion of the ingredients of the egg resulting from incubation f : Shell and membrane Albumen, &c. - Oily matter, &c. Water - - - - T /Water - 5-6") ljOSS t Oily matt.,&c. 11-4 J When an egg is examined immediately on being laid and while yet warm, or still better when taken from the egg-bag of the fowl pre- vious to laying, the yolk and white fill com- pletely the interior ; but immediately on cooling, a small space or vacuity appears ge- nerally towards the obtuse end of the egg, and this air-space increases gradually in size as the eggs are longer kept and the natural evapora- tion of water proceeds. This space is formed by the separation of the two principal layers of the lining membrane of the shell. Durin^ incubation the air-space increases much more rapidly; and indeed towards the end of this * Composition of the yolk, according to Gobley, in Journal de Pharmacie, 8e. se'r. torn. ix. p. 174. Water, about - 53' Vitelline, albumen, and protein com- pounds - - - 1C -5 Oily matters 29- Salts, &c. - 1-5 100-0 These salts are the following viz., chloride of sodium and potassium, sulphate of potassa, muriate of ammonia, phosphates of lime and magnesia, lactic, acid, colouring matter, iron, t From Sacc, loc. citat. Fresh Incubated egg- 10-67 efig. 10- 17-8 19-4 18-83 6-5 52-7 47-1 17- 100-00 100-00 62 OVUM. process, and in eggs that have been long kept, the space has extended over the whole width of the egg, and the quantity of gas contained in this space is sufficient to cause the eggs to float in water. The extent of the air-space may be ascertained in some degree by the greater or less feeling of coldness of the shell of the egg near the obtuse end, when it is applied to the skin of a delicate part, such as the eyelid, in consequence of the heat being less rapidly carried off by that part of the shell within which the air-space has been formed, than at others with which the albumen is in contact. Dr. Paris found this air, amounting to about half a cubic inch, to be nearly pure atmo- spheric air, with a small quantity of carbonic acid towards the end of the period of incuba- tion. MM. Baudrimont and St. Ange find it to contain in general more oxygen than atmospheric air, and no carbonic acid ; whence they conclude that the shell has a peculiar power of passing outwards the carbonic acid formed during incubation and taking in oxygen.* The formation of the air-space is manifestly a compensation for the loss of sub- stance in whatever way occasioned, that may take place from the egg. We shall have oc- casion afterwards to consider in how far it may be important in connection with the phenomena of incubation. The specific gravity of the whole egg, when newly laid, and before evaporation has taken place, is generally as high as 1090, being raised considerably above the common spe- cific gravity of the fluids and soft parts of animals by the superior density of the shell ; but as the air space increases, the whole spe- cific gravity will be lowered. The specific gravity of the albumen and yolk differ in a considerable degree; that of the yolk , though containing the largest amount of solid matter, being lowest in con- sequence of the large quantity of oily matter belonging to it ; and thus when the albumen becomes more fluid during incubation, the yolk naturally rises towards its upper part, or displaces some of the albumen which lay above it in the newly laid egg. It is also an interesting circumstance, that the specific gravity of the lower and upper parts of the yolk differs perceptibly ; that of the upper part being reduced by the greater quantity of oily matter contained in the cells situated in the vicinity of the cicatricula. The up- turning of the side of the yolk bearing the cicatricula, which is familiarly known, has long excited attention ; and several explana- tions have been suggested of its cause ; and, among others, the chalazae have been supposed to balance the yolk in such a manner as to secure this position. But Von Baer showed that this view was erroneous, and that the less specific gravity of the upper part, or of that portion of the yolk in which the cicatri- cula is placed, is the true cause of the phe- * Eecherches Anat. et Physiol. sur 1'CEuf des Vertebre's, Mem. Couronn. ; published in Mem. des Savans Etrangers de 1'Acad. Fran. 1850. nomenon. The measurements of the specific gravity of different parts of the egg by Messrs. Fig. 44. Position, form, and attachment of the chalazcc, yolk, and cicatricula, as shown by sections of fowls' eqqs boiled in different positions. A. Section of an egg, boiled on its side : B, with the narrow end up: c, with the wide end up These figures show the tendency of the lighter part of the yolk, on the surface of which the cicatricula, c, is situated, to be buoyed up and to expand in the white, at the same time that the movements of the yolk are to a certain extent limited by the attach- ment of the chalazre ; a, air-space. OVUM. 63 - 1042'5 - 1029'5 Bauclrimont and St. Ange* are quite confir- matory of this view. They are as follows : Sp. gr. of the External albumen - 104T Internal albumen Whole yolk- Upper part of yolk - 1027' Lower part of yolk - lOSl'S-j- The chalazae, being of greater specific gra- vity than even the inner layer of white, always float lowest ; but, being attached to the yolk near its poles, they hang down from these points. All these circumstances may be illus- trated very clearly by sections of eggs that have been boiled in different fixed positions, as on the side, on the large and small end ; in which it will be found that, while the chalazae exer- cise a certain control over the position of the yolk, that portion of its surface containing- the cicatricula rises higher and expands more fully within the white than the opposite portion, while the chalazae gravitate towards the lower side. (See fg. 44.) Structure of the external parts of the egg. The shell of the bird's egg is composed of a delicate basis of organised animal matter im- pregnated with the calcareous and earthy particles, the arrangement of which approaches to a crystalline appearance, but is probably of a different nature. This substance is porous, like concreted gypsum-plaster, and allows of evaporation and the mutual diffusion of gases through it in the same manner as that sub- stance ; while, by its strength and rigidity, it affords protection and support to the softer parts of the egg during incubation. The pores of the egg-shell may be easily stopped by any greasy or oily matter, or by melted wax or varnish ; and then all passage of moisture or air through the shell being pre- vented, the development of the embryo be- comes impossible. Eggs that have been oiled cannot, it is well known, be hatched ; but eggs may be kept for a considerable time weeks, or even for months by immersion in lime- water, which impedes the evaporation and the access of air, which might favour putrefaction, while the natural condition of the contents is thus preserved. The shell in most eggs is slightly dimpled externally, with small depressions visible to the naked eye ; but these are not the open- ings of the pores through which evaporation or exchange of gases takes place these being much more minute and numerous but merely the indication of depressions caused by the largely villous structure of that part of the oviduct (uterus) in which the calcareous shell is deposited. On removing the earthy matter by means of a dilute acid, the animal basis remains as a * Op. cit. t Dr. Wm. Aitken has, at my request, repeated these experiments, and has obtained results in ac- cordance with the above statement. He found the unboiled yolk to float indifferently in any part of a saline fluid of specific gravity 1035. By boiling, the specific gravity was reduced to 1031, and in both cases the side with the cicatricula floated upper- most. The upper half, containing the cicatricula, had a specific gravity of 1030 ; the lower half, 1032. slightly coherent, cellular, organised structure, the form of the small compartments in which corresponds with that of the calcareous par- ticles of the shell (see fig. 45. c). The m- Fig. 45. a B Structure of the shell and shell-membrane in the Fowl's egg. A. Lining membrane of the shell ; a, thick matted or felty portion ; b, thin shred of the torn margin, showing the peculiar fibrous tissue of which the various layers are composed. B. Outermost layer of the same, which is incor- porated with the shell; some of the angular cor- puscles of the shell lying upon the fibrous substance and firmly united with it. c. Small portion of the calcareous shell, which has been steeped in dilute hydrochloric acid, show- ing the remains of opaque calcareous substance in the centre, some portions of it exhibiting a granular aspect, and round the margins the animal basis or matrix from which the calcareous matter has been dissolved, presenting an irregular granular or almost amorphous aspect. Here and there clear oval cells are seen, as at a a. ternal surface is irregular and flocculent, and adheres very closely to a different kind of membrane which lines the shell. In those instances in which the shell of eggs is coloured, the pigment substance, of various hues, is generally deposited in cells, which are strewed uniformly or in patches over the external surface of the calcareous shell. In some other instances, however, the OVUM. colour seems to be merely a uniform tinge of the outermost layer of calcareous matter.* The lining membrane of the shell is a peculiar fibrous, interwoven structure, depo- sited in laminae of some thickness and tough- ness, which is readily divided by tearing into two layers over the whole surface of the egg an outer, thicker, and denser, adhering firmly to the inner surface of the shell ; and an inner, thinner, smoother, and of finer texture, which may be easily withdrawn from the outer one, and which naturally separates from it at the air-space ; but both the outer and inner layers of this membrane may be torn into a number of thinner laminae, all agreeing in their minute structure. By microscopic examination, this membrane is found to consist of a closely-interwoven network of peculiar fibres, which are of va- rious sizes, generally between ^oVo*' 1 and ^._i__th of an inch in diameter; the larger fre- quently branching into or giving off smaller fibres at acute angles, the sides rendered un- even by minute projections or knots upon them (not represented in the figure) ; the larger fibres are of a somewhat flattened or ribband-like form. The external layer of the membrane contains the largest fibres. These fibres appear to be analogous in their che- mical nature to those of the elastic yellow texture, not being soluble in strong acetic acid ; but they do not coil up in the manner of the elastic tissue {see Jig. 45. A). The parchment-like coverings of the eggs of serpents and lizards, which have no calca- reous shell, seem to be composed of a greater number of layers of the fibro-laminar texture now described. The albumen, or white of the egg, compre- hends several layers of glairy, albuminous, semifluid substance deposited round the yolk, the chalazae, or grandines, or twisted cords, and the condensed layer of albumen, forming a thin membranous investment immediately over the yolk membrane. In a perfectly-fresh egg, or in an egg taken from the oviduct pre- viously to its being laid, the whole albumen has the consistence of a moderately-firm jelly ; but very soon the outer part becomes fluid, and allows of the freer motion of the parts within the shell. This solution of the albumen proceeds to a greater extent after some hours' incubation, especially over the cicatricula. The deeper part of the albumen, or that next the yolk, is more dense in consistence. No part of it, when unchanged by reagents, presents any sensible structure either to the naked eye or when viewed microscopically. If, however, the soft contents of a fresh egg, or one removed from the oviduct, be taken from within the shell, and thrown into water either pure or with a little acetic acid mixed with it, a slight turbidity or coagulation of the albumen takes place on the surface, which brings out the appearance of a spiral arrange- * See the works of Hewetson and others on the Eggs of Birds. Cams and Otto, Eiiauterungstafeln der Vergleich. Anat. part v. ment of lamina? ; and in a boiled egg these lamina? may be torn in great numbers in suc- cession from off it, the direction of the spiral being from left to right, from the large towards the small end of the egg. With a little care, almost the whole of the albumen may thus be wound off the egg in spiral strips, the deeper ones enclosing the twisted cha- lazac (see Jig. 46. n). The coagulated albumen presents, in the microscope, a minute but indefinite granular structure. The chalazoE ( grandincs) are two irregularly- twisted cords of albumen, harder than the rest, Fig. 46. Manner in which the chalaza;, albumen, Sfc., are deposited round the ovarian ovum of the. Fowl. A. Yolk from the upper part of the oviduct soon after it has entered it, showing a thin covering of albumen on the yolk, forming the chalaziferous membrane, and the twisted chalazse extending from the opposite poles of the yolk The twisting in these is represented more strongly than it can be seen at this period. B. Sketch of the fully formed chalazte from opposite sides of the yolk, stretched to their full length, and showing the opposite direction of the spiral in each. c. Egg from above the middle of the oviduct ; the first layers of albumen deposited round the yolk and chalazte. OVUM. 65 D D. Egg from the lower part of the glandular oviiluct near the isthmus, when the deposit of albu- men is complete; the spiral arrangement of the albumen made manifest by slight coagulation. attached to the opposite ends or poles of the yolk by means of a membrane which looks ex- actly like a continuation of the twisted part of these bodies opening or expanded over the surface of the vitelline membrane. These bodies attracted considerable notice from the earlier observers of the structure of the egg, and have had various uses attributed to them ; but, if we may judge from the varieties they are subject to in the fowl and other birds, and their absence in the ova of scaly reptiles (otherwise very similar to those of birds), it would appear that they are only of secondary importance. One of the chalazse is directed towards the larger, and the other to the smaller end of the egg, and the latter usually adheres with some firmness to the inside of the shell-membrane, while that of the large end floats more freely. In this manner the yolk moves more freely at the large than at the small end of the egg. The spiral twist is in opposite direc- tions in the two chalazae ; a circumstance depending on the manner of their production, by the gradual deposit of albumen and the spiral motion of the yolk during its descent in the oviduct. The membrane which pro- ceeds from the cbalazas over the surface of the yolk has been called chalaziferous ; and the funnel-shaped dilatation of the chalazae where they join the membrane, has been sup- posed to be the opening of a tube passing through these bodies, and serving as a conduit from the white to the yolk ; but entirely without reason. The chalaziferous membrane and innermost twisted part of the chalazoe are, in fact, nothing more than the first- deposited and densest parts of the albumen ; nor is any importance to be attributed to a curved line or fold of the membrane which is often seen stretching over the yolk between the adhering parts of the opposite chalazae. The fact of the upturning of the side of the yolk which bears the cicatricula has already been adverted to, as well as the supposition that Snpp. the clialazne may be the means of securing this position ; but, although it is well ascer- tained that these bodies control, in various directions, the motions of the yolk, they can- not be the cause of the upturning of the cica- tricula ; as this is secured by the difference of specific gravity in the upper and lower parts of the yolk. The true action of the chalazae is to limit the motions of the yolk in the long axis of the egg, and control the rota- tion during a certain time ; but in incubation the relations of the chalazae, white, and yolk are very soon changed ; and, in the progress of these changes, the remains of the denser white are collected at the lower part of the egg. If a fresh egg be turned round on its long axis, the cicatricula will keep its position up- wards for one turn or a little more, and then, by the twisting of the chalazas, the yolk is carried completely round, and balances itself again with the cicatricula uppermost in its new position. The accessory parts of the egg, now de- scribed, are formed round the yolk or ovarian egg during its descent through the oviduct ; and as they may be regarded as only indirectly connected with the functions of the true ovu- lum in their relation to embryonic develop- ment, it may be best to complete their history at this place by stating what has been ob- served as to their formation, referring for this to the researches of Purkinje, Coste, and others, which I have confirmed in most parti- culars by the examination of a considerable number of fowls during the process. Formation of the external or accessory parts of the bird's egg. These parts are produced with much greater rapidity than those of the ovulum. Many fowls lay an egg every twenty- four hours for a part of the season, while others lay only every second day, or two or three days in succession, generally at a later hour on each successive day, and then intermit for a day ; other fowls lay regularly nearly every thirty-six hours. There is probably some difference in the rapidity of the descent of the egg, or at least in the length of time it remains in particular parts of the oviduct, in these various cases ; but in general the whole passage of the egg, from the time of the re- ception of the yolk by the infundibulum to its being laid, occupies about twenty-four hours. If a fowl which is laying only every second day, be killed and opened from seventeen to twenty hours, or if one which is laying daily be opened from three to six hours after the last egg was deposited, one of the ovarian capsules will sometimes be found completely enveloped by the infundibulum of the oviduct, which is thus in the act of receiving the ovulum or j'olk about to be discharged by the cleaving of the capsule along the stigmatic band.* The infun- dibulum is contracted round the neck or pedi- cle of the ovarian capsule, so that the whole is embraced by it with moderate firmness, and the yolk thus usually passes securely into the * See a later section for an account of the circum- stances which influence the discharge of the ovarian ovula. 66 OVUM. oviduct; but it occasionally happens that capsules burst without being'so embraced or that the process is disturbed, and the sub- stance of the yolk falling into the abdominal cavity of the fowl either produces serious injury by peritoneal inflammation, or mav be gradually removed by absorption. The yolk enters the infundibulum, with its long axis corresponding to that of the oviduct, consequently witli the cicatricula on its side which we shall find to be its position also in the completed egg. The passage of the yolk through the first two-thirds of the length of the oviduct, in which part the albumen is deposited, is verv rapid, scarcely occupying more than three hours, according to Coste*, before it arrives in the narrow or constricted part of a more limited extent (isthmus), in which the mem- brane of the shell is formed. About three hours moresiiffice for this process, and the ovum then enters the dilated portion, which has been called uterus, in which the substance of the shell is deposited and gradually consolidated on its surface. The albumen begins to be deposited round the yolk, immediately upon the entrance of the latter into the oviduct; at first in a thin layer, immediately investing the yolk, which subsequently becomes condensed into the chalaziferous membrane, and in two lono- narrow portions extending before and behind the yolk from its poles, which portions of albumen are at first straight and simple, but afterwards become twisted and form the chalazae. ( See. fig. 4-6, A.) In the next part of the albuminiferous part of the oviduct, in which the glandular struc- ture is most fully developed, the albumen is deposited in much greater quantity round the .yolk and chalazse, not following the form of the latter, and thus soon gives to the whole the oval shape which belongs to the egg; and we then recognise, previous to the formation ol the shell or its lining membrane, that the narrower end of the oval is placed down- wards, or advances first in the oviduct. During the passage of the egg, and the formation of the albumen, membrane, and shell, a greatly increased determination of blood is observed in the vessels of the se- veral parts of die oviduct. (See Jig. 47.) The formation of the accessory parts of the egg appears to proceed nearly in the same manner in the scaly reptiles as in birds. The accompanying figure, borrowed from the article Reptilia, is illustrative of the main features of the process. The advancing motion of the egg of the fowl is caused by the peristaltic action of the muscu- lar coat of the oviduct, which may be easily seen in any laying fowl opened immediately after death. The egg does not descend, however, in a straight line, but in a spiral direction, corresponding with that of the ridges of glands with which the mucous membrane of the oviduct is beset. Two peculiarities in the structure of the albuminous part of the egg result from this spiral motion viz., Fig. 47. * ITist. gn. et partic. du BeVel. &c. Descent of the egg in the oviduct of the Tortoise (after Bojanus"). A. Infundibular opening of the oviduct; n, o,p, canal of the oviduct laid open; s, t, ovum opened showing the yolk, albumen and shell; B, allantoid bladder; F, oviduct; c, D, kidney; E, ureter; m, termination of the opposite oviduct. the spiral laminated form of the outer layers of albumen, and the marked tortuosity of the chalazae. It is easy to understand how the spiral form is given to the deposit of the layers of albumen. The cause of the pecu- liar manner in which the chalazse are twisted is not so immediately apparent . it may be explained as follows. As already remarked, the spiral twist is in an opposite direction in the two chalaza? ; one end of each of these cords must, therefore, have remained in a state of rest as compared with the other. Either, it may be supposed, the farther ends of the two chalazas extending into the ovi- duct before and behind the descending yolk, remain comparatively at rest, while that body with the albumen forming round it being closely embraced by the oviduct has the ro- tary motion impressed upon it ; or, as is more probable, when the chalazae become attached to and involved in the deposited albumen, their outer ends move with it, while the yolk within, to which the inner ends of the chalazje are fixed, docs not rotate in the same degree; a circumstance to which it is possible thesis-' position of the side on which the cicatricula is placed to remain uppermost may in some degree contribute; and thus the yolk not turn- ing so rapidly, or so often as the white, the chalazas are twisted upon their roots attached to the surface of the yolk.* ' It ought to be observed, however, that according to Coste, the yolk does not at first rotate freely OVUM. 67 Although it can scarcely be doubted that the chalazae are produced during the de- scent of the egg, while the albumen is being deposited, it is worthy of remark, that the twisted structure of these bodies is usually not to be seen till after the shell has begun to be formed*; but it is very probable that Fig. 48. Position of tlie egg in the oviduct as it descends. A portion of the oviduct near the lower end opened, taken from a fowl killed three and a half hours after the last egg was laid. The greater part of the albumen has been deposited, and the egg has assumed its peculiar form, the small end of the oval advancing first; the cieatricula placed on the side of the yolk. this may depend on their not having pre- viously acquired sufficient opacity or conden- sation to render their tortuous structure ob- vious. Indeed, Von Baer has observed them to make their appearance by increase of their opacity from exposure while under actual observation. It has been ascertained by experimental observation that the membrane of the shell is formed in the narrow part of the oviduct, termed the isthmus, which intervenes between the albnminiferous part and the uterine dila- tation. It consists, no doubt, in the fibrillation of consolidated albumen, or some analogous substance, which must take place with great within the white, and that it is only towards the end of the period of its passing through the oviduct that a liquefaction of the albumen, which then occurs, permits this rotation: but I think it doubtful that the adhesion between the surface of the yolk and deeper albumen is so great as to prevent the degree of rotation above referred to. * Von Baer, Ubcr Entwick. p. 31. rapidity ; but we are not yet sufficiently ac- quainted with the nature of this process, for the phenomena of the solidification and fi- brillar organisation have not been minutely ex- amined, nor has any difference yet been ascer- tained between the substance secreted in the isthmus, which undergoes the fibrillation with- out calcification, and that of the uterine dila- tation, which seems to have no such tendency, remaining amorphous or cellular, and having very soon a deposit of calcareous matter formed in it. By the time the egg arrives in the uterus, it has acquired its peculiar oval form, the small end pointing downwards in the oviduct. The cause of this form, which is already ap- parent in the white previous to the formation of the shell, is somewhat obscure, on account of the complexity of the mechanical condi- tions influencing the egg in its passage. It may probably depend on the circumstance that the soft mass dilates the oviduct more gradually as it insinuates itself between its coats, in being propelled onwards, while the part of the duct through which it has passed contracts more abruptly and firmly in conse- quence of the stimulus of distension to which it has been subjected. But the variety of forms which occurs in the eggs of different birds and other animals must not be for- gotten, as indicating that the peculiarity of a lesser and greater end is not essential, and may depend on very slight or transient cir- cumstances. Perhaps, the greater density of the albumen, secreted over the end which advances first in the oviduct, may also have some effect in giving this part the smaller volume. It certainly seems remarkable that the ends of the egg should be moulded into so smooth and rounded a surface as that of the membrane and shell by a tubular organ. In some rare instances, however, I have ob- served irregularities of form at the extremities of the egg, indicating an imperfect contraction of the oviduct during the passage. The egg remains a much longer time (from twelve to eighteen or more hours) in the uterine dilatation of the oviduct during the formation of the shell. The mucous mem- brane of this part differs in structure consi- derably from the rest : it presents over its whole extent large villous-like processes, or short folds, of a flattened form, containing small follicular glands, from which the substance of the shell is secreted. As soon as the egg enters this part of the passage a thickish white fluid is poured out from the membrane, which speedily coagulates on the surface of the membrane lining the shell, and very soon we can perceive with the microscope small heaps or united groups of particles somewhat of a crystalline appearance, but in reality cal- cified blastema studded over the whole surface. These are the calcareous particles of the shell, which are deposited in a delicate matrix of animal tissue of a large cellular structure. The deposit goes on rapidly increasing: at first the shell is soft, it remains friable for a considerable time, and, subsequently, it F 2 68 OVUM. gradually acquires the peculiar dry hardness which characterises it after the egg is laid.* The view of H. Meckel that the animal basis of the shell is formed by the separation of a layer of the mucous membrane of the ute- rine part of the oviduct does not appear to be established. . During the time that the shell is forming, the distinction between the softer and thinner external albumen, and the more dense and deeper part, becomes more obvious, and, at the same time, according to M. Coste, a cer- tain degree of liquefaction occurs in a layer of albumen immediately surrounding the yolk, which allows the latter body to float more freely within the superincumbent albumen. The egg remains in the uterine dilatation till it is about to be laid. The expulsion of it from this cavity through the narrow part of the tube, leading into the cloaca, requires very strong muscular contraction for its ac- complishment ; and, although the egg always descends in the oviduct, and usually lies in the uterus, with its narrow end downwards, both Purkinje and Von Baer state that they have sometimes seen its position inverted towards the end of the time of its residence there in consequence of the force of the mus- cular contractions of the wall of the oviduct. Ovarian ovum of birds ; uvulum ; yolk and its contents. The yolk, yelk, or vitellus (Jaime, Fr. Duller, Germ.) consists in the newly laid egg of the external enclosing vitelline membrane, of the yolk substance, a mass of vesicular, cellular, and granular matter of va- rious structure, to which as a whole the membrane gives a subglobular form, and on the surface of this mass, below or within the vitelline membrane, and on that side of the yolk which naturally turns uppermost in the complete egg, the cicatricula, or embryo spot, a thin disc of organised cellular structure, in which, under the influence of heat and air, as during ordinary incubation, the embryo, and its accompanying foetal membranes, &c., are first formed. The cicatricula of the laid egg, as has al- ready been remarked, however, has, during its descent through the oviduct, undergone some part of those changes which belong to the Fig. 49. Form of the Fowl's egg and structure of the yolk as exhibited by a section. A. Sectional view of the fowl's egg ; a, yolk enclosed by its vitelline membrane ; b, U, inner and outer parts of the albumen : c, c, chalazae ; d, two principal layers of the lining membrane of the shell ; e, calcareous shell ; /, air-space between the two layers of the shell membrane. B. Outline of the yolk ; , cicatricula ; b, nucleus of the cicatricula ; c, yolk cavity or latebra, and canal ; d, concentric deposits of yolk substance or halones ; m, vitelline membrane. fecundated condition, and by which the found- ation is laid of that structure in which the future embryo is more immediately developed ; for it has now lost its germinal vesicle, and from being formed, as at first, of mere granules or simple spherules, it has acquired a true organised cellular structure. It now consists, in fact, of the delicate discoid collection of cells, which has been called blast oderma. It may be proper, therefore, to consider the mass of the yolk and the germ, in their unfecundated state, while still within the ovarian capsule, * It is to be remarked that the animal basis of the calcareous shell is of quite a different structure from the fibrous lining membrane of the shell ; and the calcareous deposit is not to be regarded as taking place in that fibrous membrane. The outer- most layer of the lining membrane adheres very firmly to the shell, which may have misled some on this point, who describe the animal basis of the calcareous shell as of the same structure with the fibrous lining membrane. next, after the ovulum has entered the ovi- duct, and, subsequently, when it is laid ; reserving, however, for a latter part of the article the account of the process by which the change in the cicatricula referred to takes place. In the newly laid egg the yolk forms an ellipsoidal mass, somewhat flattened on the upper or cicatricular surface, and with its long axis corresponding to that of the egg. Its largest diameter is about one inch and a quarter, its shortest about an inch : it floats within the white, capable of a certain degree of motion, which is controlled, as before ex- plained, by its own specific gravity, and by the attachment of the chalazse. The yolk substance is not of the same nature throughout, there being a part of a lighter colour in the centre, about one fourth of the diameter of the whole; from this, a narrower prolongation extends upwards OVUM. G9 towards the cicatriculu, near which it' again widens and spreads out like a shallow cone. This whiter internal substance constitutes what has been called the central cavity (or latebra) of the yolk : the whole of this inner part has something of the shape of a flask, with a narrowing neck and a wider mouth at the top, which is, as it were, surmounted or closed in by the cicatricula. (Seey%. 49.) The shape of the yolk, I have said, is not that of a regular ellipsoid ; the less density of the upper part, which is towards the cica- tricula, giving rise to a widening of the yolk on that side, as may be seen mflg. 44, A, which represents a vertical section of an egg boiled while lying on its side. This does not depend simply on the rising of oil globules in greater quantity to the tipper side of the yolk, but, 'as has already been noticed, on the fixed predominance of globules containing oil in the neighbourhood of the cicatricula. Neither is the outer deeper-coloured por- tion of the yolk altogether uniform in structure or appearance ; for it will be seen, both in the raw and boiled egg, but most easily in the latter, that several concentric layers surround the central cavity and canal of the yolk, as well as the funnel-shaped dilatation which lies below the cicatricula. These layers are marked by a slight variation in colour, and are attended by a difference in the minute structure of the corpuscles composing the alternate layers. They probably depend upon the growth of the coloured part of the yolk being more or less rapid at different successive periods. The cicatricula of the newly laid egg is a spot of an opaque yellowish white, easily dis- tinguished by its difference of colour from the rest of the yolk, about one sixth of an, inch in diameter, and lying immediately within the vitelline membrane, in connection at its mar- gins with the most superficial layer of the yolk substance. Examined in a favourable light* it will be found, that in the laid egg, when fecundated, the cicatricula consists of a central clearer and thinner part, and of an external more opaque annular portion. The central part is about one third the diameter of the whole, and seems as if it perforated the remainder of the disc with a circular aperture, something after the manner of the pupil of the iris. There is not, however, any perforation in reality, but only a greater thin- ness and transparency of the central part of the disc. Neither is this central part entirely clear; for there is placed below its middle a round heap of whitish granules, described by Pander as the nucleus cicatricula: (see the figure in section), which gives greater opacity to that part when viewed directly from above. The central part of the cicatricula, already * It may be here mentioned, that by far the best mode of examining the natural appearances of the parts as they lie in the opened egg, is to allow a ray of strong or of direct sunlight to fall upon the part which it is wished to investigate, through an aperture in a screen, which places the rest of the egg and the observer in comparative darkness. obvious when the egg is first laid, is the same which, after some hours of incubation, ex- pands, changes its figure, and becoming still Fig. 50. Structure of the cicatricula in a laid Fowl's egg. A. Diagrammatic section of the yolk near the cicatricula, enlarged; o, vitelline membrane; b, cicatricula ; c, nucleus ; d, canal leading to the cavity ; e, e, large yolk corpuscles of the coloured part : the corpuscles are not represented of their real proportional sizes, but more with a view to show their general difference. B. Enlarged view of the cicatricula, as seen from above on the surface of the yolk in an impregnated egg : the dark central space or transparent area surrounded by an opaque zone and one or two delicate haloes. c. Cicatricula of an unfecundated laid egg : instead of the central transparent area a number of rather irregular transparent spots are seen. more clear, receives the name of transparent area, in the centre of which the embryo be- gins to be formed; while the outer more opaque part retains its greater thickness, and is con- verted afterwards into the vascular and peri- pheral part of the germinal membrane which spreads over the yolk. Round the margin of the cicatricula the' deeper-coloured yolk sub- stance is seen even in a perfectly fresh or newly laid egg to be intersected by one or more fine circles of a lighter colour. These seem to be the same which afterwards, expanding and widening, constitute the haloes which pre- cede and accompany the extension of the ger- minal membrane over the yolk. These circles are the terminations at the surface of the concentric layers of lighter substance, which, as already mentioned, may be seen surround- ing the central cavity and canal of the yolk (see fig. 49). It seems not improbable that, the difference in the structure of the central and peripheral parts of the cicatricula just stated proceeds from, or is connected with, the peculiar process of fissuring or segmen- tation which follows the disappearance of the germinal vesicle from its central part in the fecundated egg ; but the description of this process belongs to a later section of the present chapter. v 3 70 OVUM. The cicatricula of the unfecundated egg, such as is laid by fowls secluded from the cock, differs from that now described princi- pally in the absence of the marked dis- tinction between the central clear and the peripheral opaque part. The germinal ve- sicle, which to all appearance remains the same in the ovary till the yolk leaves the ovarian capsule, is now no longer to be seen; and the cicatricula is often marked irregularly throughout, but more especially towards the circumference, with clearer intervals, or small irregular circular orjoval spaces, mingled with the opaque substance of the disc. I have, not, however, had the means in more recent times of making a sufficiently careful exami- nation of the cicatricula in this condition to enable me to state more minutely in what respects it differs from that of the fecundated egg- In the ovarian yolk, while still within its capsule, a white spot corresponding to the cicatricula also exists, and occupies the same place in relation to the yolk cavity and canal. Its structure and appearance, however, are somewhat different from that of the true cica- Fig. 51. Cicatricula, and its contents, in the ovarian egg of the Fowl. A. A square portion of the surface of the ripe ovarian yolk, showing the vitelline disc or cicatricula, with the germinal vesicle in the centre, magnified ahout six diameters. u. Lateral view of the same, to show the con- vexity produced by the thickness of the disc round the germinal vesicle. c. Vertical diagrammatic section of the same ; m, vitelline membrane; d, granular disc ; g, germinal vesicle. D, E, F. Germinal vesicles more highly magnified; D, from a yolk of about one tenth of an inch dia- meter, showing scattered globules or germinal spots; E, from a nearly ripe 3 - olk, quite clear; F, from another of the same period, exhibiting a turbid or minutely granular mass from the action of water. tricula of the egg which has passed through the oviduct ; it is covered by a layer of closely set nucleated cells which lie below the vitel- line membrane ; it contains the germinal vesicle in its centre, and, instead of being thinnest towards the middle, the mass of its granular substance is accumulated in greater quantity in that part round and below the germinal vesicle, and thins gradually off towards the margin. Nevertheless, its much lighter colour than the surrounding part of the yolk makes it always easy to distinguish it. Its margin, however, is not so well marked as that of the true cicatricula ; for the opaque whitish sub- stance seems there gradually to pass into or be continuous with the most superficial layer of cells covering the yolk. To this ovarian representative of the cicatricula, Von Baer has given the name of stratum proligcmm. It is also somewhat smaller than that of the laid egg. It is usually to be found on that part of the yolk which is next the ovary, which, as the yolk hangs within its capsule in the usual attitude of the bird, will be upper- most, and for the most part is situated close to the pedicle of the ovarian capsule. This position is not, however, a constant one ; for sometimes the cicatricula is seen on the sides of the yolk, or towards the stigmatic band of the capsule, but rarely, it would appear, towards the ends or poles of the yolk. The cicatricula may generally be perceived on the surface of the yolk when the outer- most layers of the capsule have been re- moved, and the germinal vesicle can be distin- guished in it shining through the inner layer of the capsule and the vitelline membrane. It is placed close below the nucleated cells which line the latter, and adheres along with them somewhat to its inner surface; so that in gene- ral, it is easiest to remove this disc along with a portion of the vitelline membrane, when it is desired to obtain it for separate and more mi- nute observation by transmitted light. The vitelline membrane being cut round with scis- sors at a short distance from the margin of the disc, the parts are floated off in water or serum, and then may readily be separated with a little careful manipulation. The germinal vesicle, or vesicle of Pur- kinje, may always be seen with the unassisted eye, with a good light, in the centre of the ovarian cicatricula, or proligerous disc, in all ripe ovula, and in most of those which are above a tenth of an inch in diameter. It constitutes there a well-defined shaded cir- cular spot, from ^ to -^ of an inch in diameter. When the proligerous disc alone has been removed Tor observation and laid on a flat surface, and viewed somewhat from the side, or when the granules are torn asunder with needles, so as to make a partial section of it without removing or bursting the ger- minal vesicle, it is easy to perceive that the middle part, containing the vesicle, is more elevated than the rest; and that, although the substance of the disc seems to pass quite smoothly or evenly from the sides over the germinal vesicle, the granules of the disc en- velope the vesicle only slightly, and none cover its middle part: the vesicle, there- fore, is set, as it were, in a depression of the disc, which fits round and overlaps its margins, and a considerable thickness of gra- nular substance is continued in the disc below the vesicle. (See/g. 5!, in section). If we select for examination the most ad- vanced yolk of the ovary, which, in a hen laying daily, or almost daily, would probably have OVUM. 71 been discharged from the capsule in a few hours, we may find some difficulty in isolating the vesicle of Purkinje from thegranulardi.se ; for, by this time, the vesicle has become flaccid, weak, and flattened down, and has begun to be softened and dissolved, prepara- tory to its complete disappearance, which generally occurs about the time when the stigma of the capsule opens to allow of the escape of the yolk into the infundibulum which embraces it. But, in all the other yolks down to those of -jL of an inch in dia- meter, it is quite easy to break up the granular disc with needle points, and to preserve the vesicle uninjured. We may then free it entirely from adhering granules, and cause it to roll along in the fluid in which it is immersed, or on a plate of glass ; and we may perceive that it is a simple membranous vesicle filled with fluid, and without any very obvious granules or nuclei. In the perfectly fresh state,the contents of the vesicle are almost limpid, exhibiting only a slight turbidity scarcely amounting to a granular deposit, provided it has been placed in a medium which does not change its ap- pearance ; but, if it is allowed to remain a short time in water, and still more if it is im- mersed in fluids which coagulate albumen, its interior speedily assumes a minutely granular aspect. The external wall of the vesicle then separates somewhat from the spherical gra- nular mass within ; and I have sometimes seen (as represented in fg. 51, F) a considerable condensation of the granular mass, so as to leave a large clear space between it and the external vesicle, and give it very much the ap- pearance of the yolk mass in the ova of some small animals within the vitelline membrane. This change seems to be a combined effect of the condensation of the granular mass and the imbibition of fluid by the external vesicle. In the earlier ovula this rounded molecular mass is of proportionately smaller size ; and although it differs very much from the smaller nucleus or macula contained in the germinal vesicle of the ova of many other animals, there can be little doubt that it is derived from this structure, as will appear from what is hereafter said of the progress of its development. When the yolk has passed into the ovi- duct, and, in most instances, probably even sooner, or when it has entered the infundibulum, the germinal vesicle has entirely disappeared. Sometimes it is already gone before the open- ing of the ovarian capsule. The cicatricula then presents an irregularly broken appear- ance in consequence of the want of support from the wall of the vesicle, and the dif- fusion of the contents of the vesicle over the surface of the proligerous disc. The solution of the wall of the vesicle is probably a gradual process connected with the state of complete maturation of the ovule. It occurs, as is well known, in the unfecundated as well as in the fecundated egg, and cannot, there- fore, in itself, be dependent on the action of the spermatozoa ; neither is it altogether caused by the mechanical pressure to which the yolk is subjected in issuing from the ovarian capsule, nor by the pressure of the oviduct itself; for it usually begins, and is sometimes completed before these causes can operate. The diffusion of the germinal substance from the vesicle (which in the fowl must have already received the spermatic influence in the ovary) has the effect thus of mingling with the remainder of the cicatricula, a ma- terial which, it can scarcely be doubted, ex- erts some immediate influence in inducing the change of segmentation and subsequent pro- cess of organisation by which the blastoderm is produced. Microscopic structure of the ovum. The in- vestigation of the microscopic structure of the yolk is attended with considerable difficulty, in consequence both of the variety and the deli- cacy of the organised elements of which it consists. The following parts require our separate attention viz., 1st. The yellow or external yolk substance ; 2nd, the substance of the cavity and canal ; 3rd, that of the cica- triculu and cumulus ; 4th, the vitelline mem- brane. We shall consider these both in the laid egg and in the ovarian capsule. 1. From the effect of boiling the yolk, every one is familiar with the fact that its yellow substance is coarsely granular ; but the exact nature of the small bodies giving this granular structure has not been equally well understood. The examination of this sub- stance with a microscope of moderate magni- fying power in a newly laid egg, shows that almost all of the deeply coloured part of the yolk consists of spherical corpuscles of con- siderable size, so closely set together that they are mutually compressed ; and thus, when the yolk has been hardened by boil- ing, the substance of the corpuscles being coagulated by heat, they present polyhedral forms ; but when diffused in fluid in the un- boiled state, they are all nearly or quite sphe- rical. The size of these corpuscles varies between T i^- and -gfa of an inch ; but the greater number of them are more near -i^ or 2 -iw noticed. In some animals the form is not OVUM. Fig. 52. Microscopic structure of the elements of the yolk and ovarian ovum of the fowl. A. Large granular corpuscles of the yellow part of the vitellus ; one of them quite spherical, as they are seen when free ; two others angular from mutual compression, from a boiled yolk. B. Various corpuscles found on the confines of the 3'ellow yolk and the cavity and canal, showing transition forms to the nest set. c. Clear vesicles containing oil globules and de- tached oil globules of various sizes from the cavity and canal. r>. From the cicatricula ; a, various-sized granules and globules forming the vitelline disc of the yolk before its discharge from the ovary ; b, the organised nucleated cells forming the upper layer of the cicatricula in a laid egg ; c, larger cells of the lower layer; d, cells of the cicatricula from an egg in its descent through the oviduct in process of formation. A scale with divisions of ^ of an inch is appended. spherical ; as, for example, in the cartilaginous fishes, in which a remarkable variety occurs of a cubical form, and sometimes these mixed with tetrahedral forms, as in the skate.* When free, these corpuscles in the yolk of the bird's egg roll easily on the surface of a plate of glass as perfectly distinct spherical bo- dies. They present (see jig. 52, A) a minutely molecular or granular aspect, but with quite a smooth outline or margin to the whole cor- puscle. If subjected to pressure, or cautiously ruptured with needle points, they break readily at one or more places, and allow the escape from their interior of the thick granular fluid, which flows slowly out of them in a stream. The granules are in large quantity, as compared with the fluid in which they are suspended, and are most of them of an ex- tremely minute size, probably below -5^0-0 f an inch in diameter. * See Miillcr's Physiology, vol. ii. Although the yolk corpuscles present the distinct external margin now mentioned, and thus constitute capsules containing the gra- nular fluid, yet we cannot, in most instances, detect any vesicular membranous envelope surrounding them. One may sometimes ob- serve a delicate limiting line ; but it has been impossible for me to determine whether it consisted really of a membrane or of a thin condensed layer of the granular substance or plasma containing it. At an earlier period it is probable that these corpuscles have mem- branous envelopes, but when fully formed the greater number are certainly destitute of them ; for occasionally a larger corpuscle may be observed to divide into smaller ones, the outlines of which are nearly as distinct as that of the larger corpuscles. Nor is any nucleus in general to be per- ceived in these corpuscles. I have occasion- ally seen in those from which the granular matter was escaping, and which had thus be- come less opaque than usual, a slight ap- pearance of a clear hyaline circular space, but it scarcely deserved the name of nucleus ; and if these spherical bodies are to be regarded as cells, which 1 think they ought, it must be in a somewhat different acceptation from that in which the term cell has hitherto been gene- rally applied to vesicular organised structures. But recent researches on the early condition of cells seem to have rendered it necessary that we should include under this denomina- tion several simple spherical minute forms of organised or organising matter, which were not at first regarded as true cells by the authors of the cellular view of organic struc- ture ; and when we consider the mode of their formation, it is more than probable that the vitelline corpuscles now under consideration may be included among the number.* They probably constitute, at all events, as Schwann has first shown, one stage of deve- lopment of a cellular structure ; and, in the meantime, they may with propriety be styled the larger granular yolk corpuscles. There is considerable uniformity in the ap- pearance and structure of these corpuscles in nearly the whole of the deeper-coloured por- tion of the yolk ; but immediately below the vitelline membrane, several layers of them are of a smaller size, and the outermost layer of all consists of cells which are much smaller and more compressed, distinctly nucleated and of a short cylindrical or prismatic shape. In some places also, corresponding to the con- centric lighter lines which run through the yellow yolk, some approach is seen to the next kind of yolk cells or corpuscles, which I shall have to describe viz., those of the cavity. The substance of the yolk-cavity and canal, which in the unboiled egg may be distin- guished from the other part by its lighter * The above observation has a general application to such minute spherical masses of matter as are destitute of external envelope or nucleus : but in rc'li-rence to the corpuscles of the yolk, I ought to observe that Schwann regarded them as cells in various stages of growth. OVUM. 73 colour, and in the boiled egg by its softer consistence and less granular appearance, is found by microscopic examination to consist of organised corpuscles floating in a larger portion of fluid, and different from those of the external part of the yolk. The transition from the one kind of corpuscles to the other in these two portions of the yolk, is not sudden ; but many gradations of intermediate forms are to be met with on the confines of the two regions. In the central part of the cavity or latebra, which, when boiled, appears like a thick milky fluid, corpuscles very different from those of the external part are to be found (see fig. 52, c). They are almost all of a very regular spherical form with a delicate and clear, but distinct vesicular wall ; the interior of the vesicle is occupied by a perfectly limpid fluid, and by one or several highly refracting globules of various sizes, not exactly similar to nuclei, but rather like oil globules, floating within the cell and moving with freedom from one part of it to another. The diameter of the clear vesicles varies from T ^ to -ji^ of an inch, the most being about -g^- ; therefore about half the size of the granular corpuscles of the yellow yolk. The internal oil globules are of very various sizes, the largest being generally about a third or a fourth of the diameter of the enclosing vesicle. Mingled with these vesicles, there are also floating in the fluid of the yolk cavity in considerable numbers, but in less quantity than the vesicles them- selves, a set of simple highly refracting globules, exactly similar to those contained within the vesicles from which we may sup- pose they have been set free. These oil-like globules are of every variety of size, from the minutest point up to T ^Vd or ToVo f an inch. Towards the surface of the yolk cavity and canal, and extending below the cicatricula, where the vitelline substance gradually passes into the darker yellow yolk, the microscope shows some mixture of and transitions be- tween the several cells or corpuscles before described, those of the intermediate structure being in greatest numbers ; these exhibit very various gradations of deposit within them, from the finest granular particles in some, to larger and fewer oil-like globules in others. In most of these transition corpuscles a delicate vesicular wall is perceptible. In the more ad- vanced of these transition forms, as the minute granules are in the process of uniting into larger and larger oil globules, and at last coa- lesce into a very few or into a single one, the condensation of the exterior layer increases to form a vesicular wall, and a separation of an albuminous fluid from the oil globules takes place within (see^g. 52, B). It is these vesi- cular globules of the cavity which, according to Reichert, are the more immediate source of additions to the germinal membrane in the course of development ; for the cavity and canal expand, as it were, at the expense of the yellow yolk, and as these inner globules increase the extension of the haloes ami change of colour of the yolk in the first days of incubation spreads rapidly over its surface below the germinal vesicle. 3. Cicatricula or proligerous disc. There does not appear to be any marked difference as to the minute structure of the mass of the yolk and its cavity in the newly laid egg and in the mature ovarian ovulum ; but the cica- tricula undergoes a great change during the passage of the ovum through the oviduct, which is indicated in a marked manner by the difference in its microscopic structure. During this period, besides the loss of the germinal vesicle, the cicatricula has undergone the peculiar process of segmentation and cell formation, upon the details of which it is my intention to return in connection with the special history of that process in the ovum of mammalia, batrachia, and other animals. The cicatricula of the laid egg is, in fact, after having undergone this process, the organised blastoderm or germinal membrane in which, under the influence of the heat of incubation, the rudiments of the embryo take their origin. It already consists, before incubation, of two layers of organised cells, which are the indica- tion or earliest condition of the upper or serous, and lower or mucous layers, which were de- scribed by Pander and Von Baer as taking their origin only after incubation for some hours.* (Seey?g. 52, D.) The cells of the upper layer are about - T rr3 of an inch in diameter. They are closely set and very slightly connected together in a continuous layer one cell thick, presenting a smoother upper surface next the vitelline membrane. Each cell consists of an external vesicular wall, a distinct nucleus, and some granular deposit. The nucleus is highly re- fracting. The cells of the lower layer are nearly double the size of the upper ones, more regularly spherical and less closely connected together. They do not in general present any single nucleus, but rather a small mass of granules and oil-like spherules within them, giving them much of the appearance, though smaller in size, of the corpuscles found be- tween the cavity and rest of the yolk. In the cicatricula or proligerous disc of the ovarian yolk, on the other hand, containing the germinal vesicle set in its centre, the microscope shows no truly organised cells, but only a mass of simple spherules of very various sizes, but the largest of which for the most part arc less than half the diameter of the cells in the upper layer of the blas- toderm of the laid egg. They are without any nucleus, and have all the appearance of simple solid spherules from ^L- to ^oW of an inch in diameter, of considerable ref'ractin"- power, and, indeed, very similar to the nuclei of the cells in the upper blastodermic layer. Vitelline membrane. The condensed layer of structureless membrane which has gene- * The most exact descriptions of the minute structure of the cicatricula are those of Schwann in his Microscopic Researches; of Reichert, in his Beitrage zur heutige Entwickelungsgeschichte, c. ; and Remak, in his Beitrage zur Eutwick. des Iliilmchens, &c., 1850. OVUM. rally received this name in the fowl's egg, and which I have hitherto regarded as corre- sponding with the immediate membranous in- vestment of the yolk (zona pellucida) in mammalia, and in all animals, constitutes, both in the mature ovarian yolk and in the laid egg, an entire thin transparent covering of the yolk substance, without any aperture that has been discovered in it at any time ; delicate and easily torn, but yet of such con- sistence that under water any portion of it may easily be removed and examined. In the egg which has passed through the oviduct, the vi- telline membrane floats free from the cicatri- cula and surface of the yolk substance ; but, so long as it remains in the ovarian capsule, these parts ^cohere somewhat together ; so that, in general, on removing a part of the yolk membrane, a more or less complete lining of the nucleated or outermost layer of yolk cells comes away with it. The microscopic exami- nation of this membrane in the fully formed yolk does not, as already stated, show any very distinct structure beyond an obscure fi- brillar and molecular marking, of such fineness, indeed, as to require a high magnifying power (500 to 600 diameters) to bring it into view ; and in many parts the membrane appears per- fectly homogeneous. In the earlier stages of the yolk's growth, however, we shall see that tli is membrane is not to be distinguished from the layer of closely set nucleated cells, the outer- most part of which appears to become fused together into the membrane as the yolks ad- vance to maturity (see j%. 53, K vni). We shall presently see that the vesicular envelope which is generally termed the yolk membrane in the bird's egg, and in the ova of all animals pos- sessing the large yolks, is probably a different structure from the perfectly homogeneous ve- sicle which in many other animals arises at a much earlier period of the growth of the ovule, and remains in them as the external covering of the yolk to the end. Early condition and first formation of the ovarian ovum in birds. It has already been stated that the ovula exist at a very early period of life in the female bird ; constituting in their earliest undeveloped condition minute cells closely surrounded by the simple vesi- cular capsules and the solid substance of the ovary, which at ^this period has not lost its primitive compact form. As the bird ap- proaches maturity, a considerable number of the ovula situated nearest the surface in- creasing in size make an advance in their structure by undergoing certain changes which will immediately be more particularly adverted to. Having attained various sizes from ^V to A of an inch, they project slightly as rounded bodies from the surface of the ovary, and remain in this condition till the approach of the breeding season, when some of them destined to reach their full state of develop- ment, are at last discharged from their ovarian capsules. A much greater number, however, must remain in the undeveloped condition, awaiting future seasons of evolution ; and a very considerable proportion of the whole germs of the ovary rather pass through a retrograde process and gradually disappear without having attained to any considerable size. Of the smaller or undeveloped ova, such as those of less than -^ of an inch in dia- meter, some are of a dull whitish or milky colour, the deeper-coloured external yolk substance not having been yet formed, and the yolk substance consisting almost entirely of small spherules or globules, not of true cells or of the granular corpuscles which appear at a later stage. Those between ^ and of an inch are for the most part of a lighter yellow than the larger ovula ; but above the latter size the colour has attained nearly its full in- tensity from the deposit externally of the deep-coloured yolk substance. In all the ovula above ^ of an inch it is easy to see the germinal vesicle situated on the surface of the yolk, when the capsule is opened, embedded in a more opaque and compact layer of substance which repre- sents the discus proligerus, extending at this period nearly over the whole surface of the yolk. But in those less than -J ? or -$ of an inch, the vesicle is not to be seen on the surface. On carefully opening or breaking up the substance of the yolk, the vesicle is easily found in the softer internal substance which flows out from the centre. From the central part of the small ovule, the vesicle appears gradually to pass outwards towards a deter- minate part of the surface, making its way through the proligerous layer or primitive yolk granules ; and thus, in examining ovula at this stage, I have been able to perceive occasionally that the vesicle was situated ,in a more or less deep depression on the inner surface of that layer, which therefore must be perforated, as it were, by the vesicle in its passage towards the surface. The sub- stance of the disc afterwards collects round the vesicle internally, and is accumulated in greater quantity (cumulus) in that situation. This change of place of the germinal vesicle from the centre or interior to the surface of the yolk in the progress of development of the ovula occurs in some degree throughout the animal kingdom ; but it is especially re- markable in the eggs of birds and other animals with large yolks, in consequence of the peculiar connection of the vesicle with the proligerous disc. In the batrachia also, the change is very obvious, and the progress of the vesicle outwards has been well described by Von Baer and others. In this latter class of animals the proligerous layer covers a much greater part, or indeed in most of them nearly the whole of the yolk ; but the germinal vesicle occupies always a determinate'place in the centre of the layer ; showing that the development of the various parts of the ovum proceeds from the first with a fixed relation of position between the germinal ve- sicle and other parts. In birds, as in all other animals, the ger- minal vesicle, which we shall see is the fun- damental part of the ovum, is proportion- ally large in the earlier stage of growth of the ovule, being at the first from a fourth to a OVUM. 75 half of the diameter of the whole ovule. In the progress of growth, it enlarges some- what, but only in the earlier periods, and in less proportion than the yolk, and undergoes no farther increase during the greater part of the time that the yolk acquires the greatest addition of new matter. It is worthy of remark, however, that the germinal vesicle is originally of a large size in the eggs of birds and other large-yolked ova ; that it is also of very considerable size, even proportionally larger, in the batrachia ; and that in mammalia, and other animals with the smaller and gra- nular yolk, its size bears in general a propor- tion to that of the yolk. The substance of the yolk appears, in the first place, to be simply granular, or to be composed entirely of minute molecules such as those which always form the yolk in mam- malia. These are united together by a some- what glairy fluid ; larger spherules gradually appear among them ; and next the distinction between the substance of the proligerous disc and of the yolk cavity becomes apparent. Lastly, 'the deep-coloured yolk corpuscles are produced, layer after layer being deposited from the exterior, so that the outermost are the last formed. Externally a closer-set layer of nucleated cells covers the surface, in con- nection with which the vitelline membrane is formed. The vitelline membrane is not formed at an early period in the bird's egg : it cannot indeed be perceived in ovula of a tenth of an inch in diameter. We shall presently see that its relations and mode of formation are peculiar in the bird's egg. Morphology of the bird's egg as ascertained from its first origin and development. The ovaries of the common fowl, and indeed of most large birds, are less favourable for the investigation of the first origin and earliest condition of the ovule, than those of the smaller tribes ; this arises, not so much from the dense structure of the ovary in the undeveloped state, as from the great opa- city produced in the ovules themselves, almost from the first, by the deposit of thick-set yolk granules. In some of the smaller singing birds, the thrush, yellow-hammer, or chaf- finch, the parts are clearer and more trans- parent ; and it will be found that the pheno- mena of earliest formation are most easily investigated in them. According to Dr. Martin Barry's observa- tions, in birds as well as in other animals, the germinal vesicle is the part of the ovum which is first formed. In the pigeon and common fowl, he has observed these vesicles in the ova- rian substance at a very early period*; and he believes their origin as simple cells to precede that of the ovarian vesicles, or follicles, or, as he has termed them, ovisacs, which surround them at a somewhat later period, but still in the earliest stages of the formative process. By other observers the ovarian vesicles have * See Philos. Trans, for 1838, p. 309. In this, coinciding with the opinion previously expressed by Von Baer. Fig. 53. B Earliest stages of the formation of the ovarian eqg in the Bird. A, B, c, D, E, F, actual representations of portions of the ovarian stroma and ovisacs of the thrush ; G, n, i, K, diagrammatic sections of the same. A. In the ovarian stroma are seen the earliest state of the ova and ovisacs that can be perceived, consisting, first, of minute granular spots ; next, of clear points within a minute granular mass ; and third, of small germinal vesicles, surrounded with the minutely granular dark yolk substance. Compare with G, in the diagrammatic figure. B and c. Different stages of formation of the ovi- sac round the small ova : the epithelium is seen to line the sac : the germinal vesicle with occasionally a single macula is now apparent. D. The epithe- lium of the ovisac shown iu focus over the whole surface : in the other figures it is only shown in 76 OVUM. focus at the margin. E. The ovisac and ovum more advanced ; o, v, ovisac, with epithelial lining ; v, minutely granular yolk ; g, germinal vesicle. F. Part of an ovuie of ^ of an inch in diameter highly magnified : v, minutely granular or primi- tive yolk substance ; g, germinal vesicle ; 2, thick consolidated membranous layer which formed a ve- sicular covering for the primitive ovule, and which corresponds to the zona pellucida of the niammi- ferous ovule. i and K are intended to illustrate, diagrammati- cally, the view, that after the disappearance of the zona, and the formation of larger granular yolk cells, the outer layer of the cells of this substance forms the permanent vitelline membrane of the bird's egg ; v, d, remains of minutely granular yolk, form- ing the vitelline disc round the germinal vesicle ; s,g, large corpuscles of the yolk ; v, m, outer layer of the cells of the same, on which the vitelline mem- brane is afterwards formed. been looked upon as the primitive or first- formed structures connected with the origin of the ova, the germinal vesicles subsequently making their appearance within them. We shall return to this point hereafter in con- nection with the history of the mammiferous ovum. My own observations agree with those of Barry, as I have sometimes observed very small germ-vesicles or cells in the ova- rian stroina without any follicular covering. But it must be admitted, at the same time, that in birds the ovisac or ovarian vesicle is formed so early that it is observed almost always coexisting with the germinal vesicle or rudiments of the ovule ; so that, if the latter takes the precedence of the ovisac, it must be by a very short period. According to Barry, there is seen almost from the first, in the clear germinal vesicle, a minute distinct granule or round spot, which constitutes the first state of the macula germi- nativa. Very soon the vesicle is surrounded by a small quantity of a clear fluid in which are rapidly deposited globules or granules constituting the first rudiments of yolk substance. There is no vitelline membrane, however, in birds, at the first ; nor are the larger cells which at a later period inter- vene between the ovisac and the primitive yolk, formed in the earliest stage. The smallest ovisacs which Barry observed, and which con- sisted of perfectly simple vesicular linings of the cavities containing the rudimentary ova, in the pigeon and common fowl, were from ^i n to 300' f an nicn i' 1 diameter. * At a somewhat later period, the number of maculae (nuclei) in the vesicle, and of the yolk granules externally, had increased, and a delicate membrane, which he describes as vitelline membrane, and believed apparently to be the same which afterwards surrounds; the large yolk in the fully-developed ovum, has made its appearance. At this period also * Vide loc. cit. Plate v., figs. 18, 19, and 22 of pigeon ; figs. 23 and 24 of common fowl. The mem- brane which Barry described as vitelliue in the earliest stages of growth of the bird's egg was pro- bably not so, but the outline merely of the albumi- nous substance in which the primitive yolk granules are deposited. This will be made more apparent in our description of the formation of the ova of Batrachia. there begin to be formed within the ovisac a se of larger nucleated corpuscles or cells, which are external to the true ovum, and which may be considered as corresponding with the so-called granular contents (sub- stantia and tunica granulosa) of the Graafian follicle in mammalia. The early structure and development of the ovum of birds have more recently been described, with considerable detail, from ob- servations on the chaffinch and common fowl by Dr. H. Meckel* ; and as the observations of this author have led him to take a somewhat different view of the relations of some of the parts of the ova of birds and other animals from that which has hitherto been generally adopted, it will be proper to give a particular account of them in this place. Many phy- siologists have felt the incongruity of the comparison generally made between the mi- nute and simple ovum of the mammifer, and the large and more complex yolk of the bird, and most are disposed to acknowledge the necessity of making some more marked distinction between the granular and the cellular yolk substance in the two great groups to which these ova respectively be- long. It has before been stated, that Von Baer on his discovery of the mammiferous ovum, regarded it as corresponding, not to the whole ovum of birds, but to the vesicle of Pur- kinje. The discovery, in J834, of the germinal vesicle in the mammiferous ovum, of the ex- istence of which Von Baer had no distinct knowledge, induced Valentin and others to maintain that the essential parts of the ovum are the same in the bird and the mammifer. But it may be doubted whether physiologists may not have proceeded further than they were warranted by observation in regardingthe vitelline membrane and large corpuscles of the yellow yolk of birds as essentially corre- sponding parts with the zona pellucida and the smaller granular yolk of the mammifer. For the membrana vitelli of the bird's egg may, perhaps, be more analogous to the outer- most layer of the membrana granulosa of the Graafian follicle, and the large cellular yolk to a part of the same substance or the fluid of the Graafian follicle ; while the minutely gra- nular yolk in which the cicatricula originates and the germinal vesicle together are the true representatives of the small ovum of the mam- mifer. It seems undoubted, that what we term the yolk membrane in the fowl's egg does not exist in the early stages, and is formed indeed only as the ovarian egg approaches maturity, and it is admitted that no large cells similar to those of the bird's yolk exist within the cavity of the zona pellucida of the mammi- ferous ovum. If this view is correct, we may expect to find a representative in the egg of the bird and of other animals having similar ova, of the very marked enclosing vesicle, which has received the name of zona pellucida * See his paper, Die Bildung der fur partielle Fiirclmng bestimmten Eier der Vogel, &c., in Sie- bold and Kiilliker's Zeitsch. fur Wissenschaft. Zool. vol. iii. p. 420, 1852. OVUM. 77 in the mammiferous ovum. Now, according to H. Meckel there is, not from the very first, but in the earlier stages of formation of the yolk of the fowl and of other birds, a homo- geneous vesicular membrane enclosing the primitive or granular yolk, or what he terms the true egg substance. As the cellular yolk is formed, this membrane, to which he thinks himself warranted in giving the name of zona pellucida, disappears, and already in ova above T \j of an inch there is no trace of it left. The observations of H. Meckel on this sub- ject appear to be both novel and important ; but he has not been equally successful in the theoretical deductions made from them. In the commencement of the paper before referred to, he thus announces his view of the morphology of the bird's egg : " For a ritiht and consistent nomenclature and defi- nition, \ve must designate the corresponding parts according to their analogy with those of the human body. I believe, therefore, that that alone ought to be regarded as the true egg which exists in Man, Mammalia, Naked Amphibia, and Osseous Fishes ; and that in the remaining Vertebrata the ovum consists only of the so-called vesicle of Purkinje, and that all the other parts are accessory, super- imposed, and unessential. In particular, that the yellow yolk of the bird and scaly reptile is a'nalogous to the corpus luteum of the human ovary, the albumen ovi to the uterine secretion, and the calcareous shell to the de- cidual mucous membrane of the uterus." Von Baer, ai, p. 32. of his Epistola, uses the fol- lowing words, which have been much contro- verted by some of those coming after him, but which show that he was aware of the difference in the relation of parts in the birds and mammiferous ovum : " Vesicula ergo CJraafiana cum ad ovarium generatimqne ad corpus maternum respiciamus, ovum sane est Mammalium, sed evolutionem quod attinet, vehementer discrepat a reliquorum ovo ani- malium," &c. And again, " In mammalibus vesicula innata vitellum magis excultum con- tinet, et ratione ad fetum geniturum habita verum sese probat ovum. Ovum fetale dici potest in ovo materno. Mammalia ergo ha- bent ovum in ovo ; aut, si hac dicendi formula uti licet, ovum in secunda potentia." Both in the Epistola, and the Commentary upon it, Von Baer insists strongly on the analogy be- tween the cellular substance of the Graafian follicle and the yellow yolk ; and he seems to have erred chieHy in limiting his comparison of the mammiferous ovum (within the zona) to the vesicle of Purkinje of the bird's egg. If, therefore, we modify Von Baer's view so much as to regard the vesicle of Purkinje along with the granular cicatricula of the bird's egg, as corresponding to the whole of the mammiferous ovum, and the granular cells (tunica granulosa, &c.) of the Graafian vesicle as corresponding to the yellow yolk (the zona pellucida having disappeared in the bird's egg), we shall establish a more correct relation of the parts than that suggested by H. Meckel. I am not aware of any animal in which the germinal vesicle alone, without some yolk substance and an external inclosing membrane (zona, or vitelline membrane) forms the entire ovum. I will now state the result of my own ob- servations on this subject, by which 1 con- ceive is proved the correctness of H. Meckel's view, that in birds there is a primitive ovum, enclosed within a zona, distinct from the large mass of cellular yolk, which is formed at a later period. As soon as the membranous wall of the ovisac or ovarian follicle has become distinct in the ovary of the fowl, we can perceive at the same time a layer of larger cells lining it which form a clearer ring round the opaque ovule. The ovule itself consists then merely of the germinal vesicle and a small quantity of the primitive yolk substance. The latter becomes opaque at so early a period that it in general hides completely the germinal ve- sicle. It seems to arise by the deposit of very fine granules, probably of an oily nature, in a dense albuminous fluid or blastema which col- lects round the germinal vesicle very soon after the latter is invested by the ovarian fol- licle. In follicles of T Jg- of an inch in dia- meter, the primitive ovule, the membrane of the enclosing follicle, and between them the layer of larger clearer cells, are all perceived with facility. There is not yet, however, any investment of the ovule comparable either to a zona pellucida or vitelline membrane. In ovarian follicles of $ or -fa of an inch in diameter, a farther progress is to be per- ceived. On bursting any such follicle with fine needle points, the ovum is ruptured, and the germinal vesicle usually first escapes along with the more fluid internal part of the yolk, in which it is freely suspended. This vesicle is about T J- n of an inch in diameter, presenting externally a smooth, thin and de- licate vesicular membrane of a spherical form, of which the double outline is just discernible with a magnifying power of 300 diameters. The vesicle is partly filled with fluid and partly with a finely granular spherical mass of no great opacity, which corresponds to the macula germinativa. In most instances this mass occupies about two thirds of the diameter of the vesicle. The yolk substance, which has scarcely altered from its primitive opaque finely granular condition, now con- sists of a more fluid internal part containing fewer granules, and in which the germinal vesicle floats, and of a more consistent ex- ternal part. In the latter a manifest change has occurred in this respect, that towards the outer surface there is separated a much clearer ring of substance contrasting strongly with the more opaque part. This may with correctness, it is true, be described as a mem- brane, as H. Meckel has done in comparing it with and giving it the designation of the zona pellucida. But the very remarkable structure, which the author now mentioned has had the merit of first pointing out, is one deserving of the greatest attention. It has appeared to me to be gradually formed in ovula of 78 OVUM. about ^o or J^ of an inch in diameter, by the clearing and partial consolidation of the outermost part of the albuminous basis or blastema in which the granules of the pri- mitive yolk substance are deposited. It is at first comparatively thin : it is most ap- parent by its greater clearness and consistence in ovules of ^ or -^ of an inch in diameter, in none of which have I ever failed to observe it. In those of ^n it becomes broader, but less clear and somewhat softer in its consistence, and is uneven and as if softening away or breaking up on the external edge ; and in ovules of iV f an mcn i fc nas ln general dis- appeared. At no period have I observed it to assume the glassy transparency, nor has it the distinct outline and membranous appear- ance represented by H. Meckel; but it seemed rather like a portion of the albuminous basis of the yolk substance, nearly but not quite deprived of the granules, which are thickly deposited in the rest. While these differences are stated, how- ever, it appears to me warrantable to coincide in so far with the view of H. Meckel as to regard this structure as a temporary repre- sentation in the fowl's egg of the zona pel- lucida, which in the mammiferous ovum assumes greater consistence, passes into the membranous form, and constitutes the only ovarium covering of the ovule. In ovules of ^ to ^V f an mc h m dia- meter, the layer of nucleated cells placed be- tween the primitive ovule and the membrane of the follicle, and which may be looked upon either as a cellular lining of the follicle or a peculiar investment of the ovule, has be- come more distinct and consistent, and the cells of the outermost layer have assumed the form of short compressed prisms. They have finely granular contents and clear nuclei, with one or sometimes two nucleoli, like the cells of the tunica granulosa of the mammalian follicle. It appears that these cells come afterwards to form the external part of the yolk of the bird's ovum, the cel- lular part of the yolk being formed within them, and the vitelline membrane being pro- duced on their outer surface. The bird's ovule, it is well known, usually fills completely the ovarian follicle ; but in several instances I have observed, from im- bibition of water or some other cause, the ovule to occupy not more than half the dia- meter of the follicle, the remainder being filled with a clear fluid ; and in these in- stances the prismatic layer of cells adhered closely to the surface of the zona and primi- tive granular yolk. In ovules of from J d too light in the figure.) B. Explanatory outline of the same; y,s, yolk segments ; z, zona pellucida ; a, layer of albumen, from which in connection with the zona the chorion takes its origin. perfectly distinct and smooth outline on its ex- ternal surface, becomes in the course of the descent through the Fallopian tubes and by the time of its first arrival in the uterus, not only irregularly flocculent on its surface, but also thickened ; in fact, presents all the appearance of a gramilo-mucous substance having been deposited upon it. It may be proper to explain here, that it has now been fully sho\\n by Bischoff's excellent observations, that in both the animals men- tioned, and also in the guinea-pig, the cells of the tunica granulosa, which adhere to the surface of the zona when it leaves the Graafian follicle.are completely separated from it within the first two or three days of the residence of the ovum within the tube, so as to leave the external surface of the zona perfectly smooth. Bischoff has shown, indeed, as I have also repeatedly observed, that a change has oc- curred in the cells of the proligerous disc, adherent to the ovum while it is still within the ovary, which indicates its approaching maturity. This change consists, as already stated, 'in these cells becoming somewhat spindle-shaped or pyriform, their narrow or pointed ends being attached to and radiating from the surface of the zona. (See as before fg. 55. L>.) It is quite easy, therefore, after this separation takes place, to distinguish any change bv addition of new matter or otherwise which the suiface of the zona may undergo. No one can fail to perceive the [G 3] [80] OVUM. addition to the ovum of the rabbit, the diame- ter of which is thus increased between two and three times, so as to give it somewhat the aspect of the ovum of a Batrachian in minia- ture ; and in the dog it has appeared to me that the increased thickness and more opaque and flocculent roughness of the surface of the zona were sufficient proofs of a new deposit having taken place. This deposit, no doubt, becomes very completely incorporated with the substance of the zona, and is not easily to be distinguished from it ; but in one or two instances I have thought that I was able to perceive a line of demarcation between them. Several of Bischoff's very faithful figures seem to me even to represent this deposit as it has occurred on the ova of the dog. But his statements in his work on the Development of the Guinea- Pig * are so precise against the occurrence of such a deposit, that further ob- servations will be required fully to determine the question whether it is of constant oc- currence or essential to the formation of the chorion. Later observations lead me to think that I may have been in error in supposing that the villi of the chorion grow directly from the al- buminous deposit. These villi, which, as I have said, form a most characteristic feature of the external covering of the mammiferous ovum in the course of development, begin to be formed only when the ovum has reached the cavity of the uterus. The time of the formation of these villi, as well as their size, varies, however, con- siderably in different animals, and probably also to some extent in the same animal. -j- They are developed from the external surface, and their structure is at first nearly homo- geneous, or at least only slightly granular ; they afterwards acquire a cellular structure, and in the course of foetal development be- come at an early period the seat of a compli- cated vascular growth, by which the relations of the maternal parent and offspring are main- tained through utero-gestation. But the fuller description of this part of the growth of the chorion belongs rather to the history of de- velopment after fecundation. My present object has been only to show the relation of this membrane to the zona or ovarian cover- ings of the ovum. The contents of the ovum or parts within the zona consist of the yolk-mass or yolk- substance, and the germinal vesicle. The first of these constitutes a spherical mass of varia- ble consistence, in which granules, or molecules and globules of various sizes, from the most minute up to about ^oo or 4oW> are sus- Fig. 59*. pended in a fluid. The proportion of the gra- nules to the fluid varies to a considerable ex- tent in different animals, the ovum being much more opaque, and usually of a dull-yellow co- lour when the granules are in large quantity, as is the case in most Carnivora, and may be easily seen in the dog or cat. The clear fluid in * Entwickelungsg. des Meerschweinchens, 4to. Giesseu, 1852. f Barry and Bischoff. Ovarian ovum of the Rabbit. (From Coste.) a, ovarian ovum extracted from a nearly ripe Graafian follicle, and freed from the adherent granular cells; the close set granules of the yolk substance, among which the germinal vesicle is perceived with a slightly oval macula or nucleus, are well represented. b, the same burst by pressure ; the yolk granules adhering together by a viscid clearer fluid sub- stance are seen escaping from the large aperture in the zoua along with the germinal vesicle. which the granules are suspended varies also in its consistence, being of a marked viscid qua- lity in some instances, and thin and limpid in others; so that in some animals, when the zona is punctured, the yolk-substance flows freely out, while in others, and this is the case in the human ovum, the yolk-substance holds together as one consistent mass. The yolk- substance does not adhere in the slightest to the interior of the the zona, but on the con- trary is readily detached from it ; and in some instances, in the entire unimpregnated ovum, a space is seen between the yolk-substance and the zona, formed apparently by the imbibition of water between them. This separation be- tween the yolk-substance and zona appears to be, at a later period, a constant and pro- bably important change in connection with fecundation and development. The granules of the yolk-substance are ge- nerally rather more densely set together and OVUM. [87] more firmly united to wards the external surface. This circumstance lias given rise to the belief among some observers in the existence of an additional delicate membrane enclosing the yolk-mass ; but the most attentive observa- tion by Bischoff, Wharton Jones, myself, and others has failed to detect such a membrane ; and there is reason to think that the con- fident belief in its existence has had its origin in part at least in a desire to establish a more complete analogy between the ovum of birds and Mammalia, and to find accordingly a vitelline membrane as well as a chorion present in the ova of the latter. The germinal vesicle is usually about a sixth of the diameter of the whole ovum ; but it is sometimes larger, or between a filth and a fourth. It possesses a delicate membranous wall of a spherical or spheroidal form and ho- mogeneous structure : it is barely possible to observe the double line of the thickness of this wall with the quarter of an inch lens in the microscope. In most animals the ger- minal vesicle is readily distinguishable from the rest of the ovum by its superior clear- ness, excepting in those instances in which it is hidden by the great opacity of the yolk- substance; but then it may generally be made manifest by flattening the ovum by compres- sion between plates of glass. The fluid which fills its cavity, which is generally very clear, contains some minute granules in suspension; and besides these there is apparent within it the macula germinativa or germinal nucleus. This last, which is in general well defined in the mammiferous ovum, varies slightly in different animals : in some presenting the appearance of a round globule, with a deli- cate circumscribing line almost amounting to a vesicular covering ; but more frequently it consists only of a small spherical or dis- coid mass of fine granules. In a germinal vesicle of ^1^" in diameter, such as that of the rabbit, the diameter of the macula is about one-fourth of that of the vesicle, or ^Vo"- In the earlier stages of formation of the ovum the germinal vesicle is of smaller size ; but it is then proportionally larger than the other parts. It is the part of the ovum first formed ; the yolk-substance, which is subse- quently deposited in gradually increasing quantity round it, together with the zona, grow at a more rapid rate than the vesicle, and thus the latter remains in the mature state proportionally smaller. As the yolk- substance is at first deposited nearly in equal quantity on every side of the vesicle, it for a time contains the vesicle in its cen- tre ; but as the formation of the ovum pro- ceeds the vesicle is found in general to have approached the surface at one side of the yolk-substance ; and in the mature ovum the vesicle seems to be imbedded in the most coin- mined by observation in the Mammalia, nor has any one as yet succeeded in observing a canal or pore leading from the surface of die yolk-substance towards the germinal vesicle in the mammiferous ovum. Fig. 60*. Ovum of the Rabbit from the Fallopian tube with spermatozoa. The accompanying figure is introduced to show the usual position of the spermatozoa in relation to the zona and albuminous layer in the ovum of Mammalia daring and after impregnation. This ovum is magnified '250 diameters. It was taken along with five others from the lower part of the Fallopian tube C8 or 70 hours after impregnation. The segmentation appears to have proceeded to the fifth stage. There is a thick covering of albumen over the zona, and a number of spermatozoa are represented involved in the albuminous substance; some were also seen on the surface of the zona, and some, varying in number in the different ova observed from five to seven or nine, were clearly ascertained to be situated within the zona on the surface of and in the grooves between the yolk segments. The position of these la*t is not s'uffi- ciently clearly represented in the figure. In the situation now described the germinal vesicle, though not by any means firmly fixed, is yet sufficiently embraced by the yolk- substance to prevent it from changing place when the ovum is moved in different direc- tions. In the instances of the more fluid condition of the yolk it flows freely out from within the zona when this has been broken ; but in those ova in which the yolk-substance is more viscid, as in the human ovum, we ge- nerally fail to isolate the vesicle from the rest of the substance. The macula or nucleus appears to be at- pact and superficial layer of fine granules of tached to the inner surface of the membrane the yolk-substance. This place no doubt cor- responds in Mammalia, as has been ascer- tained in Eatrachia, to the point from which after fecundation the first cleavage of the yolk proceeds; but this fact has not yet been deter- of the germinal vesicle. This is especially seen to be the case in the Pig, in which the macula seems to be somewhat pyriform or pediculated (see Jig. 61*). No important changes have been observed [G 4] [88] OVUM. Fig. 61 *. Ovarian ovum of the Sow. This ovum is represented in order to show the peculiar pyriform shape of the macula in the ger- minal vesicle, to the wall of which the macula appears to be attached. This ovum was taken from a Graafian follicle of -,L" in diameter. The following are the dimensions of its several parts which are magnified 250 diameters in the figure. The ovum across the exterior of the zona ' " f the vitellus =J g "; the germinal vesicle ^"5 the ma- cula irreo ' > thit;kn ess of the zona ^'. The ovum is surrounded by the thick layer of cells which form the granular disc, dp. A few cells of the thinner membrana granulosa are represented at tg. to occur in the germinal vesicle during the ovarian existence of the ovum. It usually disappears in Mammalia previous to the es- cape of the ovum from the Graafian follicle ; but in this class of animals the phenomena attending the disappearance have not yet been fully investigated. There are some grounds for believing that immediately pre- vious to the bursting of the vesicle there may be changes of the macula and other contents of the vesicle of a corresponding nature with those which have been more clearly observed in Batrachia and Fishes. Dr. Martin Barry seems to have observed something of this kind, and M. Coste has figured, but not so far as I am aware described, the development of cells in the germinal vesicle of a ripe ovarian human ovum.* R. Wagner states, that occasionally a double macula may be seen in the germinal vesicle.f I have on one occasion observed two germinal vesicles within the same ovum in the dog. Bischoff has on three occasions observed two ovules in the same Graafian follicle of the rabbit. if This had been previously noticed by Von B.ier in the dog and pig. And Bidder detected two ovules embedded in the same granular membrane of one Graafian vesicle of the cow. Upon the question how far these varieties in the structure of the ovum may be supposed to be related to the origin of Double- monsters and Twins, I must refer to Pro- fessor Vrohk's interesting article Teratology.* For the assistance of those who may wish to engage in researches of the same nature as those by which the above facts have been as- certained, I will state shortly the manner in which the ova of Mammalia may be procured either from the ovary or after they have left that organ. 1st. For the examination of the earlier ovarian ova and follicles, thin sec- tions of the ovarian substance are to be made, especially towards the surface of the ovary; and some of these are to be teased out with needle points, and examined with the aid of compression, &c. 2nd. For the more mature ovarian ovum, &c., the outer covering of the ovary is to be removed from the surface of one or more of the pro- minent follicles; and the latter may then, if large, be carefully dissected out of the ovary, and laid on a glass plate, where it is to be opened with a sharp-pointed knife, and its contents are to be gently pressed out on the glass. The ovum may in general be easily detected in a part of the tunica granu- losa with a low magnifying lens, or even sometimes with the naked eye. In the ovary of the dog the ovum may sometimes be seen without any dissection towards the most prominent part of the surface of the ma- ture follicles, t 3rd. To procure the ova after they have left the ovary, or while they are in the tubes, two methods may be pur- sued : either the whole length of the tube may be opened with very finely-pointed and sharp scissors, and the surface then spread out ami examined carefully with a low magnifying power under a good domination, but this must not be done under water ; or another plan may be followed according to the re- commendation of Martin Barry, founded on a suggestion thrown out by Cruickshank, as follows : The Fallopian tubes being divided into several portions, the contents of each portion are to be separately pressed out by passing a blunt instrument firmly along the outside of the tube, and, being placed on suit- able plates of glass, are to be subjected to the necessary examination. The latter method is particularly convenient in small animals : in the larger I have followed both plans. The method of Barry certainly saves much time and trouble, and is on the whole sure enough.^ 4th. Of the plan for obtaining the ova from the uterus, when of considerable size, as it belongs rather to the history of develop- ment, I will only say here that the greatest caution is necessary in cutting through the walls of the uterus in different layers so as to ,,,,,..,. * See the plate in his great work marked " Mam- miferes; Homme." PI. 1 Fig. 6. p. 973. Cyclopjed. of Anat. and * Vol. Physiol. t See Von Baer's Commentary on his Epistola, t See 1 rodromus Hist. Generations, Fig. xxxi. in Breschet's Repertoire, 1828 p 38 I 5J!i ' S A rC , hiV ' ^i'f ber l dlt ' P> 1G9> * See Bar) T' s Second Series of Researches, &c., Mttller s Archiv. 1812, p. 8(5. Phil. Trans. 1839, p. 3GG. OVUM. [89] avoid injuring the ova, and that the examina- tion must be made at first in the dry state, Origin and Formation of the Mammiferous Ovum. This subject has already been ad- verted to in the previous section in connection with the history of the formation of the bird's egg. Dr. Martin Barry was led, by his numer- ous observations, to form the conclusion, that the germinal vesicle is the first part which makes its appearance in the ovarian stroma at the commencement of the formation of the ova. All observers seem now to be agreed, that of the parts belonging strictly to the ovum itself, the germinal vesicle is the first formed ; but the observations of Valentin, BischofFand others appear rather to support the view, which is opposed to that of Barry, that the Graafian follicles may be detected in the ovarian stroma before any part of the ovule is distinguishable. The ovules are formed at n comparatively early period in the ovary. Carus was the first to point out * that in the ovary of the human female child the follicles con- taining distinct ovules are perceptible at birth. Vallisnieri had long previously, it appears, made a similar observation. Bis- choff has, with more precision, pointed out, that there is considerable variation in dif- ferent children of the same age as to the degree of advancement of the germs of ova within the ovarium ; in some nothing more than a perfectly uniform ovarian stroma is perceptible at birth, while in others the follicles are distinctly formed, even at an earlier period. By the age of ten or eleven years a number of the vesicles are found to be approaching ma- turity, and almost all have left their earliest condition. Both Barry and Bischoflf, how- ever, are of opinion that new sets of Graafian follicles and ova may continue to arise within the ovaries during the whole child-bearing period of the human female; and there can be little doubt that this takes place in most of the lower animals. Bischott' describes the Graafian follicles as taking their origin from minute heaps of granules in the ovarian stroma; but he has not been able to confirm the statement of Valentin that the earliest follicles proceed from primitive gland tubes stretching from the attached border towards the surface of the ovary, f In various animals the follicles and ova begin to be formed at an earlier period than in the human female : according to Bischoff, they arise very early both in the cow and pig. When the primary follicle can be perceived, it consists of a small vesicle scarcely more than 7oW m diameter. To this primary vesicle Martin Barry has given the appropriate name of Ovisac. Soon afterwards, when the vesicle has expanded somewhat, it is found to con- tain the rudiment of the ovum ; first in the shape of the very small germinal vesicle, gene- * Muller's Archiv. for 1832, p. 379. t Handbuch der Entwiekelungsgeschichte, 1835, p. 389. ; and Muller's Archiv for 1838, p. 529. Fig. 62 *. Development of the Ovarian ovum of Mammalia. (From Bischoff.) A represents a very small portion of the ovary of a foetal dog. The commencing Graafian follicles are visible in the granular or cellular stroma of the ovary, constituting dark heaps of more opaque granules or small cells. B, fragment of the ovary of a dog three weeks after birth. The Graafian follicles are now seen in the fibro-granular ovarian stroma, each surrounded by a homogeneous and fibrous covering, and filled with granules. c, fragment of the ovary of a pig three weeks old. The Graafian follicles are now seen to be formed of a fine transparent vesicular membrane, and round the larger ones fibres are beginning to be deposited. The wall of the follicles are lined internally wiUi delicate epithelial cells. The ger- minal vesicles now visible within consist of a fine clear cell with a nucleus or dot, and a few vitelline granules have begun to be deposited round the germinal vesicles. g, one of these Graafian follicles burst with a needle, showing the contents of the follicle ; there being as yet no zona or vitelliue membrane. rally surrounded by a small quantity of granu- lar fluid. Soon afterwards the outer follicle is lined with a few extremely delicate or hya- line hemispherical cells, which have somewhat the appearance of those of epithelium, and which thus give rise to a clear space between [90] OVUM. the membrane or wall of the follicle, and all that yet exists of the ovule. Next, the yolk substance is formed round the germinal vesicle ; first of all, as has been shown by Leuckartf, by the deposit of a clear viscid fluid, and next by the formation of dark or opaque small granules in this fluid adjacent to the germinal vesicle. Somewhat later the 2ona pellucida, Fig. 63 *. Ovarian follicle and ovum of the Rabbit at an early stage. The follicle here represented was about .ji^- in diameter: in the figure it is shown as it appears under slight pressure. All the parts of the ovum are distinct, and its large size in proportion to the follicle and tunica granulosa is apparent. In the lower of the two figures the follicle is represented as having been burst by pressure and the ovum with the tunica granulosa in the act of escaping from within: the yielding character and elasticity of the zona is shown by the change of form during the escape, and the ovum afterwards regaining its spherical shape, o, the wall of the follicle ; t vi- :\',:.'.';.-'.'.'.'.'!-'.:\^:.'~.'> :';'.'-;'':?'' ''".: ! . .'.-.'-, '-> ' : . .';:',"'''. '--" r'\ - ' ; ..'.'''' '..'" -. ; ' --'.:"..';'.'> '- '.':': ':-f^'-i-' : '':''''..''."'' ':' ':' ' '.. :':^.[- ''':. " . -', p% !.< Micropyle of the ovum in Osseous Fiskes. A. Enlarged view of a quadrangular portion of the surface of the mature ovarian egg of the Stickleback containing the micropyle from above. In the outer part of this figure the'general dotted appearance of the membrane is seen, and here and there the pedicu- luted flask-like processes attached to the membrane in this fish in the vicinity of the micropyle ; the radiated shading represents the appearance of the funnel-shaped depression leading to the aperture of the micropyle, which is seen in the centre of the space it encloses. B. Transverse section of the dotted membrane and funnel of the micropyle of the same egg some- what more enlarged, seen in profile ; the aperture of the micropyle is seen towards the point of the funnel. This view is semidiagrammatic, and the line canals passing through the membrane are re- presented fewer and wider than they are in nature. The diameter of the whole ovum was about /' ; the thickness of the external membrane T J gg " ; the width of the base of the funnel about T i g " ; the depth of the funnel 5 lg" ; the diameter of the micro- pyle aperture at the apex 3^5". c. Small portion of the membrane at the apex of the funnel containing the aperture of the micropyle pressed flat, magnified 500 diameters; from the trout's egg. D. A similar portion of the membrane magnified 1000 diameters. The lumen of the canals is seen, and an indication of hexagonal division of the spaces between them, represented somewhat too distinctly in the figure. the existence of the micropyle in the ova o* several fishes ; and though I have not yet been so fortunate as to perceive the sperma- tozoa actually passing into the ovum through this aperture, the accuracy of Ransom's obser- vations on this as well as on other points leave little doubt as to the fact stated by him. The micropyle in the Gasterosteus, as de- scribed by Ransom and observed by myself, is a considerable funnel-shaped depression in the outer membrane, which projects inwards on the granular substance of the yolk, so as to indent this layer to some depth, and pro- bably to reach near to the germinal vesicle, which lies imbedded within the germinal layer. The inner narrow end of the funnel terminates in a distinct rounded or elliptical mark, with a fine but distinct line bounding it, which has all the appearance of a foramen, and which is either an open passage or one which is closed only by an extremely delicate structure. The funnel-shaped depression leading to the micropyle may be easily seen on the surface of the egg of the salmon or trout when slightly dried of the adhering moisture, and is of such a size that it may be perceived with the naked eye or with a lens of low magnifying power. In order to perceive the micropyle itself, how- ever, or pore 'in the point of the funnel, it is necessary to remove from the egg that portion of the dotted shell membrane containing the funnel ; and having freed it from the adherent granules of the yolk-substance by careful wash- ing, for which Ransom has recommended a solution of acetate of potash, this part of it may be viewed under pressure with great ease with a magnifying power of 200 or 300 diame- ters. The porous structure of the membrane is then seen to continue very nearly up to the margin of the micropyle. This last has a diameter of from -soW' to Woo"- The ap- pearance of a double outline surrounding the micropyle proceeds from the circumstance that, in looking through the funnel we see at once two portions of the narrowing wall of the passage of different widths. In Ransom's experiments, very soon after spermatic fluid was placed in the water round the ovum of the Stickleback, several of the spermatozoa were perceived to pass in at the micropyle ; and immediately upon this water was imbibed, and the space named the respira- tory chamber was formed between the yolk surface and the external membrane ; a change which in this fish did not take place in the unfecundated ova, but which in some others occurs without impregnation. It is from this fact apparently that Ransom is inclined to the opinion that the micropyle may be closed by a very delicate membrane, which in fecundation is removed or broken through by the entrance of the spermatozoa; but with regard to this point there is still some uncer- tainty. The germinal vesicle previous to its disappearance is imbedded below the super- ficial layer of yolk-substance in a stratum of granular matter ; and Ransom conceives that at the time of the rupture of the vesicle, this srraiuilar matter being mingled with the con- til 3] [102]. OVUM. tents of the vesicle, the more immediately germinal part of the egg is formed from the mixture of the two. However this may be, it seems not improbable, from the observations now referred to, that the spermatozoa are conveyed directly to the germinal part of the egg by the funnel of the micropyle. I shall afterwards have to state the more numerous instances in which, following its first discovery by J. Mtiller in the Holo- thuria, a micropyle has been detected in the ova of Invertebrate animals ; and I may at- tempt to show the great importance of this aperture in connection with fecundation in ova with thick external coverings to which the spermatic substance does not gain access till the later periods of their formation. The ac- companying figures of the micropyle in the Stickleback will give a sufficiently clear view of this remarkable structure. At present it may be permitted to remark that, if we consider the size of this aperture, and the ease with which it may be found in the ova of fishes by an observer whose at- tention has been called to its existence, to- gether with the fact of its having been so long overlooked previously, there is much ground for caution as to negative statements as to the existence of a similar aperture in the ova of other animals. I have already made al- lusion to this subject in the previous sections, in which I have stated that Dr. Ransom has expressed to me his firm conviction, founded on observations, that the micropyle exists also in the ova of Batrachia. At the same time it is quite probable that such an aperture may only exist or be required for the admission of the spermatozoa when fecundation is of late oc- currence, and when the covering membrane of the ovum is so dense as to resist the pene- tration of the spermatozoa through its solid substance. It is right also to mention that the exist- ence of this aperture, or rather the funnel lead- ing to it, did not entirely escape the observa- tion of preceding physiologists. The accurate Von Baer, in his work on the development of Fishes*, has described in the Bream (Cy- prinus blicca) a funnel-shaped depression of the external membrane, which reached nearly to the surface of the germ ; and he ob- served that this funnel was effaced as soon as the imbibition of water took place. He considered this aperture as most probably owing to the escape of the germinal vesicle from the surface of the yolk and through the coverings of the ovum, in the same manner as he had described in the frogf, and did not therefore conceive it to serve any immediate purpose in connection with the introduction of the spermatozoa. Dr. Ransom has ob- served that the effacement of the funnel which he had seen in the Stickleback is not inva- riably the consequence of fecundation in the Fish's ovum; for in the salmon and trout * Entwickehingsgeschichte der Fische, Leipzig, 1835, p. 9. figs. 1. and 2. t i>e Ovi Mammal. &c., pi. xxv. Fig. 69*. Development of the ova of Gasterosteus. A. B. c. D. Four ova of the Stickleback in the earlier stages of their development within their ovisacs. In that figured at A, which is the earliest, ^" in diam., the germinal vesicle placed near the cen- tre has scarcely any perceptible membrane or wall, but resembles a gelatinous mass in which the small number of macula; are developed : there is as yet no yolk, but only a slightly turbid fluid substance filling the space between the ovisac and the ger- minal vesicle : delicate epithelial cells project from the inner surface of the ovisac. In B. ^ig" the macula? have increased in number, the germinal vesicle, as well as all the other parts, has increased in size, the fine granules of the yolk sub- stance have begun to be deposited towards the periphery, but there is as yet no vitelline mem- brane. The wall of the ovisac is now more distinct, and besides the internal cells, there are seen on the exterior the nuclei of external flattened cells. In c. ylg" the maculae have become more numerous and distinct ; the yolk granules are more opaque and in greater quantity, and the mass of the j-olk more circumscribed, a clear space now intervening between it and the wall of the ovisac. In D. T ! 5 ", although the number of maculre has greatly increased by endogenous multiplication, the germinal vesicle has not now undergone an enlarge- ment proportional to that of other parts of the egg and ovisac : the granules of the yolk, especially to- wards the surface, are much increased, and a narrow clear marginal space on the surface now indicates the commencement of the formation of a zona or vitelline membrane. This appearance is also slightly perceptible infy. c. The dimensions of the several parts in these dif- ferent specimens were as follows : Ovisac Yolk Germinal vesicle - Macula) A. B. C. I>. 0025 -004 -0056 -007 _ _ -0042 -005 001 -0016 -0025 -0026 00015 -00018 -00025 -0003 he found the funnel-shaped aperture to re- main for some time after the completion of fecundation, and in none of the fishes he has observed does he conceive the aperture of the micropyle to be closed. The ova of osseous fishes appear to take their origin within the rudimentary follicles or ovisacs of the ovary much in the same manner as those of the Batrachia. The ear- liest part of the ovum that can be distinctly seen within the follicle is a vesicle of about half the diameter of the primitive follicle it- self. A little later this vesicle is seen to be surrounded with a clear, jelly-like substance, in which some small dark granules are depo- sited chiefly towards the surface of the vesi- cle. There is as yet no enclosing membrane, but the follicle is seen to be lined by a layer of extremely delicate hyaline cells, often dif- ficultly perceptible. The earliest recognisable part of the ovum, therefore, is the germinal vesicle ; which, as in other animals, has soon deposited round it the clear gelatinous base- ment-substance of the yolk, in which the opaque yolk granules soon make their appear- ance. There is not at first any vitelline or other membrane enclosing the primitive parts of the egg, and indeed it is a considerable time before any such membranes are formed. The deposit of vitelline granules increases ra- pidly, so as to give the yolk considerable opa- city ; afterwards larger globules appear, and seem to increase by endogenous multiplica- tion. * The oil globules are at first small, and equally diffused through the whole yolk ; it is only in the later stages of for- mation that they unite into fewer and larger globules.-j- The granular or primitive yolk- substance continues to surround more imme- diately the germinal vesicle till the period when this vesicle is ruptured, and is probably spread over the germinal disc of the egg. Si- milar granules also occupy, however, in a layer the surface of this part of the egg pre- vious to the rupture of the germinal vesicle ; so that it is not probable that the germinal disc owes its origin, as Coste states J, entirely to the effusion of the contents of the germinal vesicle. * Lereboullet, loc. cit. t Retzius, loc. cit. j Hist. ge'n. et part, du De'velopp. des Corps organ, torn. i. OVUM. [103] The ovum receives its firm porous mem- brane while within the ovarian capsule, but only in the latter part of the time of its forma- tion. This membrane lies very close to the inside of the ovisac, is at first comparatively thin and destitute of apparent structure, and gradually increases in thickness towards the time of its approach to maturity. At the same time a remarkably thin pellicle may be distinguished close to the surface of the granular yolk-substance, scarcely meriting the name of membrane. As already remarked, it is difficult to determine what is the true homo- logical signification of these membranes. The inner one might by some be regarded as a re- presentative of the zona pellucida, or a conso- lidated pellicle on the surface of the yolk, though it must be admitted that Ransom's ob- servation, that it follows the segmentation, is opposed to this view, and makes it more probable that it is only a part of the yolk itself. The origin of the external porous membrane I am inclined to connect rather with the interior of the ovarian follicle ; but whether by exudation from it, or by amalga- mation of the innermost layer of epithelial cells of the follicle, I have not yet been able to determine. I am inclined to regard the latter as most probable, and that this is the true vitelline membrane. The manner in which the micropyle takes its origin has not yet been ascertained. It will afterwards be shown, that in a consider- able proportion of those invertebrate animals in which this aperture in the egg coverings is found, it has existed from a very early period, and proceeds from the remains of the pedicle by which the ovum is originally con- nected with the ovarian substance. Such a pediculated connection has certainly not yet been observed by most of those who have in- vestigated the ovarian ovum of fishes.* Rathke, indeed, observed the appearance of the remains of a pedicle in the detached ova of the Blennius viviparus-|- ; according to Ransom the micropyle in the Pike is not a depression, but projects from the surface like a trumpet-shaped process ; and in the earliest stage of development of the ovarian ovum of Trigla hirundo, according to Ley- dig J, the shape is somewhat pyriform or pediculated, in the same manner as in some of the invertebrate animals. On the other hand, Ransom expressly states that he has never been able to observe the slightest connection in Gasterosteus be- tween the pedicle of the ovum by which it is attached to the ovary, and the mi- cropyle. This aperture he says is always situated at that side of the ovum towards which the germinal vesicle and the germinal disc are placed ; but these parts have no regular connection with the pedicle. The pe- * The pedicle here spoken of is not that of the ovarian capsule containing the ovum, but of the ovum itself within the capsule. f Abhandlung. zur Entwick. part. ii. p. 4. I Miiller's Archiv. for 1854, p. 376. fig. 6. [H4J 1 1 01] OVUM. dicle, he affirms consists only of the ovarian structure, and of no part of the membranes of the ovum. From his observations on Gaster- osteus, in which the projecting bodies from the porous or outer membrane in the vicinity of the micropyle enable this part to be easily recognised, he feels confident that if any pediculated connection had existed it could hardly have escaped notice. When the ovarian ovum has attained ma- turity it falls into the cavity of the ovary, or that which may be regarded as ovary and oviduct united, by the rupture of the ovarian capsule in which it is contained. The walls of the ovi-capsules have by this time become extremely thin ; but according to Von Baer a small stigma or non-vascular mark may be dis- tinguished where the rupture takes place. After the ova have fallen into the common cavity they are surrounded by a considerable amount of secreted albuminous matter, by which in some fishes the ova are covered when excluded. In some this albuminous se- cretion serves to unite the spawn in chains or networks. In other fishes the ova are covered externally with villous projections ; but the manner in which these are formed has not yet, so far as I am aware, been observed. One of the most remarkable, but as yet quite unexplained, varieties in the external coverings of the ovum in one of the osseous fishes, is that discovered and recently de- scribed by Ernst Hackel, as occurring in the family of Scomberesoces.* This consists in the formation, in the space between the sur- face of the 3'olk and the vitelline membrane (that is, the porous membrane), of a layer of long and very distinct fibres, which are wound somewhat spirally, but irregularly, over the surface of the yolk. Hackel has traced the gradual formation of these in fresh specimens of Belone from points on the surface of the yolk-substance; and in other genera he has observed several varieties in the forms of the fibres. They are on an average about a^Vo" thick, and long enough to surround the egg several times; and they appear to resemble the fibres of the elastic yellow tissue more than any other animal substance, but do not entirely agree with them. In the meantime we must suspend our judgment as to this very extraordi- nary addition to the surface of the ovum until farther observations shall have been made as to their distribution in various fishes or other animals, and as to their relation to the deve- lopment of the embryo. -J- * MUller's Archiv. 1855, p. 23. See plates IV. and V. t Some time after the above was in the hands of the printer, I received the first and second parts of the seventh volume of the Zeitsch. fur Wissen. Zool., containing a notice of the discovery of the micropyle in the Salmo salar, and S. fario, by Professor Bruch of Basle. The observations leading to this discovery were made in the winter of 1854-5; and it is right to state here, that Dr. Ransom's dis- covery of the micropyle in the gasterosteus, which was communicated to the Royal Society on the 23rd of November, 1854, was made in the months of June and July previous; and these observations had been Invertebrate Animals. The ova of Inverte brata may be considered under two princi- pal divisions, according as they present more of the large-celled or of the finely granular yolk-substance. The ova of the first kind are usually of a larger size ; they possess a larger germinal vesicle, and often a divided or multiple macula ; and the process of seg- mentation in them is either partial, that is, limited to one part of the surface of the yolk, or it occurs in a different manner on the upper and lower sides of the ovum. In these there is, in fact, nutritive as well as formative yolk. In the other division of animals the yolk is finally molecular, or is mainly composed of smaller granules, and is chiefly of the formative kind ; segmentation usually involves the whole yolk, or if not so, is very nearly complete : the germinal vesicle is generally clear, and the macula most frequently single, and well marked. It is true that the form and struc- ture of the ova of Invertebrata presents many and considerable varieties, as might indeed be expected among animals of such diversity of organisation as belongs to the great divisions of the Radiata, Articulata, and Mollusca ; but still it is to be observed that as a greater degree of simplicity exists in the form and structure of the primordial elements than in the more developed textures and organs of ani- mals, so also we find that much closer analogies may be traced among these elements in the lowest classes of the animal kingdom. We meet, therefore,with little difficulty, even in the most diverse tribes of the Invertebrate animals in tracing the correspondence of the essential parts of the ovum; and we are enabled also to trace a more close analogy between these and the corresponding parts in the Vertebrata than might have been expected. We are there- fore warranted in applying to them similar designations ; and we have daily increasing reason to trust to observations made on the ovology of the lower animals as the means of extending the knowledge of the reproductive functions in Vertebrata and in Man. Thus the recent discovery of the micropyle aperture in some animals, and the certain and clear ob- servation of the penetration of the sperma- commnnicated to Professor Sharpey and myself in August and September. In the beginning of January, 1855, Dr. Ransom informed me by letter of his having found the micropyle also in the Trout, and a few days later in the Salmon. I then saw the micropyle in the ova of both of these fishes; and I have since examined it minutely in the Stickleback, and have confirmed in every particular Dr. Ransom's statements. The existence of the raicropyle in these Vertebrate animals has thus been established by several independent observations ; and I believe that no one who uses the proper means can fail to detect it in these and other fishes. Professor Bruch's ob- servations were chiefly made on the ova after im- pregnation, which may explain the reason of his having failed to perceive the connection pointed out between this aperture and the depression in the centre of the germ disc. Bruch was like myself unsuccessful in perceiving the entrance of sperma- tozoa by the micropyle. His measurement of the micropyle in the Salmon and Trout does not agree with mine, making it much smaller. tozoa into the ovum in others, suggest novel and more general and extended views of the process of fecundation, and while they add certainty to the more limited observations of the same kind made upon animals higher in the scale, tend to prevent the adoption of partial views in regard to these functions of the animal economy. It is principally among the more highly or- ganised Invertebrata that we meet with that form of ovum in which the nutritive is com- bined inconsiderable quantity with the forma- tive yolk, and in which segmentation is partial, such as the Cephalopoda, Insecta, Arachnida, Myriapoda, Crustacea, and some of the Arti- culate Worms. In by far the greater number of the Mollusca, such as Gasteropoda and Acephala, the ova belong to the smaller kind with more or less complete segmentation, as also in most of the Annelida, as Hirudinea and Lumbricina, the Nematoid, Cestoid and Trematode worms, with the Planariae, the Rotifera, Echinodermata, Bryozoa, Acalephee and Polypina. I now proceed to give a short statement of the principal facts that have been ascertained as to the structure of the ovum in these ani- mals, and to state some details with regard to some of those which are either best known or which present phenomena of the greatest interest. 1st. Large-yolked Ova with partial Cleavage. Cephalopoda. The ova of this class of ani- mals have already been referred to in connec- tion with those of birds, scaly reptiles, and cartilaginous fishes, to which they present in some respects a greater analogy than to those of almost any of the Invertebrata. The con- siderable size of the germinal vesicle with its multiple maculae, the large mass of the coloured yolk (nutritive), composed of conglomerated masses of yolk corpuscles, and the very limited extent of the process of segmentation, which affects only a round disc of the germinal part of the egg, are all characters in which the ova of the Cephalopoda, at least the Sepia and Loligo, which have been fully examined, are ascertained to be similar to those of the large- yolked group. We owe the most of our knowledge of the ova of this class and their development to Kolliker's interesting treatise, published in 1844.* The ova of the Sepia are deposited singly, but are attached in numbers close together by pedicles to the stalks of Algae and other marine productions. Those of Loligo are arranged in small masses, in which a number are enclosed in a general bag or covering of gelatinous matter, which is at- tached along with others of the same kind by means of pedicles. I have found those of Sepiola also thus enclosed in small pyriform capsules. The ovum of Cephalopoda possesses a firm laminated external covering or chorion, which in some is darkened on the surface by the colouring matter or ink, in others is trans- * Entwickelungs-gesch. der Cephalopoclen, 4to. Zurich, 1814. OVUM. [106] parent and" colourless. Immediately within this outer membrane is situated a structureless vitelline membrane, containing the mass of yolk-substance, which is separated from the membrane by a slight interval. It appears to be ascertained that the chorion is formed by superposition on the surface of the ovum dur- ing its descent through the oviduct. In the ovary the ova are contained in slender capsules, attached to the rest of the ovary by narrow pedicles. When ripe the ova escape from the capsules, in some species by an ir- regular laceration, in others by a more regular and defined opening, and, falling into the cavity of the ovary, pass thence into the oviduct, through which they are finally excluded. Fe- cundation is believed to occur soon after the escape of the ova from their ovicapsules or in the earlier part of their descent through the oviduct ; but this process has not, so far as I am aware, been directly observed. The ova of the common Sepia officinalis have an oval form, one end being much nar- rower than the other. It is at this the pointed extremity or narrow pole of the egg that the germinal vesicle is situated, while the egg is in the ovary, close under the vitelline membrane; and it is at this part also that, at a subsequent period, the process of segmentation and the first formation of the embryo take place. The narrow end is therefore the germinal pole. This extremity of the egg is always turned to the opposite side from the pedicle of the cap- sule, which is attached to the middle of the blunt or wider end. One of the most remarkable peculiarities in these ova, is the extraordinary change which the outer part of the yolk and the vitelline membrane undergo during the greater part of the time occupied by the growth of the ovum in the ovary. This change, of which the appearance had been known to some previous observers, was first accurately described and explained by Kolliker. From his observations it appears that at first the ovarian ova are quite smooth on the surface, and that at the time of complete ma- turity of the ovum, or after its escape from the ovary, the vitelline membrane and surface of the yolk are also quite smooth; but that in the intervening time, that is, during the greater part of the period of its growth, the surface of the yolk is indented or marked with peculiar grooves, into which folds of the vitel- line membrane pass so as to line them to the bottom, somewhat after the manner in which the pia mater descends into the sulci of the brain, but without the same convoluted form. This has been represented by Kolliker in the Sepia, and 1 have observed it in this genus, and have con- firmed in every particular that author's state- ments as to this change. It appears that at first these inflections of the yolk and membrane begin as longitudinal folds, extend- ing between the wide and narrow poles of the ovum, and, gradually increasing, become at last so deep as almost to meet each other in the interior of the yolk. Subsequently they are traversed by more numerous depressions. [106] OVUM. which subdivide them; and as these cross folds are formed the longitudinal ones beome gradu- ally shallower. The surface of the egg then presents the reticulated appearance which is shown in fig. 70.* On making a section through such an egg, hardened^ in alcohol or any other suitable reagent, it is easy to per- ceive that the ovicapsule takes no part in the inflections, but that they consist entirely in the grooving of the yolk, and the corresponding bending into the grooves of the vitelline mem- brane. This state is maintained till the ovum is approaching maturity, when the depth of the grooves or folds speedily diminishes; and these come at last to be completely effaced in those ova which have left the ovicapsule. In Loligo, it is stated by Kolliker, there are only the longitudinal folds. No satisfactory opinion has been offered as to the cause of this peculiar structure. Fig. 70 *. Ova of the Sepia. (.From Kolliker.') A. Three ovarian ova of the Sepia in somewhat different stages of advancement attached by their pedicles to the ovary, and represented several times magnified. They all show the reticulated mark- ings on the surface produced by the folding in of the vitelline membrane ; g, the place of the germi- nal vesicle and possibly also of a micropyle at the small pole of the egg, in which segmentation after- wards occurs. B. Direct view of this germinal pole of one of the ova, showing the absence of the folds towards the centre in which the germinal vesicle is situated. c. Cross section of one of the ova, showing at o the unfolded or smooth ovarian capsule, and at v m the folded vitelline membrane. Towards the narrow pole of the ovum, the folds now described become less marked ; and they are entirely absent just at the pole itself, so that the germinal vesicle may be seen in the smooth space which is left between them. At this place I had some expectation to find an aperture of the nature of a micropyle ; and I accordingly sought for it in some specimens of the ovarian ova of Sepia which I had pre- served in alcohol, but without success, per- haps on account of the opacity produced in the membranes by the alcohol, and the adhe- sion of the yolk substance to them. Professor Kolliker has since informed me that he believes the micropyle to exist in these ova, which I think extremely probable. The germinal vesicle, according to this ob- server, remains entire and visible till the ova are mature, as may be seen by the examination of specimens hardened in alcohol. It dis- appears just about the time of the ova leaving the ovarian capsule ; but in several instances he found it still remaining in ova that were already free. It was always gone in those ova which had regained the smoothness of their exterior. The yolk-substance of the mature ova con- sists entirely, excepting immediately at the seat of the germinal disc, of corpuscles some- what similar to the vitelline tablets of the Frog's egg. At an earlier period there are heaps of fine granules of the same size, from which the corpuscles are therefore probably formed. In the earliest stage the vitelline substance is entirely composed of fine molecules the pri- mitive yolk which appear to be formed in the same manner as in other animals. The Cephalopoda furnish a remarkable ex- ample among the Invertebrata of a very limited or partial segmentation. This process, upon the detailed description of which I will not enter here, usually commences in a spot either in or more frequently near to the germinal pole, by the formation of the primitive groove which extends across the disc. The forma- tion of the second groove, which crosses the first, the production of other radiating grooves, the separation of annular sets of segments from the periphery, and the suc- cessive steps of the process which follow, are probably determined by the same circum- stances which have been referred to as re- lated to this phenomenon in the cicatricula of the Bird's egg. Gasteropoda. In the greater number, if not almost all, of the remaining Mollusca, the ova differ greatly from those of the Cephalo- poda, and approach more nearly to those I have classed under the groups possessing the small or middle sized yolk, which is princi- pally or entirely formative, and which under- goes a more or less complete segmentation.* In the Pteropoda and in those of gasteropodous Mollusca, which have the male and fe- male organs in the same individual, there is a remarkable combination of the ovary and testis in a single hermaphrodite organ, usually * The genus Sagitta, among the Pteropoda, is, however, probably an exception to this statement, as in it, according to Darwin, the embr3 r onal part of the yolk is distinct from the rest, or rather covers it like a ring. OVUM. [107J Fig.ll Genital Organs of Phyllirhoe bucephalum, one of the Hermaphrodite Gasteropoda. (From H. Mutter and Gegenbaur.) A. The compound or hermaphrodite organs dissected out and represented several times magnified ; o t, the two productive organs each composed of ovigerous and seminiferous parts ; v d, the common excretory ducts for both kinds of organ ; v s, the seminal vesicle ; u, the uterus ; p, a part of the penis ; c, the common external vent. B. One of the lobes of the common productive organ laid open and more highly magnified. Towards the surface o o, the ova are seen in different stages of development in the ovarian stroma ; in the interior t t, the substance of the testis with spermatic cells and spermatozoa in various degrees of advancement ; some of the filaments being very long ; v d, the common excretory duct for ova and spermatozoa. enclosed in the liver, the nature of which was for along time involved in obscurity, and occa- sioned much doubt and difficulty to naturalists. The explanation of this peculiar structure we owe first to H. Meckel*, and Leuckartf ; and more recently H. Meckel and Gegenbaur have described this organ particularly in one of the heteropodous Mollusca,viz. Phyllirrhoe buce- phalum.J The outer part of this curious organ constitutes the ovary, the inner the testis ; and the products of these respective organs, in leaving the seat of their first formation, pass together into an inner common cavity, and thence downwards in the excretory duct. There is, therefore, a common outlet for both. The ova and spermatozoa most frequently pass out at different times ; but occasionally both these reproductive elements are seen to- gether in the passages. It seems probable therefore that they in general meet for im- pregnation only in the lower part of the pas- sages ; but this apparently is not yet fully de- termined, and the modes of union may be * Miiller's Archiv. for 1844, p. 483., see plates xiv. and xv. f Zur Morphologic und Anatomic der Geschlechts Organ. 1847, p. 128. J Zeitsch. fur VVissen. Zool. vol. v. p. 355. pi. xix. various in different genera or families. At all events, the primitive ova and spermatozoa seem to come into contact with each other previous to the addition of the enveloping membrane. * The ova of the Mollusca are in general of small size. The yolk consists of a viscid al- buminous substance, containing suspended in it minute granules, and a variable quantity of coloured oil globules. The germinal vesicle is proportionally of considerable size ; and the macula is distinct and granular. Leuckart -j- states that he and Nordmann have ascertained that in Lymneus, the first part of the ovum which is formed is the germinal vesicle, that the yolk-substance begins as a clear trans- parent albuminous matter surrounding the germinal vesicle, and, as we have seen in various other animals, the granular yolk matter is gradually deposited in this clearer part of the vitelline substance, occupying at first princi- * The hermaphrodite gland exists in the Ptero- poda, Apneusta, Nudibranchia, Infero-branchia, Tec- tibranchia, and Pulmonata. The Mollusca which have separate sexual organs belong chiefly to the orders Cyclobranchia, Scutibranchia.Tubulibranchia, andCirribranchia, some Heteropocla, Pectinibranchia, and operculate Pulmonata or Cyclostoma. t Article Zeugung, p. 800. [108] OVUM. pally its outer part. The vitelline membrane does not exist at first, but seems to be formed at a later period by the consolidation of an external layer of the primitive yolk substance. The time of the disappearance of the ger- minal vesicle has not been determined in many of these Mollusca. Previous to seg- mentation a phenomenon occurs, which has now been observed in a large number of ani- mals, but which first attracted special atten- tion in the gasteropodotis Mollusca; viz., the separation of one or more clear hyaline liquid globules of considerable size from the surface of the yolk substance, into the space be- tween it and the vitelline membrane. This was first observed by Dumortier*, and de- scribed by Pouchetf, by Van Beneden in the Aplysia {, by Nordmann in Tergipes Ecl- wardsiijji, by C. Vogt in Action ||, and by various others. A precisely similar phenome- non has also been observed in some of the Vertebrata, as in Mammalia by Wharton Jones, Barry, and BischofF, and in Batrachia by Newport. But though this separation of one or more hyaline globules from the yolk- substance at the time of segmentation appears to be a very general accompaniment of that process, it must be confessed that its import, either in connection with fecundation or de- velopment, has not yet been ascertained. Acephala. In Acephalous Mollusca the ova are generally of small size, the yolk-sub- stance principally finely granular, the germinal vesicle clear, with a distinct macula, which last not unfrequently presents the form of a double or elongated biscuit-shaped particle. The vitelline membrane is distinct and possesses considerable strength ; and there is generally a considerable space occupied by clear fluid between it and the surface of the yolk. The most interesting feature of the ova of these Mollusca is the funnel-shaped aperture which most of them possess, leading through the vitelline or external membrane into the space occupied by the yolk. This aperture, styled micropyle by J. Miiller in the Ho- lothuria, the first instance in which it was discovered, in 1850 If, was observed in the ova of Unio and Anodonta by Leuckart ** and Keber. -f-j- The latter author supposed that he had observed the penetration of a spermatozoon into the ovum through this aperture, and has described with great form- ality and minuteness all the phenomena which he conceived were related to that process. Although Keber was correct in asserting the existence of the micropyle in these Mollusks, * Embryol. des Mollusques, in Annal. des Scien. Nat. for 1837, p. 136. f Id. lib. for 1838, vol. x. p. 63. See also Pou- chet's further observations in his work, Theorie positive de 1'Ovulation spontanee, pi. xvi. I Annal. des Scien. Nat. 1841, p. 126. Id. lib. 1846, p. 147. || Sur 1'Embryol. des Mollusques Gasteropodes, id. lib. 1846, p. 33. ^[ Archiv. 1852, p, 19. ** Article Zeugung, p. 801, Ann. 1853. tf De Introitu spermatozoorum in ovula, &c. Kb'nigsberg, 4to. 1853. Fig. 72 *. Ova of Unio in different stages of development. (A. B. c. and D., from Hossling ; E. from Keber.) A. The early stage of the ovum, when the ger- minal vesicle alone is distinguishable lying in a bulging part of the ovarian substance. B. The same somewhat more advanced ; the ovicapsule and vitelline membrane have assumed the pediculated form, and the yolk granules sur- round the germinal vesicle. c. The ovum now enlarged and spherical in form, the yolk granules increased in quantity, and the pedicle narrowed so as to form a short micropyle tube ; s, the small body taken by Keber for a sper- matozoon, existing long previous to the occurrence of fecundation. D. The ovum, &c. at a later stage ; g, the ger- minal vesicle ; v, the yolk ; v', the separated portion of the yolk ; s, as in c. now enlarged. E. A nearly similar stage of the ovum as figured by Keber. Some of the contents of the separated portion of the yolk are escaping through the micro- pyle aperture ; s, Keber's alleged spermatozoon. it appears that the body described by him as spermatozoon cannot have been of that nature, seeing that it has been proved by other observers that the appearance on which Keber's supposition was founded existed long before fecundation, and remained long after the commencement of embryonic formation in the same condition.* The existence of a similar aperture or micro- pyle in several other Acephalous Mollusca has been ascertained by the recent investiga- tions of various authors; but the actual en- trance of the spermatozoa by the aperture, has not, so far as I aware, been satisfactorily observed. There seem, however, to be suffi- cient grounds for believing that in the Ace- phala, as in other animals in which it is found, the micropyle is immediately related to the process of impregnation, by affording a ready access of the spermatozoa to the yolk through the more resistent membranes of the ovum. The accompanying figures from Keber and his critic Hessling give a sufficiently clear view of * Hessling, in Zeitsch. fur Wissensch. Zool., 1854, vol. v. p. 380. ; and LJischoft', Wiederlegung des von Dr. Keber bei den Naiaden, &c. Giessen, 4to. 1854. OVUM. [109] the nature of this structure in the mature ovum of Anodonta. In this family of Mollusca the micropyle forms a small but very apparent funnel-shaped projection from the surface of the outer mem- brane ; and its hollow nature may easily be ascertained by the fact that the fluid and granular yolk-substance may be forced through it from within. The yolk ball is placed ex- centrically within the vitelline membrane, the inner surface of which it touches just at the place where the micropyle is situated. From a variety of observations, it has been Fig. 73 *. Structure and Formation of Ova in Acephala. (.From Lecaze Duthiers.') a. Portion of the ovary with three pediculated ovicapsules and contained ova from Cardium rusti- curn, magnified 400 diameters; the micropyle is afterwards formed at the place where the pe'dicles are detached from the secreting coaca of the ovary. b. Unripe ovum of Spondylus gtederopus magni- fied 170 diameters, showing the remains of the cap- sule at the upper part, and the projection of the vitelline membrane at the same place where the micropyle is situated. c. Kipe ovum of the same burst by pressure, showing the escape of some yolk granules through the micropyle and into the space between the yolk and the outer membrane. Iii'this and the previous figure the double state of the macula is represented. shown that the micropyle of the Acephalous Mollusca owes its origin to the early pediculated attachment of the ovum. This has been fully brought out by the observations of Hesslin'g in Unio and Anodonta, of Leydig in Venus, and of Lecaze Duthiers in Cardium and some other genera.* From these observations it appears that the ova first arise in the ova- rian stroma by the formation of the ger- minal vesicles, as in most other animals, eacli vesicle possessing a distinct single macula. These vesicles come very soon to be surrounded by some of the primitive or finely granu- lar yolk, which gradually increases in quantity. These parts are from a very early period en- closed by a membrane which may be regarded as vitelline, but which is differently disposed from that in any of the animals previously re- ferred to ; for, instead of having a regular and complete spheroidal or vesicular form, this membrane is elongated at one part into a pedicle, so as to give the whole of the early ova a pyriform shape, and so as to attach them to the ovarian substance by the pediculated parts of the vitelline membrane. In Venus de- Fig. 74*. Ovarian ova of Venus decussata. (From Leydig.~) a. A group of five ova in their earliest stage projecting from the ovary in their pediculated capsules : the germinal vesicles with single macula, the vitelline granules, vitelline membrane, and ovicapsule are all distinct. 6. Two ovicapsules within which at a more ad- vanced stage the ova have become detached from their pedicles, the remains of which at the upper ends of the ova form the micropyle. A considerable amount of albumen has been deposited between the ovum and the ovicapsule. * Annal. des Scien. Nat. 1854, ii. p. 155. [no] OVUM. cussata, according to Lej'dig*, the ova are ar- ranged in aggregated pediculated groups, from which it seems probable that they are originally produced in numbers by the multiplication or division of multiple germs, somewhat in the same manner as will afterwards be stated to have been observed by Meissner among the Gordian Nematoid Worms. An albuminous layer is afterwards formed externally, and may be instrumental at last, from its increasing thickness, in separating the ovum from its pe- diculated attachment to the ovary. There seems therefore to be little doubt that in these Mollusca, and in a certain number of other Invertebrate animals in which the micropyle has been observed, this apparatus is produced by the remains of an original or early ovarian pedicle. In the Unio and Anodonta it is certainly not formed by the peculiar process of development from within the ovum, which has been elaborately described by Keber.f It will afterwards be shown, however, as in- deed may be concluded from what has already been stated in regard to osseous fishes, that in other animals the micropyle may arise in other modes and without the early existence of the pedicle now described. When the ova are detached by the rupture of the pedicle in the Acephala, they lie, in different stages of advancement, but all pro- vided with the micropyle, in the general ovarian cavity. The coverings of the Acephalous ovum appear to be composed at least of two layers, of which the inner may per- haps be looked upon as the vitelline mem- brane, the outer as a chorion; but sufficient data have not yet been furnished to determine the homological rank of these membranes. The early connection, in a pediculated form, with the ovarian stroma might point to a different view of their nature. Leydig states that while in Unio and Anodonta the albu- men is deposited within the membranes, in Venus it is added externally. The micropyle appears to be closed previous to the com- mencement of embryonic development. Arthropoda. The ova of Articulate animals might with most propriety be classed with the large-yolked group, at least as regards the ova of Insecta, Arachnida, and the higher Crustacea. In addition to the germinal vesicle and finely granular yolk-substance, they all contain a large proportion of clear or oil glo- bules of considerable size ; and the process of segmentation is generally limited to a small portion of the yolk surface. The ova of these three classes present, however, many subordinate differences in their structure and mode of production, which renders it neces- sary to give a short separate account of them in this place. Insecta. The ova of insects are more especially distinguished by the extraordinary varieties of their external form and appearance. These varieties affect, however, principally, or depend upon modifications of the external * Muller'sArchiv. 1854, p. 320. f Hessling and BischoiF, loc. cit. covering, chorion or shell-membrane, as it has been called. They differ also from those of most other animals in a frequent departure from the regular symmetrical form. Some are nearly hemispherical, others more oval ; many are somewhat bent in an antero-posterior direction* ; many present the most curious elevations and irregularities on their external surface reticulated ridges or fringes, and de- pressions, tubercles, hairs or spines, or other long processes, sometimes single, at other times in numbers. These modifications of the external coverings of the eggs of Insects appear to have reference chiefly to the protection of the ova from the effects of external injury, and to serve various mechani- cal purposes connected with their deposition and attachment ; but they are not, in most at least, attended with any important varieties in the internal structure, which, on the whole, presents considerable uniformity throughout the whole class. The ova of all insects, we shall afterwards see, are provided with one or more apertures corresponding to the micro- pyle. , All recent observers agree that, in the ova of Insects, in addition to the external shell- covering, there is a delicate transparent vitel- line membrane. The germinal vesicle is of pro- portionately large size. Its macula is at first single ; but in the course of the growth of the ovum it becomes multiple, or diffused as a finely granular or molecular substance throughout the vesicle. -j- The germinal vesicle is situated in a vitelline or germinal area composed of fine granules, in which without doubt the limited process of segmentation afterwards takes place ; but fuller observations are still much required in regard to the segmentation of the yolk in insects, which has as yet been very rarely seen. The germinal vesicle appears to be burst and diffused at a comparatively early stage of the growth of the egg. The external membrane consists in general of more than one layer of substance. The outer and inner are described as being gene- rally more clear, dense, and homogeneous; the middle one, in some insects at least, pre- senting greater varieties of structure, and not unfrequently being composed of united nu- cleated cells. It is in these several layers of the outer membrane that the micropyle ap- paratus, recently discovered, is situated. The existence of a micropyle in the ova of Insects was first published by Meissner, in September, 18541 ; but the discovery appears to have been made simultaneously by Leuckart, who has given a most interesting and elaborate description of this apparatus, and of the minute structure of the membranes, in a great variety of insects, in a memoir recently pub- lished by him.$ Meissner described several varieties of the * This has reference to the position they occupy during their formation in the passages of the female parent. f See R. Wagner's Prodromus Hist. Generat. J Zeitsch. fur Wissen. Zool. vol. v., p. 272. Memoir on the Micropyle and Minute Structure, OVUM, Fig. 75*. tained by that author, were very large number of Insects, thousand different kinds, he tecting the existence of the less than two hundred ; and servations on this apparatus, of the membranes, extend to Fig. 7G*. till] extended over a Among nearly a succeeded in de- micropyle in not his detailed ob- and the structure one hundred and Micropyle in the ovum of Insects. (From JHeissner.) a. A portion of the upper pole of the ovum of Musca vomitoria from the Vagina. There are shown in succession the vitelline membrane, chorion and outer envelope, and at the upper part in profile the micropyle aperture situated in the middle of a nipple-like projection of the chorion, and with a number of spermatozoa involved in it. b. Direct view of the upper pole of the ovum of an insect belonging to the Pyralida. The micro- pyle aperture is seen in the centre of the radiated markings of the chorion. micropyle apparatus in the ova of Insects be- longing to the following genera, viz., Musca, Tipula, Culex, Lampyris, Elater, Teleopho- rus, Adela, Pyralida, Tortrix, Euprepia, Li- paris, Pieris, Panorpa, and in more than one species of several of these genera. The same author also observed and described in Musca vomitoria a number of spermatic filaments entangled in the micropyle. Leuckart's observations, which are fuller and more minute than those of Meissner, and differ in some of their results from those ob- &c., of the Ova of Insects, chiefly pupiparous, in Muller's Archiv. Nos. 1. 2. and 3., February and July, 1855, p. 90., et seq., with five plates, with 122 figures. There can be no doubt that both of these authors made the independent discovery of this curious structure. Perhaps the priority claimed by Leuckart, may be accorded to him, as he had pre- viously stated the probability of its existence in his article " Zeugung," published in 1852, p. 906. A ; iH ?*??; V Micropyle of the ovum of Lepidoptera. (From Leuckart,) c A. Side view of the upper part of the ovum o Sphinx Populi, showing the micropyle, the radiated markings surrounding it, and the cellular and other structure of the coverings of the ovum. B. More enlarged and direct view of the vicinity of the micropyle in the same. The dotted or punc- tated structure belonging to the chorion is here re- presented. eighty species. This must furnish ample proof of the universality of the existence of the micropyle in this class of animals, when we consider the minuteness of the object and the difficulty of obtaining specimens in a con- dition suitable for the investigation. Leuckart has stated, indeed, that in all instances in which the ova were ripe and favourable for examina- tion, he was enabled to assure himself of the presence of this apparatus. In a certain number of instances, amounting to about a dozen, Leuckart farther found that the spermatozoa adhere to the micropyle, and that a certain number of them pass into the ovum by this aperture. He observed that a [112] OVUM. Fig. 77*. Oeum and Slicropyle of Dipterous Insects. (From Leuckart.) A. Ovum of Melophagus ovinus (Muscida). 1. The entire ovum, presenting at its upper part the adherent mass of spermatozoa close to the micro- pyle. 2. This upper part more highly magnified, showing a section of the micropyle, above which the point of the conical mass of spermatozoa glued together by an albuminous substance is inserted, while externally the filaments float free. 3. The micropyle apertures seen directly from above. B. Side view of the upper part of the ovum of another insect of the same order, showing a single micropyle aperture and the dotted structure of the chorion. small mass, formed of the spermatozoa which have met with the ovum in its descent through the female passage, comes to be lodged in the depression of the micropyle, and is fixed in that situation by a lid or covering of albu- minous matter. It is somewhat remarkable that the greater part of this mass remains for a long time apparently without any change, even when embryonic development has ad- vanced to a considerable extent ; but he as- certained that a few of the spermatozoa be- longing to the mass, usually not more than three or four, really enter the ovum and effect the change of fecundation. We are, however, as yet at a loss to conjecture what farther purpose may be served by the mass of per- sistent spermatozoa near the micropyle. Leuckart has also made the novel and interesting observation, that the depression and aperture of the micropyle become at a later period converted into a deeper funnel, which is connected directly with the mouth of the embryo, and undoubtedly serves, ac- cording to this author, to convey nourishment from without to the embryo. The head of the embryo lies, according to Leuckart and other observers*, in all instances, at that end or pole of the ovum which is uppermost in the oviduct, as may be most easily observed in ova of the cylindrical form, such as those of the common house-fly ; but according to Leuckart, the micropyle is not, as Meissner had stated, always at that end, being some- times at one, sometimes at the other, and oc- casionally at both poles. The provision for the escape of the embryo, however, is usually at the upper or anterior pole, while the lower or hinder pole more generally serves to fix the ovum, as it is often pediculated or other- wise modified in its form in connection with this purpose. In some Insects, as is shown in the accom- panying figure of the ovum in Pulex irritans, the micropyle consists of a number of foramina nearly of uniform size. Fig. 78*. Ovum of Pulex irritans. (From Leuckart.) A. Entire ovum, magnified, showing the micro- pyle apparatus with a number of foramina at both poles. B. Portion of the chorion with the micropyle foramina, more highly magnified. In a previous part of this article allusion has already been made to the great facility with which the development of the ova of in- sects may be traced, in their successive stages, as they lie in different parts of the tubular ovaries and oviducts. According to the In- teresting observations of R. Wagner -{-, the upper end of the fine ovarian tubes are filled with a number of germinal vesicles. Wag- ner supposed indeed that these were at first nucleoli or germinal maculae, and that a vesicle was developed round each macula ; but Leuck- artj and Steins) were never able to detect the germinal vesicles before they already possessed the macula. The primitive yolk arises as in most other animals first, by the collection of a clear substance immediately round the germinal vesicle, and by the subsequent de- posit in this matrix of the fine granules of the vitelline substance ; later still the deli- cate vitelline membrane is formed, perhaps by the consolidation of a film of the primitive yolk-substance. As the ova attain a larger size, each one being situated in the lower part of its compartment * See Kblliker, de prima Insector. Genesi, 4to. Turici, 1842. f Prodromus, Hist. Gener. p. 9., and Beitriige zur Entwickel., &c. p. 42. See Jig. 40. Append, of CYCLOP. ANAT. AND PHYSIOL. i Zeugung, p. 803. Vergleich. Anat. und Physiol. der Insecten, Berlin, 1847. OVUM. 1113] Fig. 79* Development of the ova of Lepidopterous Insects. {From Hermann Meyer.) b. A small portion of the upper part of the ova- rian tube from the larva of Saturnia Carpini. The entire lines mark the basement membrane of the tube; externally elongated epithelial cells are placed on it ; internally a number of larger and smaller free nuclei are imbedded in an albuminous fluid. a. A similar portion of the ovarian tube from Bombyx Mori more developed. The external epi- thelial cells are visible now only as elongated nuclei; a part of the internal cells now form a lining to the wall of the tube, while others of a larger size, which have become complete cells, towards the centre, form the primitive ova ; of these last only a few undergo farther development. c. One of the loculi or chambers of the oviduct of Hyponomeuta variabilis. The wall of the tube with its external epithelial nuclei as before, enclos- ing now the entire loculus and the small portions of the adjacent ones represented in the figure. The lower half of the loculus is occupied by the deve- Sujjp. loped hemispherical ovum in which the several parts, viz. germinal vesicle with macula, yolk and vitelline membrane, are seen. The lining cells of the oviduct are seen to be elongated and modified in structure preparatory to their forming along with the albumen one of the external coverings of the ovum (chorion). In the middle of the upper half of the loculus there are the remains of five aborted primitive ova. d. Section of the coverings of the ovum of Har- pyia vinula, which may be taken as an example of the hemispherical ovum of Lepidopterous insects. of the oviduct, there are to be seen in the in- tervals between the ova numbers of large clear globules or cells, which have been supposed to furnish the materials for the growth of the ovum ; but it appears more probable that these are merely abortive ova or germinal vesicles, which, though at first similar in size and structure to those which have been farther developed, have undergone a retrograde pro- cess, and are ultimately removed by absorp- tion. The production of the chorion or shell membrane does not take place till the ovum has attained nearly its full size. It appears to proceed in part from the consolidation, over the whole surface, of one or more layers of albuminous fluid secreted from the wall of the oviduct. But the observations of Hermann Meyer * have shown, in an in- teresting manner, that a part of the outer membrane is also derived from a conversion into it of the inner cellular or epithelial lining of the oviduct, at the place where it is in closest contact with the surface of the ovum. Many of the varieties in the appearance and structure of the external covering may pro- bably depend on the different modes of deve- lopment of these cells. As to the origin of the micropyle, it does not appear to proceed, as has been supposed by Meissner, from the mere deficiency of these cells in a certain space; and it is not dependent, either, on its pre-ex- istence in the vitelline membrane. On the contrary, according to Leuckart, it is formed in the chorion before it appears in the vitel- line membrane ; and it is not in any way con- nected with an early pediculated condition of the ovum, which, as is well known, never at any time exists in insects. Before leaving the history of the ovum in this class, it may be proper to make the fol- lowing addition to what was stated in an earlier part of the article in reference to the remarkable modification of the reproductive process, by which, in the Aphides and several other insects, many individuals are produced without the formation of true ova, or the con- currence of the two different sexual products. The learned editor of the American transla- tion of Von Siebold's " Comparative Anatomy of the Invertebrate Animals," Dr. Waldo Burnett, has given, at p. 464*. of that work, a short statement of his own observations on the origin and mode of formation of the re- peated broods or colonies of Aphides, made on a large species of that insect, viz., A. * Zeitsch. fiir Wissen. Zool., vol. i. p. 190. [rj [114] OVUM. Caryse, and of his views as to the nature of this process of non-sexual reproduction in general. The viviparous Aphides, according to Dr. Burnett, are neither male nor female, and do not possess, as has been supposed, any ovaries or oviducts. The new colony already begins to be visible within the body of its parent before the latter has itself been brought forth. The substance in which the new progeny takes its origin consists, at first, either of a single nucleated cell of ^Vo" m diameter, or of a small mass of these cells at- tached in the same place as that occupied by the ovary in the oviparous females. These masses increase in quantity, are subdivided by a kind of notching into more numerous masses ; and each of these being inclosed in a capsule, the whole come to be arranged in a continuous row or series. There is not, however, any germinal vesicle nor segmenta- tion, as in the sexual ova ; and when develop- ment of the new insects is complete, it is by falling into the abdominal cavity, and by es- caping through a genital aperture (poms genitalis) that the offspring is excluded. With regard to the origin of the cellular mass or germ from which the non-sexual progeny proceeds, Dr. Burnett states that a small mass, of a different appearance from the germinal part of the ovum, is seen to be in- cluded within the arches of the embryo ; and the next colony is produced from this mass. He regards this process as analogous rather to one of internal gemmation than of true generation, coinciding therefore more nearly with the views of Leuckart and Carpenter than of Steenstrup and Owen. Arachnida. The ova of nearly all the higher Arachnida do not differ much in their internal structure from those of Insects ; but they do not present the same varieties of ex- ternal form. Their mode of first origin is also very different. All the higher Arachnida are, like Insects, of separate sexes. The Tar- digrada are hermaphrodite; and in these as well as some other simpler Arachnida, as Pycnogonida and Acari, the ovum, though proportionally of large size, is of extremely simple structure, approaching very nearly to that of the lowest classes of Invertebrate animals. The ova of the higher Arachnida are gene- rally spheroidal ; the chorion or external membrane is generally smooth ; the vitelline membrane is slender, clear, and structure- less ; the yolk-substance is not unfrequently coloured, often purplish, consisting of a consi- derable quantity of large oily-looking globules, smaller granules of various sizes, and larger corpuscles which have been looked upon as cells, but which Leuckart states are only ag- gregated masses of granules held together by a viscid substance. The germinal vesicle is proportionally large, placed eccentrically, and possesses a macula, which in some genera is simple and flattened, as in the Scorpion, in others multiple and granular, as in Epeira. The formation of the ova may be observed in Arachnida with great ease, from the manner in which they are disposed in the ovary, pro- jecting like bunches of grapes from the central part of that organ, in almost every stage or degree of advancement. The process has been carefully observed by Wittich * and others. So soon almost as the ovum begins to be formed, it causes a bulging or projec- tion of the membrane from the surface of the ovary ; and when that has somewhat increased in size, the ova hang or project from the sur- face in small pcdiculated ovi-capsules. Ac- cording to Wittich, V. Carus f, and Leuckart, the part of the ovum which earliest makes its appearance within the small ovicapsules is the germinal vesicle. At first it appears quite sim- ple and without a macula, which last soon after- Fig. SO*. Ovarian ova of the Spider. ( From Wittich.') a. Small fragment of the ovary of Epeira diadema from which three ova project in the early stage of their development previous to the formation of the yolk : the germinal vesicles are enclosed in the membrane formed by the bulging out of the ovarian substance. b. Two ova similar!}' situated, but more advanced ; the primitive granular yolk substance intervening between the germinal vesicle and vitelline mem- brane. c. An ovum still more developed ; the germinal vesicle occupies the upper part; in the finely gran- ular yolk substance below is seen the dark body regarded by some as a yolk nucleus, presenting an appearance of concentric lamellar structure ; towards c. in the figure, or close to the connecting pedicle, the large nucleated cells are seen, which usually occupy that situation, and appear to give rise to the cellular yolk substance. d. More advanced ovum greatly increased in bulk, the pedicle diminished, and the yolk com- pletely occupied by the larger cells or corpuscles ; the yolk nucleus has disappeared or is obscured. e. f. Different forms of the yolk nucleus or dark body, which for a variable time is placed within the ovum during its formation. * Die Entstehung des Arachnideneies im Eier- stock, &c., Miiller's Archiv. for 1849, p. 113. f Zcitsch. fur Wissen. Zool., vol. ii., 1850, p. 97. OVUM. [115] wards appears as a small dot or nucleolus. The yolk begins, in the same manner as we have had occasion to state in many other animals, first by the clear deposit of a basement sub- stance round the germinal vesicle, and the subsequent formation of opaque granules in it ; the vitelline membrane is of later for- mation. As the egg increases in size, the larger corpuscles and the fat globules gra- dually appear. The ovarian ova of several spiders contain besides the usual parts another body of a peculiar kind, the nature of which seems still involved in some doubt. This body is eccentrically placed near the yolk mass of the primitive ovum, and is of considerable size, viz. about ^-3", of a yellowish colour, and, during the earlier part of its existence at least, consisting of concentric layers of a hard granular matter. V. Cams* has compared this body to the yolk-nucleus of the Frog's ovum ; and both he and Von Siebold seem disposed to consider it as in some way or other the source of the granular substance of the yolk ; but according to Wittich this view is not well founded, as he has observed the body remain- ing in the ovum till it reaches maturity, though it loses its concentric laminated structure, and becomes clearer and vesicular. Von Siebold, on the contrary, states that it gradually disap- pears. The large clear or oily globules appear, according to Cams, to be produced from near the pedicle of the ovum, at a place where there is fixed a group of cells apparently destined for their formation. No observations have as yet been made, so far as I am aware, on the existence of a mi- cropyle in the ova of Arachnida. Almost all the Arachnida are oviparous. The Scorpions are an exception, however, bearing their young alive ; and it is deserving of notice that in this family the embryo is deve- loped in the ovum while it still remains in the ovary. In the greater number of this class em- bryonic development commences in a blasto- derm, which covers only a part of the surface of the yolk, situated in what may be called its lower part or pole f ; and the segmentation of the yolk is therefore limited or partial, as in Insects. In the higher Arachnida the steps of this process do not appear to have been yet satisfactorily observed. I may refer, however, to the researches of Kaufmann of Lucerne on the development of the Tardigrada, as afford- ing clear and beautiful illustrations of the process of segmentation, which is shown to be complete in the lower Arachnida.ij; Crustacea. All the animals of this class are of distinct sex ; but in the allied Cirrhipedia hermaphroditism most frequently prevails. In some of the Cirrhipedes, however, it has been shown by Mr. Darwin that the sexes are * Loc cit, p. 99. t See the Researches of Herhold De Generat. Aranearum in Ovo, 1824 ; and Rathke, zur Mor- phol. Reisebemerkung. 1837; and in Burdach's Physiologie. J Zeitsch. fur Wissen. Zool., vol. iii., p. 220. See Plate vi., figs. 3. to 11. Monograph of the Sub-class Cirrhipedia, &c., printed by the Ray Society, 1854, p. 27, &c. also distinct, as in some of the species of the genera Ibla, Scalpellum, Alcippe, and Crypto- phialus. In these instances the males are very minute, and are attached, almost like pa- rasites, to certain parts of the more developed females, the place of their attachment varying in different species. It is interesting to ob- serve that these males, as in the case of several of the Epizoa, are often of the most rudimen- tary organisation.* The ova of the greater number of Crustacea, especially the more highly organised genera, belong, like those of most of the Articulata, to the group in which a considerable amount of nutritive yolk is present along with the for- mative part, and in which the process of seg- mentation in the latter is partial. The forma- tive disc is situated on the lower surface of the ovum ; and from that part the development of the embryo emanates. Even among the higher decapodous Crustacea, however, the ova are of very various sizes f; and in the lowest genera, as among the Entomostraca, the ova are proportionally the largest, although they are of the simplest structure, and present the smallest amount of nutritive yolk ; so that, in this as in other classes of animals, magnitude alone is no true criterion of the internal structure of the ovum. The ova of this class have been described principally by RathkeJ, by Erdl, R. Wag- ner ||, LeuckartlTj Leydig**, and others ; but the knowledge both of their structure and their mode of formation is yet far from being sufficiently minute or complete. They pre- sent, indeed, many varieties, which renders it diflicult to give any general description of them. The following may however be stated. The ova of Crustacea are often variously and brilliantly coloured. The yolk-substance con- sists of a large quantity of clear globules of considerable size, having the aspect of oil globules, in which the colouring matter chiefly resides. In some ova these globules attain the size of ^^ f an inch. There is also a more fluid granular matter in the yolk, and in the more mature ova there is a layer or disc of granular corpuscles on one side which after- wards is the seat of segmentation and embry- onic formation. The germinal vesicle is of considerable size, in some instances possess- * It is also a remarkable fact, pointed out by Mr. Darwin in his interesting Researches, that even among the hermaphrodite species there are some- times distinct male individuals attached parasiti- cally to the hermaphrodite animals ; these have been called complementary males. f Thus, for example, the ova of the river craw- fish (Astacus fluviatilis) are twice as large as those of the common lobster. J Entwickel. des Flusskrebses, 1829 ; in Bur- dach's Physiologie, vol. ii. 1837 ; in his Abhandl. zur Bildung vind Entwickel. Gesch. &c., 1833 ; in Dissert, de Animal. Crustac. Generat. 1844 ; and various other treatises. Entwickel. des Hummereies, 1843. || Prodrom. Hist. Generat., 1836. f Article Zeugung. ** On Argulus foliaceus in Zeitsch. fur Wissen. Zool., vol. ii. [i 2] [116] OVUM. ing a single nucleus, in others multiple ma- culae. The formation of the ova may be observed with ease in any of the smaller isopodous Crustacea. According to Leuckartin Oniscus or Armadillo, it is essentially the same as in the Arachnida. The ova consist at first of . germinal vesicles, originating below the epi- thelial lining membrane of the ovarian sac. The yolk-substance first appears as a clear deposit round each germinal vesicle ; minute opaque granules are then formed in this sub- stance, and subsequently the larger albuminous and oil globules gradually make their appear- ance. The vitelline membrane, which is very delicate and structureless, is added at a com- paratively late period, in the Oniscus for ex- ample, when the ova are about T o" in diameter. In many of the Crustacea the ova also acquire a chorion or shell membrane of con- siderable strength. On arriving at the lower part of the female passages, the ova of many genera also receive an addition of a peculiar so-called albuminous secretion, which becomes coagulated in water, and thus, when the eggs are laid, serves to glue them together in heaps or to cause them to adhere to the hinder feet, caudal plates, &c., of the parent, where, as is well known, they remain during the whole Fig. 81*. Ephippial ovum of the Daphtiidce. ( From Baird.) The figure represents a profile view of the female of Moina rectirostris (one of the Daphnidfe) show- ing at a. the ephippial ovum in its usual place on the hack of the animal. progress of embryonic development. In the Monoculi and some other Entomostraca, there are marsupia or pouches appended to the genital orifices of the parent, in which the ova are retained during the formation of the young. In all the bisexual Crustacea the ova are fecundated while still within the body of tlie female parent ; but the phenomena and period of this process have not yet been acurately determined, partly perhaps in consequence of the peculiar form and motionless condition of the spermatic corpuscles belonging to the greater number of this class. No micropyle has yet been observed in the crustacean ovum. From the observations of several naturalists it is now well ascertained that in the ento- mostracous Crustacea, there commonly occurs a production of young individuals without impregnation, somewhat in the same manner as previously described in the Aphides. " In the Daphnia," says Dr. Baird *, "it is now clearly ascertained that a single copulation is sufficient, not only to fecundate the mother for life, but all her female descendants for several successive generations ;" and it was considered probable by Jurine, that in some species this might extend to the fifteenth gene- ration. In the Daphnia and other similar Ento- mostraca, the ova are transferred from the ovary into a cavity situated below the shell on the back of the animal, which has been called uterus, perhaps erroneously, and there undergo development. But at certain seasons many of these Entomostraca produce ova of a different kind from those now referred to. To these the name of winter or hybernating ova has been given, as they appear to be adapted, from the strength and impermeability of their external coverings, to resist the in- jurious effects of cold and other atmospheric influences during the winter season. These ova are generally in smaller number than those of the ordinary kind, frequently two, some- times only one; and they are contained and undergo development in a peculiar case, which Ls formed on the back of the animal below the shell, nearly in the same situation as the matrix for the ordinary ova. This case, which afterwards separates from or is abandoned by the animal, forms a sort of hump or saddle on its back, and has hence been named the ephippium, and the eggs have been called ephippial ova. These ephippial ova, according to Baird, are already fecundated by the original impregnation of the female parent, and do not require, for themselves nor for their progeny for several generations, any renewed or special impregnation. It appears from the observations of Jurine, Strauss, and Baird, that at the time when the ephippial ova are about to be formed, a sudden change takes place in the appearance of the ova, by the deposit of a quantity of dark granu- lar substance. This appears to be transferred * Nat. Hist, of the British Entomostraca, Ray Soc. Public., p. 79. OVUM. into the cavity behind, in which an increased growth of substance round the ova and within the shell gives rise to the production of a two- valved case for containing the ova. Accord- ing to S. Fisher of St. Petersburg *, the for- mation of the ephippial ova may be noticed during the whole season, from the middle of July onwards ; and it may therefore be inferred that these ova have for their object the pre- servation of the species in the heat of summer when the ponds are liable to be dried up, as well as by resisting the cold of winter. Von Sieboldf states that these hybernating ova contain no germinal vesicle ; and Dr. Burnett, in his translation of Von Siebold's work, has adduced various arguments in fa- vour of the view that this is an instance of "internal gemmiparity" (as he regards the corresponding phenomenon in Aphides) rather than the production of true ova. Sufficient data are still wanting, however, to form a de- cided opinion on this subject, as we cannot at present distinguish between the ova of the Entomostraca which are the result of fecun- dation, and those which are formed and de- veloped independently of the concurrence of the male.J Annulata. In the class of Annulate Worms, including the Leeches, Earthworms, Nereids, and Amphitrites$ , although considerable va- rieties present themselves in the modes of reproduction, there is yet a greater degree of uniformity in the structure of the ova than in some of the classes previously referred to. In the greater number the ova are nearly spherical in form, of rather small size ; the yolk- substance is generally finely granular, and segmentation is complete; the germinal vesicle is clear, with a distinct single macula, or one which is elongated or only slightly divided into subordinate particles. In most ova of Annulata there is, in addition to the inner transparent vitelline membrane, a cho- rion or external membrane of considerable strength, and not unfrequeutly a superadded layer of albuminous substance, which unites the ova in groups or cocoons, or serves to attach them to other bodies. || In Clepsine, among the Hirudinea, the yolk- substance differs from the common form above described, being composed rather of larger- sized globules ; and in another genus belong- ing to the same order, Piscicola, according to Leydig If, t.iere are peculiarities oi' structure * Mem. of the St. Petersburg Acad., 1848, torn, vi., p. 162. f Compar. Anat. j See Burnett, loc. cit., p. 353 ; Zencker, iiber die Daplmoidaj in Miiller's Archiv., 1851, p. 112; and Leydig, liber Anemia salina und Branchipus stagnalis, in Zeitsch. fiir VVissen. Zool., vol. iii. 1851, p. 297. Suctoria, terricola, errantia, and tubicola. || For a clear and comprehensive account of the reproduction of the Annelida in general, and with special reference to the genus Hermella. one of the suctorial Annulata, the excellent memoir of Quatre- fages, iu the Annal. des Scien. Nat., 1848, vol. x. p. 153, in which, in addition to his own researches, are duly recorded those of previous observers. ^f Zeitsch. fur Wissen. Zool., vol. i. p. 123. which have not as yet been referred to any general law. In the ova of these animals the covering is double, consisting of a delicate in- ternal vitelline membrane, and an external envelope or chorion, to which a layer of dis- tinct flattened and nucleated cells is adherent; and within the vitelline membrane there is a collection of nucleated cells which displace and partially surround the usual finely granu- lar or formative yolk-substance. Leuckart* informs us that the same peculiarity exists in Pontobdella; out the nature and destination of this inner cellular part of the ovum does not appear as yet to be understood in either of the animals mentioned. In the Piscicola, Leydig observed the ovum, while within the ovarian cavity, to be com- pletely surrounded for a time and enclosed by a consistent mass or covering of spermato- zoa ; and it has been observed that in this animal the germinal vesicle has not in general disappeared till some time after the ovum has thus encountered and been enveloped by the mass of spermatic substance. In the Lumbricus, Meissner-j- has made the novel and interesting observation, that pre- vious to the encounter of the spermatozoa with the ovum, the latter loses the vitelline membrane which before covered it, and that the spermatozoa then penetrate, in great numbers, the whole surface of the exposed yolk. Fig. 82*. Ova of the Lumbricus during fecundation. (From Meissner.) The figures represent three views of the ova of Lumbricus agricola, a. Sf b. on their flat sides, c. seen edgeways. Over the surface spermatozoa are seen penetrating the vitelline substance, giving to it on a large scale the appearance of a ciliated surface. The ovum which has now reached the receptaculum seminis is without vitelline membrane, the yolk being thus directly exposed to the action of the spermatic masses ; but the vitelline membrane ex- isted at an earlier period and disappeared by solu- tion in the course of the descent of the ovum. The development of the ova in Hermella has been minutely described by Quatrefages ; and this may be taken as an example of the general nature of this process among the * Article Zeugung, p. 809. t On. the penetration of spermatozoa, &c., in Zeitsch. fiir Wissen. Zool. vol. vi. [31 [118] OVUM. Annelida. According to this description, the first germs of the ova consist of minute ger- minal vesicles formed in the ovarian sub- stance; they soon acquire the single macula or nucleus. After undergoing some enlarge- ment, these germs fall into the abdominal cavity, and there acquire, by deposit round them, the clear primitive vitelline substance. In this substance opaque granules, which are at first colourless, are subsequently deposited; and as these extend outwards from the ger- minal vesicle, and accumulate in quantity so as to increase the bulk of the whole ovum, a delicate vitelline membrane is added exter- nally. The germinal vesicle attains a diameter of about -$\v", and its macula of ^-^3"; and when the several parts of the ovum which have been mentioned have appeared, and the yolk is now coloured, the whole ovum has a diameter of about 75-^". The superficial part of the yolk consists of minute coloured granules. Within this there are larger oil-like globules free of colour, and in the innermost part a somewhat viscous transparent fluid.* According to Leydig, the germinal vesicle in Piscicola becomes enveloped by a second vesicle or cell-wall before the formation of the yolk-substance ; but it is suggested by Leuckart that he may have been misled in this by the appearance often presented by the clear and somewhat highly refracting substance which in many animals precedes the formation of the opaque yolk. If" this is not so, the fact observed by Leydig would constitute a marked departure from the usual homological relations of the ovum.f Rotifera. Although most zoologists are now disposed, on the ground of the analogies in the most important parts of their general structure, to place the Rotifera among or close to the Articulate Worms, yet in some re- spects their mode of reproduction presents a marked correspondence with that of the lower Crustacea. Thus they have, in com- mon with some of the lower Crustacea, the occasional separate condition of the sexes, the preponderance of females, the imperfect development of the males, the proportionally large size of the ova, and the production of winter ova as well as the ordinary kind ; on the other hand, the simpler structure of the ovum and its complete segmenta- tion are more similar to what is observed among the Vermes.f * Quatrefages, it is to be observed, designates the enveloping membrane ovarian and not vitel- line membrane, which last he holds is wanting in these ova. f Farther interesting views of the ova of this class will be found in Milne Edward's memoir in theAnnal.des Scien. Nat. for 1845, vol. xxiii. p. 145 ; and in his article ANNELIDA in this Cyclopaedia, to which I must refer the reader ; in Grube's Unter- such, iiber die Eutwickel. der Clepsine, Konigsberg, 1844. H. Koch, Ein Worte zur Entwick. von Eunice, with an Appendix by Kolliker, on Exogone and Cystonereis. | See Leydig, On the Structure and Systematic Position of the Rotifera, &c., in Zeitsch, fur Wissen. The relation of the sexes in Rotifera has only recently been in any degree under- stood, and that only in a few genera ; and there are still many points requiring elucida- tion. The greater number of the animals, in fact, which till lately have been known or de- scribed in this class have been females ; and as yet the males or male organs have been as- certained only in a few genera. Some are certainly of separate sexes, as Notommata, and the allied Rotifer of which the male was first discovered by Brightwell*, and of which the development was described by Dalrymplef, Others seem to be hermaphro- dite, as in Megalotrocha, described by Kol- liker^; ; in Euchlanis, by Schmidt^ ; and in Lacinularia socialis, by Leydig. || But ac- cording to Huxley If, there may still be some doubts as to the bodies described as spermatozoa, and as to the arrangement of the male organs in the Lacinularia. Fig. 83*. Ovarian ova of Rotifera. (From Huxley,') The figures represent the formation and develop- ment of the true or ovarian ova of Lacinularia socialis (one of the Rotifera). A. and B. are small fragments of the ovarian substance showing the primitive ova with their germinal vesicles and maculae ; in B. one of the ova more advanced than the rest. c. represents the mature ovum. D. the same undergoing the first stage of segmentation. The ova of Rotifera have been observed by Ehrenberg and many other microscopists. They are of comparatively large size, but yet belong to the group of ova possessing the simpler kind of structure, the yolk substance being quite finely granular, and undergoing a complete segmentation. The germinal vesicle is large, and possesses a distinct single ma- cula ; and the whole ovum is inclosed in a clear vitelline membrane. No micropyle has yet been discovered, nor have the time and Zool., vol. vi. ; and C. Vogt on the same subject in vol. vii. of the same work. * Ann. of Nat. Hist, for Sept. 1848, p. 153. t Philos. Transact, 1849, p. 331. t Froriep's Neue Notizen, 1843, p. 17. Vergleich. Anat. p. 268. || Zeitsch. fur Wissen. Zool., vol. iii. ^f Microscop. Soc. Trans, p. 1. in vol. i. of Microsc. Journal, 1853. OVUM. [119] phenomena of fecundation been minutely ob- served. The formation of these ova may be traced with facility in the substance of the ovary, in consequence of the transparency of the ani- mals. The nucleated germinal vesicle seems first to make its appearance ; the granular 3 oik substance follows; and the vitelline membrane is last formed.* The Rotifera present another example of the formation in the autumn season or before winter, of that variety of the reproductive body which has been called winter egg, and which has already been noticed under the Entomostraca. These bodies were observed by Ehrenberg in Hydatina and Brachionus, by Dairy mple in Notommata, and by Huxley and Leydig in Lacinularia. They are twice the size of the ordinary ova, are formed in very small numbers, probably only two, as is most common in Daphnia, and contain no apparent germinal vesicle. Mr. Huxley-)- ap- pears to have pointed out very clearly the dis- tinction between true or ordinary ova and these reproductive bodies. He says, at p. 16 of his paper, " The true ova are single cells which have undergone a special development, the ephippial ova are aggregations of cells, in fact larger or smaller portions, sometimes the whole of the ovary, which become enveloped in a shell and simulate true ova." Mr. Huxley Fig. 84:*. Formation of Ephippial ovum in Lacinularia Socialis. (From Huxley.) A. represents a portion of the ovary massed together and undergoing a change of structure pre- paratory to its conversion into the ephippial ovum. B. the ovum now complete, the external invest- ment distinct. c. the same having now its contents divided into two portions. The ephippial ova differ from the ordinary ones in their mode of formation, and in having three investments. has traced minutely the process of conversion of the substance of the ovary into such an ephippial ovum, or rather the protecting covering of the two ova which are contained in the epbippium ; and his observations seem to show a manifest difference between these and the ordinary ova. The same follows also from Mr. Dalrymple's researches on No- tommata. The correspondence of the num- * Leuckart, loc. cit. f Loc. cit. ber and general structure of these ova in Daphnia and the Rotifera is also deserving of notice. These winter ova, besides being much larger than the ordinary ones, differ from them also in structure, having three investing membranes ; and they appear designed, like those of the same kind in other animals, to resist the cold of winter and other hurtful influences. It would appear that these ephipphial ova, like those of Daphnia, do not require fecunda- tion. Leydig, though distinguishing the two kinds of ova, regards the hybernating ova as only modifications of the ordinary ones ; while Huxley considers them rather as pecu- liar buds like those of Aphis or Gyrodactylus.* Turbellaria. Under this class three orders of the animals allied to the Planaria maybe brought, according to the researches of Qua- trefages and others, viz., the two kinds of Planaria with simple and ramified alimen- tary canal, or Rhabdocoela and Dendroccela, and the Nemertides or Miocosla. The first two orders are hermaphrodite ; in the third the sexes are distinct. The ovology of this class is known principally from the interesting and beautiful researches of Quatrefages -j-; but the history of the structure and formation of the ova is still far from being complete. The ova of the Planariae are of various magnitudes, and present some differences in their structure. For the most part they con- tain only the finely granular yolk, but with oc- casionally some oil globules interspersed. It is only in the earliest stages that the germinal vesicle is perceived with ease, in consequence, probably, of the opacity of the yolk-substance, and the dark colour of the external envelopes. In most of the genera the germinal vesicles and the yolks are formed in separate organs, as in the trematode animals, to which the Planarise are nearly allied, but in some, as Macrostomum, these two organs come to be combined in one. At first the yolk-mass, in descending and meeting with the germinal vesicles, unites a number of them into a con- nected chain ; but somewhat later the ova are separated into distinct spheres, and a vitelline membrane is formed to enclose each of them. Just as occurs in the body of the adult Planarias, there is also in the ova a remarkable tendency to subdivision by fission. Thus, in the commencement of the development of the ovum, it is liable to become divided into distinct masses, so as to give rise to the de- velopment of a number of embryoes from one ovum. Such, at least, is the view entertained by some ; but there may be doubts as to whether the ovum so divided is really simple, * See on this subject also, Burnett's translation of Von Siebold's Comparative Anatomy, p. 150. t Mem. sur quelques Planarieea Marines, in Annal. des Scien. Nat. 1845, torn. iv. p. 169 ; and Me'm. sur la Famille des Nemertiens (Nemertea), id. lib. 1846, torn, vi., p. 209. The Khabdocwla are known chiefly by the researches of Schmidt, Die Ilhabdocoelen Strudehviirmer des siissen Was- sers, Jena, 1848; and of Schultz, Beitrage zur Geschichte der Turbellarien, 1851. [i 4] [120] OVUM. or is rather a collection or aggregation of a number of germs surrounded by a common yolk ; in fact, as has been suggested, an ova- rian sac containing a number of ova.* The manner in which the spermatozoa reach the ova for fecundation does not appear to have been ascertained with accuracy. Entozoa. The ovology of the Helmintha or Entozoa has received considerable atten- tion from physiologists, both on account of the interesting nature of the phenomena pre- sented by its study, and because of the anxiety to discover the mode of production of these parasites within the bodies of other animals. From the researches on this subject which have been prosecuted with great assiduity by a number of observers in recent times, not only have many doubtful points been solved as to the origin of the Entozoa, and the views of naturalists greatly modified in regard to the history of these animals, but considerable assistance has also been received in the elu- cidation of general questions in ovology. I will give a short sketch of what has been most recently ascertained on this subject under the three divisions of the Nematoidea, including all the Round Worms, the Trematoda, and the Cestoidea including the Cystica. All the animals belonging to the first division are bisexual, and the production of the embryo is direct from the ovum, without metagenesis or metamorphosis ; in the two other divisions hermaphroditism prevails, and development is indirect, or accompanied by metagenesis and metamorphosis in the greater number. Nematoidea. The genital organs in the first of these orders present the same favourable circumstances as those of Insects for the ob- servation of the structure and formation of the reproductive elements in their successive stages, as in the different parts of these tubu- lar organs there are to be found at once the spermatic cells and spermatozoa, and the ger- minal cells and ova in every conceivable de- gree of advancement from their earliest ap- parition to the state of maturity. In the Ascarides and most of the round worms, the upper closed extremities of the two genital tubes of the female correspond with an ovary, or rather as a portion of it which may be regarded as a germ-form- ing organ ; for in this upper part of the tube are produced only the nuclei or nucleated cells, from which the germinal vesicles derive their origin. A second portion of the tube, in which the granular yolk substance is added, is to be looked upon also as a con- stituent part of the ovary, and may be called the yolk-forming or vitelligenous organ. Next follows a constricted part of the tube, which may be termed oviduct, in which the ova meet with the spermatic corpuscles and undergo fecundation. From this the ova pass into the fourth compartment, a dilated portion which has been called a uterus, and below this * Burnett's transl. of Siebold's Compar. Anat. p. 140. the two genital tubes finally unite into a com- mon vagina. In the Ascarides, the process of formation Fig. 85*. O "O o C O" * O'o Development and fecundation of the ova of Ascaris mystax. A. Earliest stage of the ova as they are found in the coecal or uppermost part of the ovarian tube ; some from the highest part are mere molecules, others a little farther down are minute nucleated cells (germinal vesicles or germs of the ova), and round these the primary yolk granules are be- ginning to collect. B. Ova from the second part of the ovarian tube in which they are closely pressed together and arranged in a radiated manner round the axis or centre of the tube. To the right, four of these ova are represented adhering together ; to the left, two ova are shown with their flat surfaces, and one with its thin edge towards the observer. The ex- ternal dotted line represents the surface of the basement substance of the yolk in which the opaque vitelline granules are deposited ; there is as yet no vitelline membrane ; the germinal vesicle and macula are very distinct. c. An ovum from the oviduct ; a faint marginal line indicates the place where the vitelline mem- brane is afterwards formed. The germinal vesicle still visible, though obscured by the yolk granules ; the ovum has now assumed an ovoid shape. D. Softened state of the ovum at a slightly later stage, when it has met with the spermatic cor- OVUM. [121] puscles ; which are held by Nelson thus to pene- trate or gain access to the vitelline substance. E. Ovum more advanced ; the vitelline and albuminous membranes formed ; clear highly re- fracting spaces resembling altered spermatic cor- puscles are seen in the yolk substance. F. Ovum after fecundation ; uniform structure of the yolk substance previous to the appearance of the embryonic cell and commencement of segmenta- tion. The chorion has now become tuberculated. of the ova appears to consist, first, in the production of minute cell-germs in the upper- most part of the ovarian tube immediately adjoining its coecal termination. It does not appear to be fully ascertained whether these germs are originally, as some have supposed, the maculae or nuclei, or rather, as others hold, the germinal cells or vesicles themselves : the latter opinion appears to be the most probable. Second, the granules of the yolk-substance very soon collect round the exterior of the ger- minal vesicles. These granules appear at first to be suspended in fluid ; but a little later, as they come to collect round the germinal vesicles, they are united together in a mass by a firmer but clear basement substance, and when the minute ova have somewhat in- creased in size, the outline of this clearer basement substance of the yolk is distinguish- able. There is not, however, at first any ex- ternal or vitelline membrane ; of this Dr. Nelson and I have convinced ourselves by re- peated observations in Ascaris mystax.* The ova, as they continue to descend in the vitelligenous part of the tube in immense numbers closely pressed together, assume the form of subtriangular flattened bodies, and come to be arranged in series of three, four, or more, in a short spiral round the centre of that part of the ovarian tube which constitutes the yolk organ, as round a central axis, but without being united together by any com- mon stalk or other structure. A prodigious number of ova are thus packed together in a very small space. In passing through the next part of tube, which forms an oviduct, the ova are detached from the spiral and closely-set position, and being surrounded by fluid, which must here be secreted within the tube, descend one by one through its narrower part. At this place they encounter the spermatic corpuscles when they are present, and undergo the change of fecundation ; but whether fecundated or not, the ova now lose their germinal vesicles, alter their form from that of flattened triangles to oval, become for a time much more yielding and soft, and somewhat later begin to acquire an external covering which they have not previously possessed. The peculiar motionless and tailless sper- matic corpuscles appear, therefore, to come into contact with the ova when the yolk is exposed directly to their action. According to the interesting observations of Dr. Henry Fig. 86*. * See Nelson's paper on the Reproduction of the Ascaris Mystax in the Trans. Roy Soc. of Loud. 1852, p. 5G3., pi. 28, figs. 48. and 60. Development of Spermatic Corpuscles in Ascaris mystax. This figure is introduced to show the several stages of development of the peculiar acaudal and motionless spermatic corpuscles of the Ascaris mystax. A. shows various stages of the primary sperm- cells or rather sperm-germs ; in the more advanced of which towards the right, internal cells are seen forming by endogenous production within the primary germ-cells. B. & c. show the second stage, in which the sepa- rated germ-cells have each become covered bv a finely granular mass collected round them ; in B. this process is beginning ; in c. it is completed, and the sperm cells thus formed have assumed an ovoid shape. D. Two views of sperm-cells in the third stage, in which a quadrifid division of the whole cell has taken place preparatory to the escape or separation of the spermatozoon-cells, usually fuur in number, proceeding from each sperm-cell. E. Various views of these spermatozoon cells in which the radiated linear marking (seen in D.) has disappeared, and is again resolved into granules ; the nucleus is seen from above in the left-hand figure ; in the three others being viewed in profile the appearance of the bell-shaped spermatic cor- puscle with the nucleolus is perceptible. F. Exhibits from right to left the various pro- gressive stages of the bell-shaped corpuscle into the test tube form ; the remains of the nucleolus and granular substance are seen towards the mouth of the flask-shaped bodies. G. Illustrates the effect of water in developing " Sarcode " on the exterior of these corpuscles in two different stages of their advancement. [122] OVUM. Nelson,* a peculiar softening of the ova, which may be caused by the rapid imbibition of fluid at the time the changes above mentioned are taking place, renders them peculiarly liable to be impressed by the spermatic corpuscles at Fig. 87*. Development of ova in ]\Iermis albicans, belonging to the Gordiacei. (From Meissner.} a. Germ -cells from the upper or ccecal end of the ovarian tube, their nuclei undergoing subdivision. b. Various stages of farther multiplication of the internal cells, which in the more advanced are seen to approach the surface of the original cell, and to cause the bulging of its membrane by the enlarge- ment of the internal cells, which last constitute the primitive ova. * Loc. cit., p. 576. c. $ d. Groups of primitive ova thus formed ; some of them much more developed than others, present- ing internally the nucleated germinal vesicles and yolkjgranules and attached in pediculated capsules, which are formed by the extension of the membrane of the primary germ cells. e. A group of these ova more advanced ; the opaque granular yolk increased in quantity so as to obscure in part the germinal vesicles ; the pedicles much narrowed and somewhat elongated ; the ex- ternal ova are nearly mature ; those in the centre remain abortive. /. Two similar ova now ripe, a part of one of them is artificially burst, showing the escape of the yolk granules and germinal vesicle with a double macula. The remains of the pedicles when detached from the central mass constitute, according to Meissner, the micropyle aperture. this period; and Nelson is of opinion that these corpuscles even penetrate completely into the yolk-substance, and ultimately com- bine with it. Little doubt can be entertained that a combination of the spermatic and vitel- line elements in some manner takes place at this time, whether by the direct interpene- tration after the mode described by Nelson, some may be inclined to doubt ; but at all events the spermatozoa act immediately on the vitelline substance at this stage of the progress of the ovum. * As the ovum descends in the next part of the tube or uterus, the external membrane becomes more dense, additional layers are deposited upon it, and at last it acquires more * Professor Bischoff has, in his recently published tract " Wiederlegung des von D. Keber bei den Naiaden und Dr. Nelson bei den Ascariden behaupte- teu Eindringens der Spermatozoiden in das Ei," &c., Giessen, 4to., 1854, called in question the accuracy of Nelson's observations, and asserted that Nelson's spermatozoa are only epithelial particles belonging to the female passages. In a subsequently pub- lished paper, entitled, " Bestatigung des von Dr. Newport bei den Batrachiern und Dr. Barry bei den Kaninchen behaupteten Eindringens der Sper- matozoiden in das Ei, Giessen, 25th March, 1854," although Bischoff has seen reason to alter his pre- vious views as to the phenomena of fecundation in the Ascaris mystax, he still in that paper, and in a special memoir on the subject, published in the Zeitsch. fur Wissensch. Zool., 1854, vol. vi. p. 377. adheres to the view that the bodies which I, along with Nelson and Meissner, regard as spermatozoa are no more than epithelial cells. I have elsewhere shown that this view is altogether untenable, and that no doubt can now prevail as to the corpuscles in question being the product of development from the spermatic cells of the male Ascaris, and as to the possibility of their direct action on the ova within the fe'male previous to the formation of the vitelline membrane. Meissner has also given the most satisfactory evidence on the same point in his memoir on the penetration of the sperma- tozoa into the ova of animals, contained in the same volume of the last quoted work, though this author takes a different view from Nelson and my- self as to the manner in which the spermatozoa are admitted into the ovum in Ascaris mystax, believing in the existence of a vitelline membrane and micro- pyle, in the same manner as in Mermis and other Gordiacei, which he has so well described. With regard to this view as applied to the Ascaris mystax, Bischofi's observations, Nelson's, and my own, give me the greatest, confidence in asserting that there is at first no vitelline membrane in this animal at the time when the ova first meet with the spermatic corpuscles. or less of a minutely tuberculated structure on its external surface. The ovum becomes of a regular short oval or nearly spherical form. If fecundation shall have occurred, the embryonic vesicle or cell makes its ap- pearance, and the phenomena of segmentation follow in rapid succession. Fig. 88*. OVUM. [123] In others of the Nematoid Worms and more especially in Strongylus and the Gordiacei, it would appear from the researches of Meiss- ner, that the first germs of ova which take origin in the uppermost part of the ovarian tube multiply by an endogenous production, and that in this manner groups or bunches of the primitive ova are produced which are con- nected together by pedicles arising from the Fig. 89*. Formation and fecundation of the ova of Nematoid Worms. {According to Meissner.) a. A portion of the ovarian axis and early ova attached to it from the ovarian tube of Strongylus armatus. The axis column occupies the centre of the tube, and the ova are suspended to it by pedicles, supposed by Meissner to form micropyle apertures when they are detached. b. View given by Meissner of a set of the nearly ripe ova of Ascaris mystax, which he conceives are thus connected by pedicles to a central axis. c. Two mature ova of the same surrounded and in part penetrated by spermatic corpuscles. At the narrow angles of these ova a spermatozoon is seen passing into the interior by what Meissner has regarded as a micropyle formed by the detached pedicle. In the ovum to the right a spermatic cor- puscle is seen in the vitelline substance. The existence of such a micropyle aperture and pedicu- lated attachment of the ova in the Ascarides 1 re- gard as doubtful. <7 Formation of ova and fecundation in Gordlus Sub- bifurcus. {From Meissner.) a. A small portion of the ovarian tube with groups of the ova partly within and partly escaping from it. b. Three of the mature ova from the lower part of the oviduct surrounded by the spermatozoa. The ova are now isolated, and "the pedicle of each is open, and is regarded by Meissner as a micropyle, by which spermatozoa, as represented in two of them, enter the ova. The germinal vesicle is still to be seen. elongated membrane of the original germ-cell which remains as a covering of the whole. A certain number of these ova make progress in development while others probably become abortive. As the ova enlarge they are more spread out in the tube and take something of the spiral disposition which exists in the Asca- rides, but with this difference, as already noted, that the various ova remain connected to- gether by the attachment of their pedicles to a central axis or stem running down the middle of the ovarian tube. On the subse- quent detachment of the ova by the. break- ing of these pedicles, according to Meissner, a micropyle aperture is formed in each ovum for the admission of the spermatozoa. The accompanying drawings from Meissner's Memoir will give a sufficiently clear idea of his views on this subject. The ova of the nematoid worms constitute a marked example of the simpler kind of ovum in which the formative yolk is present, and [124] OVUM. in most but not in all of which segmentation is complete. This process was first made known through the interesting researches of Kolliker*, in Miiller's Archiv., 1843, p. 68, and Bagge, in his Inaugural Dissertation.f The memoir of Reichert in Miiller's Archiv., 1847, contains very correct views as to the formation of the spermatic cells. The accompanying figure from MeissnerJ, gives a representation of a remarkable form of the external capsule of the ova occurring in some of the Gordiacei (Mermis nigrescens}. Fig. 90*. Mature ova of Mermis nigrescens. (From Meissner.') This figure is introduced to show the very pe- culiar capsule in which the ovum is enclosed. a. Ovum taken from the uterus with embryo enclosed ; the chorion and shell capsule with cha- lazse or brush-like processes attached to the latter. b, c. The sholl capsule <. burst across the equa- torial groove, allows the ovum b to escape with the chorion and embryo contained within it. The ova of Trematoda are generally of a long-oval form, and of middle size. They are enveloped by a shell membrane of consider- able firmness, and winch is not unfrequently of a dark brown colour. The yolk-sub- stance contains fat corpuscles simple and compound ; and there is a germinal vesicle present, which, however, from the deep colouration and other circumstances, is often very difficult of detection. In these animals an interesting peculiarity in the arrangement of the reproductive organs exists, in the separation of the germ-forming and yolk-forming portions from each other ; in the first of these organs germinal vesicles or clear nucleated cells alone being produced, in the other the opaque granular fatty matter which furnishes the vitellus. This arrange- ment was first described by Von Siebold in 1836.$ The germ organ is generally in the form of a rounded sac, which is filled with the nucleated germ-cells or vesicles in various * See his admirable memoir on the first changes in the fecundated ovum, principally referring to the Entozoa. t Dissert, inaug. de Evolutione Strongyli auri- cularis et Ascaridis acuminatse, Erlangae, 1841. I Zeitseh. fur Wissen. Zool. vii. pi. ii. Wiegmann's Archiv., 1836, p. 217, Tan. vi and Mttller's Archiv. 1836, p. 232, Tan. x., fig. 1 stages of development. The vitelline organ is double, each one consisting of crecal tubes, in which the opaque granular yolk-substance is secreted.* The ducts of these two organs meet in a common cavity or uterus, and the germs descending into this cavity are there enveloped by a portion of the vitelline mass, and shortly afterwards an enclosing vitelline membrane is formed. These animals being hermaphrodite, the vas deferens of the male organ or testicle leads into the uterine cavity ; and it would appear, therefore, that in many cases, if not in all, impregnation takes place by the access of the spermatic corpuscles to the elements of the yolk and germinal vesicle, just at the time when they are brought toge- ther to form the ovum. This separation of the germ-forming and yolk-forming parts of the ovarian organ, which is so apparent in the Trematoda, is not in truth so great a departure from the more familiar structure of other animals as might at first be thought ; for, as Leuckart has well observed, there are other examples of the same disposition, or an approach to it. Thus in Insects and in Nematoid Worms, as we have seen, it is from distinct parts of the genital tube that the germs and yolk are produced ; and more or less of the same arrangement exists in all instances in which the form of the ovary is tubular. The Cestoidea present a great similarity to the Trematoda in the arrangement of the organs by which the ovum is formed. Indeed, notwithstanding the difference of their antece- dent stages of development, the structure of the mature sexual joint or proglottis of the tapeworm, offers so great a resemblance to that of some of the Trematoda, that they have been regarded as identical by several recent obser- vers. In each sexual joint of the tapeworm, the testicle and the two par is o" the ovarian organ coexist, and, PS stated in an earlier part of this article, arrive at maturity simul- taneously in the posterior or oldest segments of the body. Van Beneden has, in his recent work on the Cestoid Worms-f-, described very clearly the structure and relations o." t'le ger- migenous and vitelligenous parts of the epro- ductive organs in the complete segments or proglottides of a variety of Cestoid worms. The ova originate in the first mentioned of these organs as germinal vesicles, ami, passing into the vitelligenous part, meet with the yolk-masses formed there. Near the same place is situated the seminal vHcle, from which, doubtless, the spermatic substance easily reaches the ovum as 'i descends in the course of its formation. The ova then ac- quire an external envelope, and pass into the cavity termed a uterus. As they come to be accumulated in gradually increasing quantity in the latter cavity, they distend it to a great degree, so as to cau^e it to pervade in various forms, ramified and others, the whole body of * See also Thaer on this subject, in Miiller's Archiv., 1850, p. 626. t Me'm. sur les Vers Cestoides. Acad. Koy. de Belgique, torn. xxv. 1850, see plate B. OVUM. [125] the proglottis ; and finally they are dis- charged from this, usually after the separation of the joint from the main tapeworm, by the irregular rupture of the outer wall, or by a genital aperture. Here, then, we have another instance of the combination of the several com- ponent elements of the ovum together with the sperm, previous to the enclosure of the whole by a membrane so as to give the form of a complete ovum. The ova of most of the Cestoidea, as in the common tapeworms, are of proportionally small size. The external envelope is firm, thick, and nearly homogeneous ; sometimes, however, presenting a slight appearance of fine radiated striae passing through it, which recalls the structure of the thick membrane of the Fish's ovum. The vitelline substance is very finely granular, or almost clear ; the germinal vesicle is perceived with difficulty, but is of large size.* In some Cestoids the external envelope is of a brown colour, as in the Trematoda, and in others presents pecu- liar forms and prolongations from its surface. A delicate vitelline membrane is described within the outer covering by some authors. -j- The segmentation of the yolk appears to he complete ; but this process has been observed only in a few instances. Of the ova of the Cystic Entozoa nothing need here be said, seeing that it has already been shown that the several genera of this order, viz., Cysticercus, Ccenurns, and Echi- nococcus, are only larval and aberrant forms of the Cestoid worms, and being immature animals, never produce ova, excepting through their more advanced stage of cestoid develop- ment. Echinodermata. The different orders and families of this class are all of distinct sex, so far as is yet known, with the single exception of one of the Holothurida, viz., Synapta(S. Duvernsea), described by QuatrefagesJ as presenting a combination of the testicles and ovaries in one organ, resembling in some measure that which exists in the Gasteropo- dous Mollusca. In the females of Echinus, Asterias, and Holothuria, the ova have been studied with care by different observers. In all of them the ova present.'when mature, more or less of a deep yellow, orange, or red colour, which belongs to the yolk-substance. This sub- stance is finely granular, and is enclosed, at least in some, as Echinus, by a delicate vitelline membrane ; but in others, as Holo- thuria, there is a considerable deposit of an albuminous layer of a peculiar structure, which, from its adhering closely to the vitel- * See Kolliker in Muller's Archiv. for 1843, p. 92 ; Tafl. vii., fig. 44. f Details as to the structure of these ova will be found in the work of Von Siebold in Burclach's Physiologic, vol. ii. ; in Dujardin's Hist. Nat. des Helminthes, see pi. ix. and xii. ; in Blanchard's memoirs in the Annal. des Scien. Nat. for 1848, p. 321 ; in Van Beneden's work ; and in Kuchen- meister's more recent Handbuch der Parasiten des Menschen, &c., Leipzig, 1855. { Anual. des Scien. Nat., 1842, xvii. line membrane, obscures the latter envelope, and thus has made its existence doubtful to some observers. This albuminous deposit also exists in Echinus, but is in that animal distinguishable from the vitelline membrane.* The colour and opacity of the yolk-sub- stance in the mature state of the ovum usually prevent our perceiving the germinal vesicle ; but in the earlier stages of formation, when the ovum is of lighter colour or even quite clear and transparent, a germinal vesicle with a single distinct macula is easily per- ceived. This vesicle has disappeared in the ova which are deposited. The segmentation of the yolk is complete in the Echinodermata : the process has been fully traced by Sars in Asterias \, and by various observers in some other genera. It was in the ovum of Holothuria tubulosa that Professor Johannes Mliller first made the novel and interesting discovery of an aperture leading through the thick external membrane towards the yolk ; an observation which has been confirmed by various other physiologists J , and has been productive of important con- sequences in its extension to a number of other animals in which such an aperture was not previously suspected to exist. Muller brought this observation before the Berlin Academy, and it was noticed in the printed report of the proceedings in 1851. A more detailed account of his observations on this subject is given by Miiller in his Archiv. for 1854 (p. 60.). The very thick covering of the ovum of Holothuria presents an appear- ance of radiated lines running through it per- pendicularly to the surface, which resembles in some degree the marking in the membrane of the Fish's ovum, but is not so distinct, and does not appear, as in it, to be produced by visible tubes or pores passing through the membrane. The canal of the micropyle pierces the whole thickness of the radiated membrane; but Muller conceived that it did not perforate the delicate vitelline membrane placed on its inner surface. Leydig, however, and Leuck- art are of opinion that the canal passes com- pletely into the interior of all the egg-coverings;, and reaches the surface of the yolk, so that it may convey the spermatozoa to that body. The entrance of the spermatozoa has not, however, as yet be^n actually observed. According to Leydig, the thick membrane may consist of several layers united together, such as, internally the vitelline membrane, the thick albuminous part in the middle, and ex- ternally the nucleated layer formed by the remains of the ovarian capsule. Leuckart and Leydig have also pointed out the fact that the formation of the canal of the micropyle in the eeg of Holothuria proceeds from or is con- nected with the original attached and pedicu- * Derbes, in Annal. des Scien. Nat. 1847, 3 C Sc'r. vol. viii., p. 80, and Leydig in Muller's Archiv. for 1854, p. 312. f Wiegmann's Archiv. 1844, and Annal. des Scien. Nat, 3 e sen, vol. ii. p. 190. J Leuckart in BischofF's Wiederlegung, &c., 1854, and Leydig, loc. cit. [126] OVUM. lated condition of the ovum in the ovary, that it is in fact the remains of the divided pedicle after the ovum is separated from the place of its original formation. Fig. 91*. Ovum and Mlcropyh in Holothuria tubulosa. (From Ley dig.) a, b. A small portion of the ovary from the inner surface, containing ova in various earlier stages of their development ; three of them project from the inner surface, of which a is the most de- veloped. In this one the pediculated attachment and enclosure of the ovum by the nucleated ovarian membrane is seen, the yolk granules and the ger- minal vesicle with its macula. c. A more advanced ovum now separated from the ovary. Externally the nucleated remains of the ovicapsule are represented ; inside this the thick albuminous layer marked with radiated lines, and lined closely by the vitelline membrane ; both these, as well as the ovicapsule, being perforated by the micropyle formed at the place where the pedicle formerly existed. The micropyle aperture has also been ob- served in other Echinodermata, viz. by J. Miiller in Ophiothrix fragilis, in which he states its diameter to be y^n/'* anc * b . v nis son Max Miiller in Sternaspis thalassemoides.* This aperture has not yet been observed in the ovum of Echinus. In the fecundated ova of this genus, however, Derbcs observed spermatozoa to have passed through the thick external albuminous covering, but not within the more delicate vitelline membrane ; but in this animal the external covering is more like a layer of soft albumen than a dense mem- brane as in Holothuria. The ova of Echinodermata take their origin, like those of other animals, by the formation of the germinal vesicles. These have been * The micropyle was represented in the ovum of Holothuria txibulosa by R. Wagner in his Icones Zootomicffl, tab. xxxii., fig. 12., before its nature was known. The first discovery of a micropyle in the animal ovum is therefore due to J. Miiller. The next observations of a similar nature are those of Leuckart and Keber. observed by Leuckart in the Holothuria tu- bulosa, beginning to be formed in the ovarian substance, which they cause to bulge or pro- ject when they enlarge, so as to hang into the ovarian cavity. The yolk-granules then come to be deposited round the vesicles, rendering the ova opaque, but during all this time the ovum is attached and enveloped by the original capsule derived from the ovary ; the albu- minous layer is then deposited, and the ovum being detached, the micropyle remains, as al- ready stated, as the perforation in the pedicle of attachment.* Polyp'ma. Although the greater number of the Polypi are commonly multiplied by a process of gemmation, as has already been stated in a former part of this article, yet they are all capable of attaining sexual complete- ness, and are also reproduced by means of fecundated ova. From the varieties, however, presented by the form both of the gemmules and true ova in different genera of Polypes, considerable difficulty has been experienced in determining the exact circumstances in which the ova are produced, and the distinc- tion between the germs from which true ova and those from which gemma? are formed. This is more especially the case among the ciliobrachiate Polypes or Bryozoa, which in their general organisation approach very nearly the tunicate Mollusca, but which in their mode of reproduction resemble closely some of the Polypes. The ova of the common Hydra, already re- ferred to in a previous part of this article, present the character common to the class, of being enveloped by a firm covering or shell membrane, which seems to be formed from modified cells, and which is sometimes beset with rough processes or projecting bristles or barbed spines somewhat like those of the Bryozoa. In the Tubularidae and Sertularidae the ova are formed in ovigerous capsules, which may be regarded as modified individuals or polype-heads of the compound animal formed by gemmation. In some instances these are detached from the parent stem, as in Tubu- laria indivisa-f- ; in other genera they remain attached, and their ova, or the ciliated em- bryos developed from them, are discharged from the cavities in which they are formed J ; but as the phenomena of the production of these ova have been fully described by Pro- fessor Rymer Jones in the article POLYPIFERA, it is unnecessary to enter into farther details with regard to the process in this place. * In addition to the memoirs previously quoted, descriptions of the ova of Echinodermata will be found in the following : viz., those of Comatula by J. Miiller, in Mem. of the Berlin Academy for 1841 ; of Asteracanthion, in Wagner's Prodromus, and in the 5th Part of Carus and Otto's Tabula} Anat. Compar. ; those of Echinus by Derbes, loc. cit. ; and by Krolm in Beitr. zur Entwick. der Seeigel- larven, Heidelberg, 1849, &c. f Sir John Dalzell, Remarkable Animals of Scotland, &c. J Dumortier and Van Beneden's Researches, in Mem. of the Acad. of Belgium, 1842, torn. xvi. OVUM. [127] In Hydractinia rosea, Van Beneden ascer- tained the existence of the germinal vesicle and nucleus within the ova while still con- tained in the capsule; and it appears that in all true ova of the Hydrozoa the vitellus, which consists of finely granular substance, undergoes a complete segmentation in the same manner as in other animals in which it presents a similar structure. In the common fresh-water polype, in which ovigerous capsules, or ova, and spermatic cap- sules were found coexistent on the same in- dividuals, I observed sometimes the spermatic capsules brought into contact with the surface of the ova by the bending round of the body of the polype at the time when the spermatozoa were being discharged. This took place pre- vious to the formation of the firm external covering ; but I could not determine whether fecundation had thus taken place or whether any spermatozoa had penetrated the ovum. In some of the Hydrozoa, as in the com- mon green polype, the ova are single, while in others as in Hydra fusca, figured by R. Wag- ner*, there are several ova enclosed in the same capsule. It is remarkable that, while in some Hy- drozoa the ova are developed from animals which retain the polype form in their com- plete sexual condition, or from modified po- lype heads, in others, as in Coryne, FritiU laria and Campanularia dichotoma, it is only from a medusoid progeny separated from the polype stock that the true fecundated ova are produced. In Anthozoa, the most of which, as Actinia, Alcyonium, Veretillum, Gorgonia, and the Corallines are hermaphrodite, the ova consist of finely granular yolk, germinal vesicle and macula, and undergo complete segmentation. The Bryozoa may be most appropriately considered in this place, as they present con- siderable analogy to the compound polypes in the mode of their reproduction. They are of separate sexes, and appear to be propagated in three modes, viz. : 1st, by gemmation ; 2nd, by true fecundated ova ; and 3rdly, by bodies which have long been regarded as ova, but which according to Professor Allman's recent researches may rather be considered as peculiar encysted gemmnles, and may pro- bably be analogous to the so-called winter ova of Daphnia and Lacinularia to which reference has previously been made. The development of the true ova of Pedi- cellina observed by Van Beneden has been already described.-f- In this instance the ova are arranged in clusters surrounded by a transparent capsule. In each ovum the finely granular yolk undergoes a complete segmen- tation. The germinal vesicle possesses a sin- gle macula. According to Van Beneden and Dumor- tierf, the ova of Alcyonella are developed in ovarian sacs connected with the inner end of * Icones Zootomicse. t See p. 23. andjfy. 19. of this article, j Mem. sur les Polypes d'Eau douce. Acad. de Belgique, 1842. the stomach. They are described as com- mencing by the formation of germinal vesicles with nuclei or maculae, and as having subse- quently the granular yolk-substance deposited round each vesicle ; and these authors de- scribe the same ova as acquiring at a later period the peculiar horny or cellular covering which forms the two-valved shell membrane long known as belonging to the winter ova of this and several other genera of fresh- water polypes. But with regard to the na- ture of these bodies and the mode of their formation some doubts may arise in conse- quence of the researches of Professor Allman. The bodies in question are at first nearly spherical and of a light or milky colour ; they become later of an oval form, and flattened or discoid, and the cells of the shell -covering are then developed, and acquire the deep brown colour which very generally prevails among these bodies when arrived at maturity, and which makes it impossible to trace farther the changes within the ovum. These cells are developed to a greater extent round the widest margin of the disc, so as to form there a thick ring or border, which is afterwards cleft in two when the valves of the shell open to allow the escape of the embryo. The same authors have described the pro- pagation of the Paludicella to take place in summer by means of buds, and in winter by 92*. Formation and Structure of the ova of Lophopus Bakeri. (From Van Beneden.) These represent, according to Professor Allman, not the true ova, but the Winter ova or " Stato- blasts." a. The ovum previous to the deposit of the cellular covering and marginal plate, b. This co- vering now in progress of formation, c. and d. pro- file and front view of the ovum, when completed, showing the structure of the cellular border which is afterwards cleft in two at the edge, when the era biyo is about to escape. e. An ovum at an earlier stage showing the ovi- capsule in part removed from one side of the ovum and its cellular covering. [128] OVUM. means of true ova, as well as by attached buds, which last are then covered by a strong corneous envelope, and have received Fig. 93*. Formation of buds in Paludicella. (From Van Beneden and Dumortier.~) a. One of the Polypes of Paludicella Ehrenbergii contracted within its cell, showing at the upper part towards the right the commencement of the formation of the bud by the growth of cells be- tween the outer and inner layers of the cell-wall. b. The same bud a little more advanced and more highly magnified, represented by itself. The vesicular cells which separate the ectocyst and en- docyst are seen more distinctly. c. A more advanced stage of the same, internally ; the part from which the embryo polyped arises is seen bulging out from the rest. This figure has been introduced to show the difference between the process by which a true bud arises and that by which ova are produced. the name of propagula. In Fredericella they describe a propagation by means of buds and by ova provided witli the strong horny envelope. In Alcyonella and Lopho- pus, besides the usual propagation by buds, and by the common ova, these authors have stated that there is also a viviparous produc- tion of ciliated embryos from ova which re- main within the parent animals ; but they have not stated particularly the manner in which these ova originate, nor their difference from those which receive the corneous en- velope. The difficulties presented by these varieties seem to be in some measure re- moved by the view offered by Professor All- man of the nature of the bodies last men- tioned, to which I will now advert. It has long been known that the so-called winter ova, or the bodies provided with the corneous envelope, are formed chiefly towards the autumn and winter sei'son ; and the strength of their covering lias gene-'all v been re- garded as a provision for the protection of the germ from the hurtful influences of the winter season. During iwo seasons I have observed the production of these bodies from the Plu- matella repens ; ana I have kepi ill em through the winter till the polypes were developed, and issued from them in the ensuing summer. From his careful observation of these bo- Fig. 94*. Winter ovum and embryo of Lophopus Crystallinus. (From Van Beneden and Dumortier.) This is the same as that represented by Turpin under the name of " Cristatella mucedo." In A. the flat surface, and in B, the narrow edge of the ovum, is represented. The two valves of the egg cover- ing have opened superiorly, and the embryo, which already possesses three crowns of tentacles, is seeu escaping. dies in several genera, Professor Allman has arrived at the conclusion that they are not, as was previously supposed, true ova, but rather separated gemmules ; and he conceives that Van Beneden, who has described their form and structure so well, must have confounded them with some other bodies in their first or earlier stages, or has failed to distinguish be- O ' t O tween them and the true ova. This distinc- tion Allman has succeeded in making by as- certaining that the true ova and these bodies do not arise in the same situation, and that these winter ova or gemmules do not in their earliest stages present any germinal vesicle or macula as the true ova do, and do not after- wards undergo any segmentation. They are formed, according to Allman, in the funiculus which connects the bottom of the stomach with the inside of the cell of the polypide, the same body which was described by Van Be- neden and Dumortier as an ovary, but which Allman regards rather as analogous to the gemmiferous stolon of the solitary Salpae. These bodies Professor Allman proposes to call stato-blasts. He farther discovered that there is a true ovary with genuine ova which may be distinctly observed in Alcyonella, and which is situated in the walls of the endocyst near the anterior extremity of the cell. A number of ova were found in the ovary con- taining the distinct germinal vesicle with macula. He also observed the segmentation of these ova in the usual manner, and the conversion of the segmented mass into a ciliated embryo, within which the new polype is subsequently developed.-j- Should these observations prove correct and be applicable to the other instances of similar winter ova among the Bryozoa, they may tend to remove some of the difficulties which exist in regard to the various repro- ductive bodies occurring in these animals ; but farther researches seem still necessary to point out in these and in other polypine ani- mals more fully and minutely the relation be- tween the three kinds of reproductive bodies, viz., true ova, separated gemmules, and at- tached buds. Acalephce. It is remarkable that notwith- standing the very close relation in which these animals stand to the Anthozoid Polypes, the form of their ova is not the same. The Dis- cophora (Medusas) are of distinct sexes : the Ctenophora (Beroes) are hermaphrodite ; the Siphonophora (Diphyidse) are various, or bear, in the manner of compound animal stocks, a variety of zoids, sometimes of one sex alone, at other times of different sexes on the same stem. The structure of the ova in Medusae is extremely simple. They are originally formed from minute cytoblasts which soon acquire a single nucleus or macula, and are enclosed in a delicate external membrane. These consti- Fig. 95*. Development of the ova of Acalepha. These figures give magnified views of the diffe- rent stages of formation of the ova taken from the ovary of a large Ehizostoma. a. The primitive germ. b. The germinal vesicle now present in the primitive ovum. c. d. The same more advanced and enlarged, the macula has appeared in the ger- minal vesicle, and a few yolk granules are deposited in the clear vitelline substance, e. The yolk gra- nules greatly increased in quantity and becoming opaque, a vitelline membrane is now formed. /'. The same somewhat more advanced, the yolk gra- nules are now collecting together to form cor- puscles. The macula is assuming the elongated form. t Proceedings of British Association for 1855. See also Professor Allman's interesting Pieport on the Polyzoa to the British Association. See Trans, for 1850, p. 320. Slip}!. OVUM. [i 29] tute the germinal vesicles, round which the granular yolk-substance is gradually deposited in increasing quantity. The complete segmen- tation of the yolk has been observed by Von Siebold in Cyanea aurita.* The yolk-sub- stance is often highly coloured, violet or yellow. In the former part of this article I have referred to the manner in which some compound Hydroida are propagated through their medusoid progeny. These medusoid individuals, like the ordinary Medusa;, are of separate sex j and they must therefore be looked upon as the complete stage of the polypine animals from which they have proceeded, whether they have their young developed while the parent remains at- tached to the nursing polype stock, or have assumed the separate and independent mode of life in a more complete state of develop- ment. There are many varieties in the de- gree of perfection to which they attain even while remaining attached to the polype ; but the general principle of formation is the same throughout the whole of the hydroid animals, the remarkable and constant fact with regard to the mode of their reproduction being this, that the immediate product of development from (he ovum which has been formed by sexual generation from a Medusa or medusoid animal is invariably an attached Polype, and that the medusa or medusoid is the product of a non-sexual process of gemmation from this polype stem. Protozoa. With regard to the Protozoa, or Infusoria and Rhizopoda, it is unnecessary to add anything here to what has been stated in the several articles on these subjects and in a former part of this one, excepting the remark, that continued researches appear to show that as the sexual distinction has not been de- tected, and may probabl} 7 be absent in these animals, the nucleus of the monocellular forms of these beings may hold the place of the germinal vesicle in them, and that the processes of division and production of in- ternal gemmules takes the place of true ovu- lation. At the same time it must be admitted that it is by no means improbable that the sexual relations may yet be discovered in the lowest monocellular animal bodies, as has re- cently been the case in some of the simpler and monocellular Algae, and that as our knowledge of the process of reproduction in these beings is still very limited, it may be destined to un- dergo even greater progressive changes than those which it has suffered from the researches of the last few years.-j- Porifera. The bodies which have usually been regarded as the ova of Sponges, and to which a reference was made in the earlier part of this article, are of two kinds, viz. gem- mules or detached ciliated portions of the * Beitr. zur Naturgesch. der Wirbcllos. Thicre, 1839. f See the papers of Focke, Colin, and Stein re- ferred to in the first part of this article, and the more recent work of Stein, " Die Infusionsthiere auf ihre Eutwickelungsgeschichte untersucht." 4to. Leipzig, 1854. [130] OVUM. substance of the sponge, and certain spherical bodies enclosed by dense capsules, which are produced towards winter, and which appear to contain a number of germs, each of which is capable of being developed into a Protean animalcule, from which probably a sponge may proceed.* But it may be doubted whe- ther these last-mentioned capsules are true ova or may not rather be of the nature of the gem- mules, winter ova, or statoblasts of Professor Allman; and it is important to notice that Mr. Huxley has recently discovered in Te- thya a different set of bodies, which contain all the essential parts of true ova, viz. vitel- line membrane, yolk, germinal vesicle, and macula, and that these bodies, which are si- tuated between the cortical and central sub- stance, are imbedded in a mass of cells together with spermatozoa.-]- Although the individual living particles of the sponge closely resemble simple ciliated infusoria, and the mass may, therefore, be viewed as an aggregate of these minute beings, yet its analogies with and transitions towards the fungiform polypes are so great, that we may expect ere long that the phenomena of its reproduction may be placed in a new and clearer aspect by the continuation of the researches now noticed, and by others of a similar kind. RECAPITULATION AND CONCLUSION. Having now stated in detail the principal facts that have come under our knowledge with regard to the form, structure, and mode of origin of the ova of different animals, it may be proper, in bringing this article to a close, to endeavour shortly to deduce from these facts the most general results to which they appear to lead. These results, together with some re- flections on our subject, may be stated under the following heads, viz. 1. Definition of the ovum, as related to its own structure, and its history in connection with the reproduction of the species. 2. Recapitulation of the most general facts ascertained by the comparison of the ova of different animals. 3. Morphology of the ovum ; homology of its parts ; and rela- tion of the ovum to other organic structures. 4. Phenomena attendant on the maturation of the ovum. 5. Relation of the ovum to fecundation by the male sperm. 6. Immediate effects of fecundation on the ovum ; and re- lation of the ovum after fecundation to the first commencement of the process of em- bryonic development. 1. Definition of the ovum, as related to its own structure, and its history in connection with the reproduction of the species. In the commencement of this article the ovum was shortly defined as " the product of parental sexual generation from which the young of animals are developed (produced)." This definition appears correct and sufficiently comprehensive ; but should it appear desirable to substitute for it a more precise description of the characteristics of the animal ovum, the * See Carter in Annals of Nat. Hist, vol, iv. p. 89. f See Mr. Huxley's paper in Annals of Nat. Hist., 2nd series, vol. vii. p. 370. following may be proposed as applicable to the ovum throughout the whole animal king- dom, without involving any merely theore- tical view of its structure and constitution, viz. " the ovum may be shortly described as a detached spheroidal mass of organised substance, of variable size, enclosed by a vesicular membrane, and containing in the earlier periods of its existence an internal cell or nucleus ; these parts, formed by the female individual or organ of animals, are capable, when fecundated by the male sperm of the same species, of giving rise, by the series of histogenetic and organogenetic changes known under the general term of develop- ment, to an embryo, from which either directly or mediately the individuals of the animal species to which the parents belong are re- produced." We thus separate from the category of true ova all those bodies of an apparently reproductive kind which are not the direct product of an act of sexual generation. To such bodies, the nature of which is as yet doubtful, and probably somewhat various, the indefinite appellations of buds, bud-germs, gemmae, spores, winter ova, ephippial ova, statoblasts, &c., have been given according to the circumstances in which they are se- verally produced. In all animals, then, with the exception of the Polygastric Infusoria and Rhizopoda, the occurrence of sexual generation and the for- mation of true ova are proved to be the regular and constant means for the permanent reproduction or maintenance of the species. In the exceptional instances now mentioned, and even in some others possessed of the sexual distinction, the best known and most common multiplication of individuals takes place by a subdivision of the parent body, either by fissiparous cleaving or by gemma- tion; but in them also it can scarcely be doubted that there are other means by which the permanence of the species is maintained. Ail the most accurate recent investigations lead to the conclusion that the production of the young of all organised beings, even the simplest of the Protozoa, does only occur by direct connection through some organised medium with other beings of a similar kind or species. We are forced, therefore, to con- clude that in the propagation or production of these simple beings, in circumstances where their more ordinary fissiparous or gemmi- parous mode of multiplication cannot be ad- mitted to have taken place, there must have passed from the bodies of the progenitors minute particles of organised substance (ca- pable, as we know, of being suspended in the atmosphere, and of resisting during a long period many of those influences which gene- rally prove inimical to animal development), which particles, when brought into circum- stances favourable to the progress of the vital processes, undergo the cycle of changes ne- cessary for the reproduction of beings similar to those from which they sprang. If there is any constant law which seems more certainly OVUM. [131] than others to result from all recent researches into the history of organic nature, it is this necessary connection by descent of one being or set of beings from another. In all animals, with the exception of the simplest tribes already referred to, the descent from parent to offspring is through a product formed and perfected only by the concurrence of male and female organs ; but we are still at a loss to determine whether the unseen germinal bodies by which the Protozoa are reproduced are of the same or of a different nature. The structure of some of these ger- minal bodies as described in the earlier part of this article (p. 7., &c.), bears a very great resemblance to that of true ova ; but yet the sexual distinction of the parent animals has not yet been discovered. The recent re- searches of naturalists indeed show that our whole knowledge of the history of the Pro- tozoa may be considered as only in its infancy. The discoveries as to the encysted stage of existence among the Vorticellae and Gre- garinoe and others, the phenomena of conju- gation observed in Gregarina and Actino- phrys, the entire knowledge lately gained of the form, structure, and habits of the Fora- minifera, all point to important future dis- coveries and modifications of our hitherto crude and imperfect views of these tribes of beings, and must make us refrain from at- tempting at present to form any opinion or even conjecture as to the modes of their re- production ; while at the same time the recent discoveries as to the existence of the sexual distinction in the simplest forms of plants encourage the hope that ere long the repro- duction of the Protozoa may, in a similar manner, be removed from the obscurity in which it now lies hidden. It does not appear necessary from these considerations that our definition should make any direct reference to animal bodies of the nature of which our knowledge is still so imperfect. The result of development from a fecun- dated ovum in all vertebrate and in a con- siderable number of invertebrate animals, is the formation of an embryo which, by a pro- cess of progressive growth, arrives at matu- rity, and assumes the form, structure, and habits, either, as the case may be, of a her- maphrodite animal, or of the parent of either sex. In a certain number of these instances, as in Batrachia, Insects, Crustacea, and others, growth is not altogether continuously pro- gressive, but is subject to one or more breaks or changes as it were, which are marked by some change in the mode of life, or some difference in structure of the individual. To such marked changes in the course of the development or growth of an individual ani- mal proceeding from a fecundated ovum, the name of Metamorphosis is given. But from the facts narrated in the earlier part of this article, it appears that in a cer- tain number of the invertebrate animals, such as those which have been referred to under the heads of Echinodermata, Polypina, Aca- lepha, Tunicate Mollusca, Trematode and Cestoid Entozoa, Annelida and Insecta, a very different result may, either regularly and constantly in some, or only occasionally in others, attend the first development from the fecundated ovum. To this modification of the developing and reproductive process the appellations of Alternate Generation or Meta- genesis have been given, of which terms the latter may perhaps be the most appropriate. The phenomena which have been described under this head are so very various, that it is difficult, if not impossible, to give a short and general statement of their nature. The dif- ference between this and the better known form of direct generation may, however, be stated nearly as follows : In the Metagenetic form of reproduction the individual formed by the development of the fecundated ovum is generally different in aspect, structure, and mode of life from the parent or parents by which the ova were produced ; this individual, or zoi'd, though possessed, in many instances, of an organisation and of powers which fit it for the efficient performance of many of the most important acts of independent animal existence, is yet wanting in the attribute of perfect animal maturity, viz., the sexual or- gans and activity, and is consequently incapa- ble by itself of an act of true generation, or, in other words, of the formation of fecundated ova, by which alone the species can be per- manently reproduced. In such instances, then, it is only by the formation from these intermediate beings of others which are sexu- ally perfect, that the generative act can be repeated. There are two phenomena re- quiring to be distinguished in connection with the most common forms of this process ; the one the frequent multiplication of the im- perfect intermediate beings, or zoi'ds ; and the other the production either directly or by a succession of acts of development from the intermediate beings of those which are sexu- ally perfect, or which resume the form be- longing to the parents from which the fecun- dated ova were derived. It seems proper, therefore, to distinguish between an act of true sexual generation, and that by which new beings are formed from the intermediate individuals (or so-called nurses of Steen- strup, or zoi'ds of other authors) ; the first consisting invariably in development from a fecundated ovum ; the second being probably more analogous to a process of budding or gemmation from a parent stock. It must be confessed, however, that we have still much to learn regarding the phenomena of this pro- cess, before we can form any general notion of its nature. The whole subject is replete with the deepest interest not only in connec- tion with the history of reproduction, but in its influence, as stated in some parts of the preceding article, on the whole range of zoo- logical classification and distinction. Our extended definition comprehends an allusion to these phenomena. Lastly, the ovum may be considered as having two phases or stages of existence ; the one in connection only with the female [K 21 [132] OVUM. parent or female organ, in which the greater part of the organised material first to he employed in development is provided, and in which the ovum arrives at a certain stage of maturity ; and the other in its relation to fe- cundation, or to the influence of the product of the male by which its developing powers are awakened or called forth. The mature ovarian ovum may therefore, in one sense, be looked upon as complete, if we regard only its own structure ; but here its progress would be arrested without the occurrence of fe- cundation, and if we view it, therefore, with reference to its more important destination as the means of continuing the animal species, the ovum can only be regarded as perfect when that hitherto inscrutable change has been effected on its substance by admixture with the minute elements of the sperm in fecundation. The constancy of this law in the whole animal king- dom, with the exception of those of the Pro- tozoa already referred to, makes it proper that our definition should make reference to fecund- ation as the means of perfecting the ovum. To the nature of this process itself a further al- lusion will hereafter be made. 2. Recapitulation of the most general facts ascertained by the comparison of the ova of different animals. The ova of animals in their complete state may be considered as consisting of two sets of parts which are of very different relative importance in connection with the develop- ment of the embryo : the first of these sets of parts belong to the ovarian ovum, and are formed previous to their quitting that organ ; the others are subsequently formed, and may be looked upon as accessory. These last often present great varieties, so as to cause the ex- ternal form and appearance of the ova of ani- mals to differ widely, while the ovarian part much more nearly corresponds. To this ovarium ovum we shall principally confine our present remarks. An extended comparison of the ovarian ova of animals belonging to almost every family of the animal kingdom has shown that, notwithstanding great differences in size, and some variation in form and structure, they all agree in consisting of three essential and nearly similar parts before the period of their detach- ment from the ovary : these are, 1st, The in- ternal nucleated cell or germinal vesicle with its macula or maculas ; 2nd, The vitellus, or yolk-substance ; and 3rd, The enclosing vesi- cular envelope, or vitelline membrane. In all animals there is, also, a general similarity in the manner in which these parts are formed and combined so as to constitute the ovarian ovum ; the germinal vesicle is the first produced, and may be regarded as the ovigerm ; the yolk- substance next gradually envelopes it or is deposited round the germinal vesicle, and in general the vitelline membrane which encloses the whole is the latest formed. The most marked differences among the ova of animals are connected with the struc- ture of the yolk and the relation which it bears to the formation of the germinal part out of which the embryo is afterwards developed' Founding upon this difference, three groups' two principal and one subordinate, may be distinguished among the ova of animals: 1st, The group of small-yolked ova, to which belong those of Mammalia and a considerable number of invertebrate animals, such as most Mollusca, the lower Crustacea, most Anne- lida, the Entozoa, Rotifera, Echinodermata, Acalepha, and Polypina. In this group, the ovum is generally of small or of moderate size, as a whole ; the vitelline substance con- sists entirely or chiefly of fluid with fine gra- nular particles, and the entire yolk undergoes segmentation : the entire yolk mass, therefore, is directly formative, or is employed from the first in the formation of the blastoderm or organised substratum in which the embryo is developed : the germinal vesicle is in this group usually of small size, and has only a single macula, or one composed of very few particles. The second principal group comprehends the large-yolked ova, such as those of Birds, Scaly Reptiles, Cartilaginous Fishes, and the Cephalopoda, to which, perhaps, may be added Insects, Arachnida, and most Crustacea. In this group, the largely developed yolk contains, suspended in its basement, homogeneous sub- stance, two kinds of organised corpuscles, viz., 1st, A certain portion of the small granular part, similar to that of the small yolked ova, which occupies a limited but determinate place in the ovum, and in its centre the germinal ve- sicle is situated ; and 2nd, A larger portion of spherules, cell-like or other corpuscles of greater magnitude. It is the first or finely granular portion only which is immediately germinal, or which is subject to segmentation and forms the basis of the blastoderm ; the remainder, or large cellular portion, is only secondarily employed in supplying nourishment to the embryo or its accompanying organised parts in the progress of their development. In the ova of this group, therefore, we distinguish the formative or directly germinal portion of the yolk-substance from the indirectly nutritive portion. In these ova, the germinal vesicle is also proportionally large, and the contents of the vesicle, though consisting in the earliest stages of their formation of a single macula, or of a very small number, very soon become converted into very numerous maculaa, or into a fine granular pulp. The third, or subordinate group, may com- prehend the ova of Amphibia, or scaleless rep- tiles, and osseous fishes, to which, perhaps, may be added some of the invertebrate ani- mals mentioned under the second group. The ova of this group are intermediate in their structure between those of the first and se- cond : they have certainly the greatest affinity with the large-yolked group, but there are many gradations among the ova of this kind, among allied species of animals, and it is chiefly on the ground of the incompleteness of the segmentation that I have thought it proper to arrange them in a separate group. It may be remarked further, that in all ani- OVUM. [133] mals, whatever may be the ultimate structure of the yolk, the primitive yolk, or that which is first formed, is invariably of the finely gra- nular kind, the cellular or large corpuscular yolk-substance is of later formation. These two parts remain distinct from each other, and the finely granular or formative yolk is that in which the germinal vesicle is invariably im- bedded. In those instances, such as the Bird, Reptile, &c,, in which the large cellular yolk greatly preponderates over the formative yolk- substance, the latter assumes in the later stages of formation the shape of a flattish disc on one side of the greater mass of the yolk, with the germinal vesicle placed in its centre. The vitelline membrane presents some va- rieties in structure, being in some animals very delicate and homogeneous; in others, as Mammalia, remarkably thick, tough, and elastic, but without visible structure ; in a third set, exhibiting peculiar structure, such as the finely tubular perforations of the ex- ternal membrane of the fishes' ovum, or the radiated markings in the ova of Holothuria or Cestoidea ; but in these last three in- stances the vitelline membrane is probably associated with additional layers of substance derived from a different source from that which forms the homogeneous membrane. A remarkable peculiarity has recently been discovered in the enclosing membrane of the ovarian ovum of some animals, in the aper- ture or micropyle which has been observed in osseous fishes, insects, some Crustacea*, the Acephalous Mollusca, some Annelida, Holothuria, and some other Echinodermata. There seems reason to believe that a similar aperture exists in the ovum of Batrachia and Cephalopoda; and it is very probable that it may yet be discovered in other animals. At the same time it is right to state that in Mammalia and several other animals it has been most carefully sought for without suc- cess. This aperture appears to be designed to admit the spermatozoa into the cavity of the ovum, or into contact with the yolk-sub- stance and germ, in those instances especially in which the egg coverings are thick and tough, and fecundation is late of occurring. The relation of the ova to the ovaries or organs in which they are produced, exhibits considerable varieties in different animals. 1. The most common is that in which the germs of the ova arise within minute close follicles or vesicles, which are imbedded in the more or less solid or loose stroma of the ovary ; the follicle enlarging with the ovum as its other parts are added till the period of of maturity, when, periodically, the follicles open for the escape of the ova. 2. In a second form, as in Nematoid Worms and Insects, the germs of the ova are produced free in the upper part of an ovarian tube, * It has been inadvertently stated in a preceding part of this article (p. 116.) that the micropyle had not been observed in the ova of Crustacea, whereas Meissner has ascertained its presence in that of Gammarus. (Sec his Memoir in Zeitsch. fur Wissen. Zool. vol. v. p. 284.) and the yolk-substance, &c. arc added gra- dually as the egg germs descend through suc- cessive portions of the tube : here no true dc- hiscence is necessary to allow of the escape of the ova. 3. In a third form, as in Trema- tode and Cestoid Entozoa, distinct organs are provided for the formation of the ovigerms and the yolk-substance, and these last are brought together and combined into the sphe- rical form of an ovum in another part of the genital apparatus. 4. In the greater number of animals the germs for each ovum appear to arise singly, and the ova are thus isolated from the first ; but it would appear that in some animals these germs arise in groups, perhaps by development from a common germ, so that they are from the earliest period connected together by pedicles. Yet, with all these differences, there is to be perceived, on the whole, a general similarity in the plan of formation of the parts of the ovum itself in different animals. This plan maybe generally stated as follows. The germinal vesicle is universally the first part of the ovum which makes its appearance ; it does not appear to be nucleated or to pos- sess its macula from the first in all instances, and this macula cannot therefore be regarded as the centre of its formation. The germinal vesicle is generally at first only a minute point ; it soon enlarges, however, and either possesses from the first, or at a very early period acquires, its macula or nucleus. In animals with the solid follicular ovary, each follicle is occupied by a single ovum, which begins within it as a minute germinal vesicle. The delicate wall of the follicle is also per- ceptible at the same time as the ovigerm ; in- deed, there is reason to believe that it even precedes the commencement of the formation of the ovum, though this is a point not yet fully determined. In those animals, on the other hand, in which the ovary is tubular, the ovigerms appear, in some instances at least, to arise in groups within cells; and it may be a question whether these cells bear to the ovi- germs arising within them the relation of the ovarian follicles of solid or closed ovaries. Whether this be so or not, that relation is in most instances speedily changed, as the ova soon become free, or, in others, are attached by a pedicle to a common stalk. The wall of the ovarian follicle consists at first of an extremely delicate vesicular mem- brane, which is the same as that to which the name of ovicapsule or ovisac has been given. At a very early period,and while the ovum con- sists of no more than the germinal vesicle, the homogeneous wall of the follicle is lined with a layer of flat cells somewhat analogous to some forms of epithelium : this is the com- mencement of the structure which in Mam- malia afterwards forms the tunica granulosa, and the fluid and cellular contents of the Graafian follicle. It appears to have various destinations in different animals. The second stage in the formation of the ovum is the deposit of the vitelline substance around the germinal vesicle. In most ani- mals the yolk-substance, when it first begins IK 31 1134J OVUM to be formed, is scarcely granular, and in some instances quite clear, consisting of a viscous blastema, and as it increases separating the ger- minal vesicle within from the ovarian follicle, which expands proportionally. Very soon, however, and in many animals indeed from the first, fine opaque granules make their ap- pearance, as if by precipitation or deposit, in the clearer basement substance, and thus the primitive yolk-substance of the ovum in all animals is formed. In most instances there is a time during which the ovum, consisting of germinal vesicle, with a small quantity of primitive yolk, exists, without any other co- vering than that given to it by the ovarian follicle ; but as the deposit of the finely granular yolk increases, and at a very variable period in different animals, the vitelline mem- brane is formed round its exterior. The ad- dition of this covering may be regarded as the third stage in the formation of the ovum. The manner of the origin of the vitelline membrane has not yet been accurately ob- served ; and it is probable (as will be hereafter stated) that the coverings known under this name may have different modes of origin ; but if we restrict our attention at present to such simple ova as those of Mammalia, I believe it may be stated as extremely probable that the so-called zona pellucida which constitutes the vitelline membrane of the Mammiferous ovum, takes its origin by the consodidation of the superficial part of the basement substance of the primitive yolk. It appears probable that in the large-yolked ova, such as those of the bird, the vitelline membrane, which we find enclosing the whole mass of the yolk, owes its origin to a dif- ferent source ; and I am inclined to believe that in this and in many other animals the membrane which we term vitelline, as being the immediate investment of the yolk, is not of the same nature with the zona pellucida, or the simple homogeneous vesicle of the smaller ova, but rather a structure of later formation, which owes its origin to the fusion, or amalgamation, or to some other change in the outermost layer of cells which form the nutritive yolk of these animals. In connection with this view, it is import- ant to remark, that at that earlier stage of formation of the bird's egg when it consists entirely of formative or primitive yolk, there is an approach to the formation of a zona, in the existence of a very distinct, clear, and consistent marginal portion of the yolk blas- tema, from which the yolk granules seem to retire. When the large cellular or nutritive yolk is formed, this temporary zona seems to disappear, and to be replaced externally by the permanent vitelline membrane already mentioned. In those animals in which the ovigerms arise by development within cells so as to be connected in groups (Gordiacei), and in some others, the vitelline membrane, or a substitute for it, seems to be formed from the earliest period in a different manner from that now described. The germinal vesicle is unimacular in ge- neral in the small-yolked ova, and multima- cular in the large-yolked ova, and also in the intermediate kinds. In the latter it is rare to observe the earliest stage in which the ma- cula is still single : the multiplication of the maculae takes place with remarkable rapidity, and apparently by a process of endogenous development, or possibly by division. The ultimate destination of these maculae is still a subject of doubt. 3. Morphology of the ovum ; homology of its parts, and relation of the ovum to other organic structures. Should the views be correct which have now been stated with regard to the relations of the parts in the mature ovarian ovum, ami the manner in which they are formed, it will be apparent that a strict homology or ana- tomical correspondence can be pointed out in regard only to some of the parts which are recognised under similar designations, as re- spectively belonging to the ova of different animals. All physiologists will probably be disposed to look upon the germinal vesicle or ovigerm as corresponding or homologous in the ova of all animals, and, notwithstanding the great differences known as to its more simple or multiple condition, the same view may also be taken of the structure known as nucleus or macula. The primitive or finely granular yolk-substance, more especially that which immediately surrounds the germinal vesicle, and is afterwards employed in the formation of the blastoderm or embryogerm, seems also to have a similar origin, structure, and relation in all animals. But beyond this it is more difficult to trace the homological correspondence ; for under the names of cellular yolk-substance and vitelline mem- brane it appears that there have been brought together parts of which the origin, structure, and relations may be dissimilar in different animals. There seems at least to be sufficient reason, from what is already known of the varieties of the enclosing membrane, or so- called vitelline membrane, to establish a dis- tinction between several forms of that struc- ture; as, for example, between the vitelline membrane, which exists from the earliest period as a pediculated sac in connection with the ovarium, as in Holothuria ; that which is derived from the extension of the wall of the original germ-cell in grouped ova, such as have been described by Meissner in Gordiacei ; that which is later formed round the ovum of Mammalia as a zona pellucida, by the consolidation of the outer layer of the primitive basement substance of the yolk ; and that which in the bird and other animals whose ova are similarly constituted, appears to derive its origin in part, at least, from coalesced cells corresponding to those of the tunica granulosa of the ovarian capsule on the exterior of the cellular yolk. With regard to the cellular yolk itself, we must refrain from any attempt to establish its homology till we shall be more fully ac- quainted with the mode of its production ; for it is still undetermined whether it arises by cell formation within the primitive vitelline OVUM. [13J] membrane through some change in the sub- stance of the primitive yolk, or whether it is derived, as I am inclined to believe may be the case in birds and some other animals, in a space external to these parts, and more in connection with the cellular contents of the ovarian follicle. In limiting, then, our comparison to the parts of the ovum in a bird and a mammifer, we may regard the germinal vesicles as homo- logous in both ; the finely granular germinal disc of the bird's ovarian egg as homologous with the whole vitellus of the mammiferous ovum ; the zona pellucida of the mammiferous ovum as temporarily represented by the clear outer band of the primitive yolk, which is seen in the bird's ovarian ovum when of a diameter of from ^ to J^ of an inch ; the cellular yolk of the bird's egg, and its enclosing vitelline mem- brane as probably homologous with the fluid and granular contents and lining tunica granu- losa of the ovarian follicle of the mammifer, and not by any means with the corpus luteum of a later period, as has been suggested by some. The albumen of the bird's egg has its homo- logue perhaps in the similar deposit which the ova of several Mammalia acquire in their descent through the Fallopian tube. The chorion of the ovum of Mammalia, being an after growth, has probably no true homologue in the bird's egg, unless we should regard the shell membrane and shell as occupying this place. Many ovologists have thought it of import- ance to establish a comparison between the ovum or its parts, and some one or other of those microscopic structures which, since the publication of the discoveries of Schleiden and Schwann, have been known as organised cells. Schwann himself, though looking upon the entire animal ovum as a cell, entertained some doubts as to the exact nature of the comparison to be instituted for its several parts. These doubts are not yet removed, and the progress of knowledge has tended rather to diminish than to increase the ap- propriateness of the comparison, more espe- cially from the somewhat various and indefinite nature of the bodies which are now recognised as organised cells. It cannot be denied that, if we regard merely the structure of the simpler ova of small animals, we find in them the general characteristics of an organised cell, as these have been usually understood ; that is, we find the external structureless vesicular cell- wall, the internal granular contents, and the internal nucleus or inner cell with its nu- cleolus. But when we consider more fully the whole history even of the most simple ova, and extend our regard to the structure and history of the more complex forms of ova, we perceive many circumstances which render the comparison with detached animal cells inapplicable. Leuckart remarks, in his article Zeugung, previously referred to (p. 815.), that if we attempt to deduce the most general result from what has been ascertained as to the formation of the ovum, it is this, that " the animal ovum is formed by deposit round the germinal vesicle." The progressive forma- tion of the parts of the ovum, therefore, would appear to differ widely from that which Schwann held to occur in most, cells. But our whole knowledge of the various forms and modes of production of cell-like struc- tures has been extended, and has undergone some modification since the time of Schwann ; and there are now known to be not a few cell structures which are formed by external de- posit of matter round a nucleus, nearly in the same manner as occurs in the ovum. In this view, therefore, the simpler kinds of ova might be regarded as examples of " deposit cells." The great variation in the magnitude of different ova, and the prodigious size which some of them attain, as compared with the minute and generally microscopic size of the organised cells of the animal body, cannot by itself be received as a conclusive argument against the cellular constitution of the ovum; but the complexity of its structure, its rela- tion to fecundation, the peculiar micropyle of the outer wall in some, the separation of the germinal from the nutritive part of the yolk- substance, and the new formation of cells after segmentation in a limited or more ex- tended space over the yolk in the interior of the vitelline membrane, are so widely different from any thing belonging to the history of other cells in the animal body, that we are forced to regard the ovum rather as a struc- ture of a peculiar kind than as a mere modi- fication of a cell. The germinal vesicle it might be held, both in its structure and its mode of origin, merits, more justly than the whole ovum, the com- parison with an organised cell. But even in its history there are points of difference, and we are still too little acquainted with the mode and consequences of its disappearance at the time of the maturation of the ovum, to warrant our making more than a vague and general comparison of the germinal vesicle to an organised cell. In admitting that the ovum, or its germinal vesicle, present some of the features of the organic cellular structure, we ought always to bear in mind that they are the source of all the other cells from which the animal body is developed. The manner of the very first origin of the germ of the ovum is still involved in obscurity, for we only know of the existence of an ovi- germ when the germinal vesicle has attained an appreciable size. Whence the first germs of the germinal vesicles proceed can as yet be matter only of conjecture. Without enter- ing here upon the debated ground of the origin of organised cells in general *, I would venture to express the opinion, that as there is no ovigerm without a parent, so there is no new organisation without previously existing, and at some time or other connected, orga- nisation. Hence, notwithstanding the appa- * See upon this subject the very interesting and suggestive Review by Mr. Huxley iu the Brit, and For. Med. Chir. Review for October, 1853. [K41 1136] OVUM. rent isolation of the origin of cells in blastema or intercellular substance, it might still be held that the unseen germs of new cells con- tained in that blastema may have derived their origin from other cells or organised parts proceeding from cells. And thus, in regard to the first origin of the ova of animals, it is fair to conjecture that the germs from which they spring have taken their descent from parent cells or structures derived from cells through the organs appropriate to their form- ation. But here observation fails to assist us further, and we are lost in the region of speculation. If, however, with the reservations now stated, it should be thought desirable to compare the ovum to the organic cellular structures, the germinal vesicle may be re- garded as the simple cell of the ovum, the whole ovum as a complex cell ; the first of these being formed probably by expansion from a minute point or molecule, the second by superposition or external deposit round the internal cell ; but both at the same time presenting features which are peculiar to themselves, and different from those which characterise other cells of the animal eco- nomy. The different and separate formation of the germinal vesicle and yolk, which is perceptible to some extent in the ova of most animals, is placed in its most striking point of view by those instances in which, as in Tre- matode and Cestoid Entozoa, there are dis- tinct germigenous and vitelligenous organs, and those in which, as in Isematoidea and Insecta, the ovary is tubular, and the forma- tion of the several parts of the ovum goes on progressively in different parts of the tube. 4. Phenomena attendant on the maturation of the ovum, and its discharge from the ovary. The ovum naturally undergoes in the ovary a progressive development till it arrives at the state of maturity, when it is usually separated from the ovary by a process of dehiscence, is conducted through the female passages either to be excluded or laid, as in oviparous ani- mals, or to be retained in a uterus or other part of the female organs in viviparous ani- mals during uterogestation. The maturation of the ova and their separation from the ovary is in many animals periodical and inde- pendent of fecundation. This natural peri- odical separation of the ova has been termed Ovulation by some authors.* The change which the germinal vesicle undergoes at the period of the maturation of * The observations of Bischoff had long ago shown that in the periodical dehiscence of ova which accompanies the heat of female quadrupeds, the ova may be detected, though unfecundated, in the course of their descent through the Fallopian tubes and uterus (Periodische Losreifung, &c. , 1842), and some observations appear also to have shown that the same is the case in the human female at the periodical return of menstruation. (See a paper by H. Letheby, M. B. in the Philos. Trans, for 1851, p. 57., where two cases are described in which ovules or their remains were detected in the Fallopian tubes of unimpregnated women who had died at or ubout the menstrual period.) the ovum has naturally attracted much at- tention, from the hope that through the ob- servation of this phenomenon some explanation might, be obtained of the first origin of the germ round which, after fecundation has taken place, the segmenting and organising stratum is collected, from which the blastoderm is produced ; but it must be allowed that as yet little success has attended our efforts to de- tect the connection, if it exists, between these two processes. In almost all animals the germinal vesicle is lost to view at the time of the maturation of the ovum, and generally before or about the time when the ovum leaves the ovary. In large-yolked ova the macuke of the germinal vesicle become very numerous by their multiplication and sub-divi- sion at an early period j while in the small- yolked ova, as has been observed in a few animals at least, the increase in the number of the maculas does not take place till imme- diately before the diffluence or disappearance of the vesicle. The more minute phenomena of this diffluence are as yet very imperfectly- known. In some animals, as Mammalia and Birds, it has been observed that shortly before the diffluence of the vesicle, its delicate wall undergoes a softening or approaching solution, so as to make it impossible to separate the vesicle entire. After this, when the diffluence is complete, the contents dis- appear from the situation they have previously occupied, but what becomes of them has not yet been determined. In some instances, as Birds and Batrachia, it has been observed that, after the diffluence of the germinal ve- sicle, the germinal part of the yolk, which previously consisted exclusively of small opaque granules, is now mingled throughout with clear hyaline spherules, somewhat similar to the dispersed macula; of the germinal vesicle ; but it is only matter of conjecture that these clear spherules have been derived from the germinal vesicle or its maculas. In a few instances, as in Ascaris, it has been thought that the entire nucleus or macula of the germinal vesicle has remained undivided, and it has been supposed that it has of itself constituted the germ of the embryo-cell, which afterwards occupies the centre of the first segmenting mass of the yolk, and whose progeny by division exists as nuclei in the interior of the successively in- creasing segments of the cleaving germinal portion of the yolk. A recent observation by J. Miiller seems to lead the way to a different view of this phenomenon. He has observed * in one of the Mollusca, the Ento- choncha mirabilis, that the germinal vesicle does not disappear or undergo a change at the time of the maturation of the ovum, but remains discernible as the foundation of the clear embryonic-cell which occupies the centre of the yolk mass when segmentation is about to take place. Reinak f has been led, by his observations on the Batrachian ovum, to * Archiv. dor Physiol. 1852. Leydig in the same. + Untemich. iiber die Entwickel. der Wir- berthiere. OVUM. [137] doubt the correctness of the view hitherto generally adopted as to the entire disappear- ance of the germinal vesicle in that instance, and holds it as probable that a part of it at least remains in connection with the forma- tion of the embryonic cell. These statements are sufficient to show that the phenomena of the dehiscence of the germinal vesicle and its relation to the subsequent changes in the ovum induced by fecundation are as yet very imperfectly understood, and that the discovery has still to be made of the link in the chain of connection between the last stage of existence of the ovigerm, and the first origin of the nucleus round which the subsequent organising process of segmentation begins. But that some such connection exists, all who have made a study of this part of the his- tory of the ovum are inclined to believe, 5. Relation of the ovum to fecundation by the male sperm. The act of fecundation is necessary for the perfection of all true ova. In the production of gemmae or buds, in the multiplication of nonsexual individuals, and in the various examples of Metagenesis previously referred to, the germs from which the new products arise may be nucleated cells or groups of these, and may without doubt be the descend- ants of the original cell-germs of ova ; but for their development into the new beings produced from them, no combination, so far as is yet known, with the product of cells of a different kind, as in fecundation, is necessary. It is otherwise with all true ova. Their germs may be the descendants through the ovary of an original cell-germ, from which the animal bearing the ovary was produced ; but for the generation of an ovum the ovigerm must be subjected to the influence of the sperm, and for its development there is re- quired a new process of organisation, inaugu- rated by segmentation, which is the invariable consequence of fecundation, and is the first obvious change in a fecundated ovum leading to embryonic formation. The developed form of the spermatic sub- stance * is in by far the greater number of ani- mals that of minute ovoid or rounded particles of various form, with each of which is connected a long and extremely delicate filament, which moves with vivacity in a vibrating or oscil- latory manner when immersed in water and various bland animal solutions. There are other less common forms of spermatozoa, such as those of Crustacea and Nematoidea, which have not the filamentous appendage, and are motionless. The vibratory motion of filamentous spermatozoa bears some resem- blance to that of some kinds of fine cilia, and is the most apparent indication of the active state of their vitality.f It is now ascertained beyond doubt that in a number of animals the spermatozoa come into direct contact with the yolk substance * See the article SEMEN. t See especially the recent researches of Kullikcr on the Sperm iu Zeitsch. fur. Wigsensch. Zool. v ol. vii. and embryogerm, or with the internal con- tents of the ovum. The actual entrance of the spermatozoa into the ovum has been observed in Mammalia, Batrachia, Osseous Fishes, Insects, Nematoid Worms, some Mollusca, and Echinodermata ; and there have been ascertained circumstances regarding the ova of other animals which warrant the inference that the spermatozoa enter the ovum in many more than those in which the phenomenon has already been actually ob- served. After long continued doubt and much discussion of this point, physiologists are therefore now generally agreed that in all instances a direct action of the sperma- tozoa in substance on the contents of the egg is necessary to fecundation. The manner of access of the spermatozoa to the interior of the ovum is probably various in different animals. In a few, as Trematode and Cestoid Entozoa, the sperm is mixed with the contents of the ovum, viz., the germinal vesicle and yolk, at the time when these are brought together from the separate organs in which they are formed : in some, as the Nematoid Worms, and probably also in some other animals, the sperm comes in contact with the ovum previous to the formation of an en- veloping membrane ; in a third set it seems probable that, as in Lumbricus, and perhaps in some Mollusca and Hirudinea, the vitelline membrane which had existed at an earlier period is dissolved or removed previous to fecundation, and the ovum or yolk substance and germ are thus left directly exposed to the action of the spermatozoa, which in Lum- bricus have been observed in great numbers penetrating the substance of the yolk. In the majority of animals, however, the sperm only reaches the ovum at a later stage of its formation, when it is already covered by the vitelline membrane or other envelopes, and through these coverings, therefore, the spermatozoa must pass to gain access to the yolk and germ. In a certain number of ani- mals the vitelline or enveloping membrane appears to be quite entire and closed on all sides, so that, as in Mammalia, in which Martin Barry was the first in 1843 to perceive with certainty the entrance of the spermatozoa into the ovum, these bodies must in some way, not yet fully known, pass through the consistent wall of the enclosing membrane ; but in other animals, as first discovered by J. Miiller, a special aperture or perforation of the egg-covering exists, apparently destined to allow of the more rapid entrance of the spermatic bodies. This micropylc apparatus, sometimes consisting of one, and at others of a number of apertures, has now been observed in several Echinodermata, in Acephalous Mollusca, in all Insects, and in Osseous Fishes ; and it is more than probable that it exists in a considerable number of other animals in which it has not yet been detected. But still, making clue allowance for the probable extension of discovery in this direction, the care and accuracy with which the micropyle apparatus has since its first discovery been sought for without success in Mammalia and [138] OVUM. some other animals, in which, had it been present, it could scarcely have escaped so careful a scrutiny, warrant the belief that in a certain number of animals the spermatozoa do actually penetrate the apparently entire egg- covering. It is not my design to enter here upon the consideration of the mode and nature of the action exerted by the spermatic matter or the spermatozoa in producing the changes of fecundation. Upon this subject the reader may with great advantage and interest consult the latter part of the article Semen in this Cyclopedia by R. Wagner and Leuckart, the papers of the late Mr. Newport in several recent volumes of the Philosophical Trans- actions, and the learned article by Professor Leuckart on Generation contained in the fourth volume of R. Wagner's Handbuch der Physiologic. I will only remark in passing that from Mr. Newport's and other researches it appears that while the actual mixture of an appreciable quantity of the spermatic sub- stance is necessary to induce fecundation, the extreme rapidity with which the action takes place, the minuteness of the quantity of the spermatic matter which is sufficient to induce it, and the fact now observed in a variety of instances that the spermatozoa which have entered the ovum remain apparently little changed for a considerable time after the changes of the ovum consequent on fecunda- tion have made some progress, lead to the conclusion that there is something in the nature of this action inconsistent with the idea that it is one of mere combination in substance of the developed contents of the male and female generative products. But whether this is to be referred to the class of " contact actions " of which themselves so little is known, or to what other kind of action it may be compared, the ascertained facts do not enable us in the least to deter- mine. The almost universal presence of vi- bratory motion in the spermatozoa during the time in which they retain their fecundating power, naturally led physiologists to connect that motion with the fecundating action ; but on the other hand, the occasional, though rare examples in which the spermatozoa are en- tirely motionless, seem sufficient to cause the rejection of the view that the force which produces the vibratory motion is identical with that which calls forth the series of histogenetic and organogenetic changes which result from fecundation. But the consideration of this subject would lead us into the discussion of the whole question of vital forces, which in its present unsatisfactory state it is desirable to avoid. The physiologist agrees, for the sake of con- venience of expression, to adopt the terms of power, property, force, &c., to denote the con- ditions necessary for the occurrence of certain actions or changes. He employs the term vital force merely as the indication of the supposed cause or causes of an ascertained regular sequence of vital phenomena ; but all philosophical accuracy rejects the idea of any unseen separate and single force which is at work in bringing about the sequence in ques- tion. The fecundating power of the semen is an expression used only for convenience to denote the invariable sequence or relation as cause and effect which has been observed to subsist between the contact of spermatic matter with the ovum, and the changes in the latter which follow on the act of fecundation. We might with as much propriety have given a name to a separate power residing in the egg or its germ which render it susceptible of fecundation, as of a special power belonging to the semen by which that susceptibility of the ovum is acted upon. The efficient cause of the process of fecundation can only be educed, as in all physical as well as vital changes, from a perfect knowledge of all its phenomena, and the statement of the efficient cause of such actions is only the expression of the most general and best known law to which a full acquaintance with the phenomena enables them to be reduced. Fecundation is to be regarded as a purely vital change, seeing that it takes place only in the usual conditions of vitality; but, like all other vital changes, it appears more probable that a variety of conditions of the organic matter rather than any one known property or condition are necessary for its occurrence. In endeavouring to deduce the most ge- neral phenomena which accompany this re- markable change, it may be said that fe- cundation consists essentially in the mutual action of two different organised bodies, which are respectively formed from two different cells ; the ovigerm and the sperm- germ. If we may form any general con- clusion from what may be so well observed in Nematoid Worms, the development of the ovum and the spermatic cells from their re- spective germs is remarkably similar, for in both the internal cell is developed from a minute molecule from within, while the ex- ternal part is deposited from without. The spermatozoa are formed in connection with the nucleus or nuclei of the sperm-cell ; and the germinal part of the ovum, though it con- sists mainly of the granular part of the yolk, which is directly formative, very probably comprehends also in some shape or other the effused contents of the germinal vesicle. In this way, then, we may conjecture that in the act of fecundation the products of the original cell-germs meet and combine or mutually influence each other. The cell-germs, then, are the links in the chain of organic connec- tion between either or both the parents and the progeny capable of being developed from the fecundated ovum. Such a view, though still in a great measure speculative, seems to be in accordance with the facts known as to the perfect transmission of the structure and qualities of either or of both parents to the offspring.' 6. Immediate effects of fecundation on the ovum ; segmentation, and first changes of the ovum related to the commencement of em- bryonic development. It does riot come within the scope of the present article to describe in detail the pro- OVUM. [I39J cess of fecundation, or the phenomena which follow it, but it may be proper to state here in a general way the relation which subsists between the earliest changes occurring after fecundation, and the commencement of those phenomena of a histogenetic nature which precede the formation of the embryo itself. The most obvious and constant of these changes is that known as the cleavage or segmentation of the yolk, a process which has been observed in the ova of all animals, and is not less interesting from its own na- ture, than from the bearing of its phenomena upon the explanation of the earliest organising process of embryonic development, and upon the whole subject of histogenesis. The segmentation affects only that part of the ovum of animals which is directly ger- minal or formative ; and it results in the pro- duction of that layer of organised cells, of variable extent, in the centre of which, in a determinate position and direction, the rudi- ments of the embryo are first formed. The process of segmentation is, therefore, the pre- lude to the formation of the Blastoderma or germinal membrane of Pander. The extent, therefore, to which segmenta- tion affects the yolk differs greatly according to the amount of the yolk-substance which is directly germinal ; that being in some animals the whole, and in others only a fraction of the yolk, in proportion to the part which is only indirectly nutritive. In that group of ova, then, to which those of Mammalia belong, and which we have called the small-yolked, the entire yolk, or, at all events, its superficial layer, being directly formative, or being in- volved from the first in the production of the Blastoderm, the segmentation is complete, or the process of cleavage affects the whole mass of the finely granular yolk within the zona or vitelline membrane. In those ova again, such as we find in the bird among vertebrate, and the cuttlefish among the invertebrate animals, in which the formative yolk has the most limited extent, and consists only of a finely granular disc near the surface of the much larger mass of the cellular nutritive yolk, the segmentation is confined to that disc alone, and is therefore, in some respects, widely different from that which occurs in Mam- malia. In the intermediate group of ova be- longing to Batrachia and Osseous Fishes, there are many gradations of transition from the complete to the partial cleavage, so that in some, as the common frog, it is nearly, but not entirely, over the whole yolk; while in others, as in the salmon or osseous fishes, it does not extend over more than a third of the surface of the yolk.* * The more important phenomena of the yolk- germ cleavage or segmentation have been ascer- tained principally by the following observations: viz. 1st. of Frevost and Dumas in Batrachia, as early as the year 1824, and subsequently of Rusconi and Von Baer in the same animal ; 2nd. of Bischoff and Barry in Mammalia, in 1838-39, their obser- vations being confirmed by myself in 1811), and greatly extended by Bischoff before the publication of his work upon the development of the rabbit, in 1842; 3rd. of Bagge in 1841, and of Kolliker in In the greater number of instances there is recognised in the mass of the whole yolk, or in its germinal part, immediately previous to its undergoing segmentation, a clear simple cell, generally nucleated, which was not be- fore perceptible ; to this the name of embryo- cell has been given, in order to distinguish it from the germinal vesicle, from which it has hitherto been believed it is in some measure distinct. In other instances a clear spherule or space only is observed in the place of the embryo-cell, and in a few animals no clear part of this nature has yet been detected. The division of the embryo-cell accompanies, or rather immediately precedes, that of the germ-yolk, so that each mass formed by the cleavage, grooving, or segmentation, as the case may be, contains as its nucleus or centre an embryo-cell, or clear spherule of its own, descended from the first cell or spherule of the same description. The process of segmentation, whether it involves the whole ovum, or is limited to a larger or smaller disc of the yolk, proceeds in most animals with a certain degree of geo- metric regularity, so that the number of germ- yolk segments are successively multiplied so as to be in the numbers 2, 4, 8, 16, 32, 64, &c., until by the ultimate division a vast number of small globular masses are formed, which occupy principally the surface of the yolk over all its germinal portion.* The last result of the segmentation is the production of the blastoderma or germinal membrane in which, by other changes, the rudiments of the embryo subsequently make 1843, on Nematoid Worms ; 4th. of C. Vogt in the Salmonidie and in the Alytes Obstetricans in 1842 ; 5th. of the same author, of Quatrefages, and many others in various invertebrate animals ; 6th. in its most limited form the phenomenon was first well described by Kolliker in Cephalopoda in 1844 ; and 7th. in birds by Bergmann in 1846, by Coste in 1848, and by myself in the following years. The observations on this subject are far too numerous for quotation ; those especially which have been made in experiments by artificial fecundation are most favourable to the" investigation of the seg- mentation and other phenomena which follow immediately on fecundation. And in all these instances, as well as in very numerous others, the occurrence of segmentation and the regularity of its phenomena are so constant that we may regard it as one of the best established series of facts in organic nature. The observations with regard to segmentation in the ova of insects, which are still imperfect, form the only exception to the foregoing statement with which I am acquainted. ' Reference is here made chiefly to the best- known and most common kind of segmentation, in which this process consists in the massing of the granular and fluid substance of the yolk round the embiyo-cells or clear spheres as centres ; but it is right to state that there is another form of this process, as yet only observed in some of the Cestoid and Nematoid Entozoa, in which the yolk, either clear or granular in its structure, does not appear to follow the divisions of the embryo-cells, but the gradually increasing progeny of the latter assi- milate or combine more and more with the yolk, so that at last, when the germinal part of the ovum is entirely occupied with new cells, the original yolk has quite disappeared. The nature of this process, as compared with the more common form of yolk segmentation, is not perhaps as yet fully understood. [HO] OVUM. their appearance. According to most ovo- logists, the last globules formed by segmenta- tion are the nucleated organised cells im- mediately constituting the blastoderma. A different view of the process, however, in Mammalia, has been taken by BischofF, very decidedly set forth in his two most recent works on the development of the guinea-pig and the deer ; according to which the last resulting spherules formed by segmentation are not true cells, and that previous to the formation of the blastodermic cells, the yolk- germ falls completely into an amorphous or homogeneous finely granular substance, out of which, secondarily, the blastodermic cells are produced by a process of cytogenesis. It seems probable that, in the different classes of animals, there may be considerable variety in the degree of perfection in organisation or ad- vance in cell-structure to which the segments of the yolk have attained at the period when the development of the embryo begins to ma- nifest itself. But in the higher animals at least the weight of evidence appears to me in favour of the view that the process of segmentation results directly in the formation of blastodermic cells. The fact now established by the obser- vations of Reichert in Eutozoa, in 184-1, of Ransom in osseous fishes, and more particu- larly those of Remak in Batrachia, that a de- licate membrane is formed over the surface of each of the segments as they appear, and that the last and smallest segments possess a deli- cate membranous envelope, appear to show that, in these animals, each segment has the structure of an organised cell, and is very si- milar to, if not identical with, those of the blastodermic lamina. The origin of the embryo-cell is still in- volved in obscurity. Most ovologists are dis- posed to connect it in some way or other with the previously existing germinal vesicle, or some part of its contents, and more especially the nucleus. For the solution of this ques- tion, as already remarked, a more accurate knowledge of what happens to the germinal vesicle at the time of that disappearance which has been so commonly observed at the period of the maturation of the ova of almost all ani- mals, will be required. Does the macula re- main, as has been alleged by some, to form the nucleus or the whole of the embryo-cell? Or, in other cases, if the multiplied maculae are dispersed among the granules of the ger- minal volk, are they again collected together into a mass or spherule to form the embryo- cell ? Or, again is the embryo-cell formed out of other materials, and not necessarily either partially connected with, or wholly de- rived from, the germinal vesicle ? And finally, might it not be, according to some recent ob- servations, such as those of J. Mu'ller on En- tochoncha and those of Remak on the frog, that the disappearance of the germinal vesicle is not attended with the dispersion of its con- tents, but is a phenomenon caused only in a certain number of animals by the solution of the delicate external wall of the vesicle, and by some change in the position and consist- ence of its contents ? Further observations will be required to determine this point ; but if in the meantime we regard it as most pro- bable that the embryo-cell is in some way or other connected in its origin with the germinal vesicle, we might further found upon this the speculative view that the blastodermic cells and the blastema from which unques- tionably, by a histogenetic process of cell-di- vision and multiplication, the various textures and organs of the animal body are produced, may be regarded as the descendants of the original cell-germ from which the ovum was developed combined with the sperm. We should thus trace the organic cellular connec- tion between the succession of parents and offspring, which I have stated to be one of the most general facts in organised nature. The observations respecting the very re- markable movements of the yolk, before and during the earlier stages of the segmenting process which have now been recorded by several physiologists, must excite the liveliest interest, and suggest subject for much reflection a.s to the evidence they may afford of the causes of this change, or, if we may use the expression, of the forces by which segmenta- tion is brought about. There seems to be little doubt that the embryo-cell (and its nu- cleus first of all) is the earliest to become di- vided, and that the process of cleavage then proceeds from the surface of the segmenting mass inwards towards the cell ; but in what relation the nucleus, granular substance of the yolk, and ovicell-membrane stand to each other in this process, must be left to be de- termined by future researches. Of the other early changes in the ovum which immediately follow fecundation and precede embryonic development little need here be said. They consist principally in the greater degree of consolidation and compact- ness acquired by the germinal part of the yolk, and in the formation in most animals of a clear space between the surface of the yolk- substance and the enclosing vitelline mem- brane. It is in this clear space, or, as it has been called by Newport, respiratory chamber, that the spermatozoa have been observed in those instances in which they have been as- certained to penetrate into the cavity of the ovum. There is another phenomenon of the same period, which has now been so frequently observed, and which is of so peculiar a nature, that it must not be passed over without no- tice ; I allude to the appearance in the re- spiratory space of one or more clear and highly refracting spherules, nearly of the size of the germinal vesicle, but quite independent of it. These clear hyaline-like globules have been observed in the ova of Gasteropodous Mol- lusca after fecundation by almost all those who have attended to the ovology of this class of animals, among whom may be mentioned Dumortier, Pouchet, Van Bcneden, Nord- maun, and Vogt ; in the Annelida by Quatre- fages ; in Mammalia by Bischoff and Barry ; and in Batrachia by Newport. From the observations of Quatrefages in Hermella they appear to be excluded or expressed, as it were, from the clear basement-substance of OVUM. the yolk ; and Bischoff states that they gra- dually disappear, or are dissolved without obvious change. We are at a loss to deter- mine what office these globules may have in connection with the changes of the ovum at the time they appear. Lastly, I would notice the interesting re- lation which appears to subsist between the situation of the germinal vesicle and the cen- tre of the germinal membrane afterwards formed, or the germinal pole of the ovum, and the conformity in the direction of the line of the first cleavage of the yolk with that of the principal axis of the embryo in verte- brate animals. The first fact has been observed in all animals, and the latter has been ascer- tained by Mr. Newport's researches in Ba- trachia, and by observations which I have myself made in the bird's egg during its de- scent through the oviduct. These facts, as yet inscrutable in their nature, point to in- teresting laws relating to the connection of the first phenomena of development, which may be worked out by the further prosecution of these inquiries. In the preceding part of this article we have considered chiefly the anatomical struc- ture of the ova of animals, and have made little mention of their chemical composition. The knowledge of the latter subject is as yet very imperfect. In a recent Memoir* Messrs. Valenciennes and Fremy have given an ac- count of an extended series of experimental researches in which they have been engaged, with a view to determine the differences in the chemical composition of the ova of dif- ferent animals, and although this investigation is still necessarily incomplete and fragmentary, they appear already to have arrived at some interesting results. The following are some of the more important of these results. The albumen or white is not exactly of the same composition in the eggs of different birds ; but it generally contains albumen with salts and a compound of sulphur in solution. In the yolk of birds' eggs they recognise the principle first distinguished by Dumas and Cahours as Vitelline, a substance precipitable by mixture with a large quantity of water, and apparently more nearly resembling fibrine than albumen in its composition and some of its properties. The phosphuretted fat of the yolk is somewhat similar to the cerebral fatty matter. The glairy white of the eggs of cartilagi- nous fishes is very different from that of birds' eggs, being neither soluble in water nor coa- gulable by heat nor acids to the same degree. It seems to contain only traces of organic matter. The angular and tabular particles of the yolk of cartilaginous fishes are com- posed of a principle which these authors re- gard as peculiar, and have named Iclithinc. This substance is insoluble in water, alcohol, and ether, and, on being dissolved by hydro- chloric acid, gives no violet colour, as albu- men does. It is dissolved by all the concen- * See Journal cle Pharmacie, &c., vol. xxv. pp. 321. and 415., and vol. xxvi. p. 5., 185"4. [141] trated acids, and by dilute acetic and phos- phoric acids. Its composition is stated to be as follows ; carb. 51 ; hyd.^6'7 ; nit. 15; ox. 25' Pancreas, Fr. ; die Bauchspeicheldruse, Germ. ; Pancrea, Ital.). The pancreas is an azygous, non-symmetrical, glandular organ, possessing a duct, and, therefore, belonging to the category of true glands; connected anatomically with the alimentary canal, and physiologically with the function of digestion. On taking even the most superficial and general view of this organ, two or three things cannot fail to strike the attention : one is, the close affinity and strong contrariety which it at the same time displays to the salivary glands f affinity in point of appearance and structure, being, like them, a typical represen- tation of a conglomerate gland, contrariety in point of situation and function, the one being placed at the very threshold of the ali- mentary canal, the other at an advanced po- sition in it ; the one acting on raw material, and having in part at least a mechanical use J, the other having to do with material far gone in the process of assimilation, and possessing a function, whatever it may be, certainly not mechanical in any degree. Another striking circumstance is its wide diffusion. It exists in all vertebrata ; mam- malia, birds, reptiles, and most fishes, all possess a pancreas, and that quite independent of what the nature of their food may be, animal, vegetable, or mixed ; a fact that one would have imagined would itself have pre- vented the adoption of the old views with regard to its function. Another circumstance, not less striking, is its constant relation to the duodenum : whatever may be the other modifications of the alimentary canal, from the straight and simple tube of some carnivora to the volu- minous apparatus of the vegetable feeders, or whatever may be the modified form of the pancreas itself, still, if the organ exists, its relation to the duodenum is invariable ; if there is a duodenal fold, it is placed in it ; and if there is not, it makes the closest approxi- mation to an analogous position that is pos- sible : indeed the uniformity of this relation is so invariable, even under circumstances where it would appear to be indifferent, that one cannot but regard it as one of those in- stances of conformity to type in which uni- formity appears to exist for uniformity's sake. The arrangement of the subject of this paper that most naturally suggests itself, is to treat first of the structure, and then of the functions of the organ. I shall therefore arrange my observations under the following heads : 1st. The descriptive anatomy of the human pancreas, including an account of so much of its structure as may be made out by a naked- eye examination. 2nd. Its minute or general anatomy. f, all flesh. This striking resemblance suggested to the older anatomists the name of "salivary gland of the abdomen," an appellation first given to it by Siebold. Historia Systematis Salivalis. J See Bernard's experiments on the secretion of saliva. Supp. 3rd. Its comparative anatomy, including those modifications both of the form and ulti- mate structure of the gland that the animal series exhibits. 4-th. The physiology of the pancreas, the role that it plays in the function of digestion. Lastly. A short account of some of the most striking pathological changes that the organ is liable to. I. HUMAN ANATOMY. Situation. The pancreas is so placed that for its display it is necessary to open the ca- vity of the great omentum. This may be done either by dividing the gastro-hepatic omentum and depressing the stomach, or by detaching the gastric layer of the epiploon and turning the stomach up, or by dividing the transverse mesocolon and turning up both transverse colon and stomach. In either of these ways the cavity of the omentum is opened, and the organs concealing the pancreas are removed. Placed transversely across the upper part of the abdominal cavity, and closely applied to its posterior wall, the pancreas extends from the duodenal fold on the right to the hilum of the spleen on the left, across, therefore, the epigastric into both hypochondriac regions. It is not perfectly transverse, however, but ex- tends from the right a little upwards as well as to the left; it corresponds to the level of the first and second lumbar vertebrae and to the splitting of the two laminae of the trans- verse mesocolon : it is post-peritoneal, being invested by that membrane only on its an- terior surface. Relations, By its right extremity it is closely engaged in the curvature of the duo- denum, to the inner border of which it is intimately attached, and which it often re- ceives into a groove more or less deep, formed by a projection of the gland to a slight extent on the anterior and posterior surface of the intestine. Sometimes this groove is very slight, and the relation of the margins of the gland and intestine merely that of apposi- tion ; at others, the inner margin of the duo- denal fold will be deeply imbedded in the gland substance, the projection both in front and behind being considerable. More fre- quently, however, the gland trespasses much further behind the intestine than it does in front, so much so occasionally as to separate it in a great degree from its posterior rela- tions. The structures on which the posterior surface of the pancreas rests are, the vena cava, the bodies of the vertebrae, the aorta, the crura of the diaphragm, the left kidney, its supra-renal capsule and renal vessels, the lower part of the solar plexus with the com- mencement of the plexuses thence pro- ceeding, as the aortic and superior mesenteric ; the splenic vein passing from left to right, the superior mesenteric vein and artery, the vena porlae, the common bile duct, many lym- phatic vessels and glands, and the commence- ment of the thoracic duct and vena azygos. To all these structures it is intimately at- tached by cellular tissue, and to the irregular surface which they form it is, as it were, moulded or modelled, so that when it is care- G 82 PANCREAS. fully dissected away, it presents eminences for instance, the longitudinal furrow occupied and depressions corresponding to them, as, by the splenic vein, and the deep groove in Fig. 51. Human Pancreas, sliou-n in situ by throwing up the suliject in which the curvature of the head of the ticularly well shown. which the superior mesenteric artery and the vena portae are received. In front it is in relation with the posterior surface of the stomach, which rests on it when empty (tanquam pulvinar, Scemm.), and moves freely upon it; but when this organ is distended with food, it recedes from it, and its lesser curvature comes into more immediate relation with the gland. In cases in which the stomach is situated lower down than usual, as in emaciated individuals, where a great part of the small intestine occupies the cavity of the pelvis, the pancreas conies into relation in front either with the liver, or with the anterior wall of the abdomen, from which the gastro-hepatic omentmn alone separates it, and through which it may be easily felt. This disturbance of the normal relations al- ways exists, according to Cruveilhier, when- ever the vertebral column can be felt im- mediately behind the walls of the abdomen. The pancreas is also in relation, in front, with the angle formed by the ascending and trans- verse colon and with the commencement of the duodenum. The upper border is in relation with the splenic artery, for the reception of which it is grooved, and which often runs in a canal formed in the substance of the gland through stomach. This dftiwing was taken from a young pancreas, following that of the duodenum, was par- its entire length ; it is in relation also with the Spigelian lobe of the liver, with the first portion of the duodenum, and the cceliac axis. The lower border is bounded by the inferior horizontal portion of the duodenum, from which it is separated, near the middle line, by the superior mesenteric vein and artery, which notch it, and which also separate it from its reflected portion or head. The right extremity is engaged in the duo- denal fold in the manner described, and is in relation with the ductus choledochus. From the intimacy of its attachments to the duode- num, it always accompanies this intestine in its displacements, so that when the duodenum is situated lower down than usual, which happens in displacements of the stomach downwards, the head of the pancreas is al- ways removed in the same direction. The left extremity is in relation with the left kidney, and with the spleen upon which it is sometimes flattened and blunted, and sometimes slightly enlarged, and to which it is attached by the intervention of the splenic veins, which send many branches into its sub- stance : sometimes it does not extend quite so far as the spleen by half an inch or an inch. Shape. From its elongated form, the pan- PANCREAS. 83 creas has been described by anatomists as possessing a body and two extremities ; and of these extremities one, which is enlarged and clubbed, has been called the head; the other, tapering and acuminated, has been called the tail; the middle portion, the great mass of the gland, being the body : other describers* have suppressed the body altogether, and de- scribed as the tail all that portion which is not included in the curved intumescence at the right extremity, which they designate the head. Indeed the imaginations of anatomists have been largely drawn on to supply analogies whereby to illustrate the shape of this organ. Some have compared it to a hammer, some to a dog's tongue; among them all I think the best is that which compares it to a pistol. But, passing by these fanciful resemblances, the pancreas, from its transverse elongation and antero-posterior flattening, presents for description a right and left extremity, an upper and lower border, and an anterior and posterior surface ; and these parts 1 shall de- scribe in succession in the order in which I have mentioned them. The right extremity, to which the name head has been assigned, is that portion which is engaged in the duodenal curvature, and to which, from its occasional separation from the rest of the gland, the name of lesser pancreas has also been givenf : it differs from the rest of the gland in being thicker and more mas- sive, curved instead of straight, and situated on a more posterior plane. It is thus formed : when the pancreas, in passing from left to right, has arrived at the duodenum, it be- comes closely attached to that viscus, and follows its course, first downwards, and then to the left, passing by its extremity, behind the superior mesenteric vessels, for which it thus forms a sort of groove or channel. It is by the fusion and massing of this curvature that the head is formed ; but in very young subjects, and in the lower animals, the curva- ture of the pancreas is as conspicuous as that of the duodenum, and by separating it from its attachments, and straightening it out, all ap- pearance of head vanishes, and it becomes a long prism, or flattened cylinder of even thick- ness throughout. In the human adult, however, it is impos- sible thus to unravel and straighten the right extremity ; and the fusion of the parts has often proceeded to such an extent as entirely to obliterate the original curvature, the groove for the vena portae and superior mesenteric vessels being the only trace of its concavity. The left extremity gradually tapers, getting both narrower and thinner ; it is sometimes bifurcated, sometimes blunted and flattened against the spleen, and sometimes slightly enlarged ; it presents nothing for special de- scription. The upper border is much thicker than the lower, so much so, that some anatomists have described the gland as being prismatic. * Meckel, Manuel d'Anatomie. f Some anatomists, as Professor Ellis, make the head synonymous with the lesser pancreas, inde- pendently of this separation. In the middle the coeliac axis rests upon it ; to the right the hepatic artery and first portion of the duodenum are in contact with it, and to the left it is deeply grooved by the splenic artery. This groove does not run along the top of the border, parallel to it, but crosses it ob- liquely from behind forwards as it passes to the left extremity, curving over it, as it were, so that while the commencement of the ar- tery is behind the pancreas, its terminal branches are in front (see fig. 54.). Some- times this groove is converted into a canal, by the gland closing over it. The lower border, much thinner, is tilted rather forwards to the right, by the passage of the superior mesenteric vessels beneath it, which separate it from the third portion of the duodenum ; on the left the inferior me- senteric vein passes beneath it to join the splenic. The anterior surface looks upwards as well as forwards, and is convex both transversely and longitudinally ; it is the only portion of the gland that is covered by peritoneum : from this circumstance, as well as from its being the free surface, it is very smooth. The posterior surface contrasts strongly with the anterior, for being uncovered by peri- toneum, and closely applied to all those struc- tures against which it lies, it presents many irregular elevations and depressions, corre- sponding with the uneven surface which these structures contribute to form. Size and weight. The size and weight of the pancreas are liable to great variety, and hence different authors have stated them very variously. Wharton* gives its weight as five ounces ; Meckel, from four to six ; Cruveilhier states its limit as six ounces, but thinks that a healthy pancreas may be as small as two ounces and a half; Soemmerring also considers six ounces a maximum, but carries his minimum as low as an ounce and a half. According to Krause and Glendinniug, its usual weight is from two and a quarter to three and a half ounces. Its size is stated by Meckel as six inches long and one thick ; Ellis gives its length as seven inches ; Quain and Sharpey, from six to eight inches, with an average breadth of an inch and a half ; according to Wharton, its length is about five inches, its greatest breadth one and a half, and its thickness one inch. Of all these weights and measurements I think that of Wharton, which is the earliest, comes nearest the truth; only his measurement of length is too little. From a large number of obser- vations I find the average weight to be from four to five ounces, the length seven inches, the greatest breadth an inch and a half, and the thickness three quarters of an inch. It is smaller in women than men, but only in proportion to the difference of size. Scemmerring says-)- that it is proportionately larger in the foetus and the new-born infant * Adenographia, p. 72. f " In nondum nato homine et brevi adeo post parttim, majus pro corporis mole videtur quaiu in adulto." Corp. Hum. Fab. PANCREAS. than in the adult ; a statement that my own frequent observations have verified. The specific gravity of this gland, accord- ing to Muschenbroeck*, is, compared to water, as 2029 to 1000. General appearance. The best view of the external appearance of the pancreas is obtained on its anterior surface where it can be seen, through the peritoneum covering it, without any disarrangement of its structure. It is seen to be of a pale, clear, flesh colour, in which it strongly contrasts with the white cellular tissue investing it, with the yellow fat with which it is often surrounded, and with the grey and dingy coloured absorbent glands which lie contiguous. On looking more closely, it is seen to be mapped out into lobules, and this mapping out is sometimes more conspi- cuously marked by the septa of areolar tissue that separate the lobules being loaded with fat. The lobules are of very various shapes and sizes, from an eighth to three quarters of an inch in diameter, closely packed so as to fit into one another, and presenting an even general surface. But on a closer examination we see that these lobules are themselves sub- divided by less conspicuous septa into a great number of smaller lobes ; and these again, especially if assisted by a little separation with a fine knife or needle, are seen to consist of numerous minute granules, or acini, as they are termed, which, as far as the scrutiny of the naked eye goes, appear to constitute the ultimate structure of the gland ; but, as we shall see more fully presently, the microscope shows these in their turn to consist of aggre- gations of follicles, and therefore to be truly compound. Thus a mere inspection of the external surface of the pancreas gives an indication of its internal structure ; we see the acini by their aggregation constituting the lobules, the lobules the lobes, and the lobes the whole organ ; the association by which these parts are bound together being more and more intimate as we descend from the greater to the less ; but all of them, even the smallest, that come under the cognisance of the unassisted vision, being truly composite : this is a common character, and indeed a general description, of all conglomerate glands. The pancreas in consistence is moderately firm, the lobules having a considerable degree of hardness, but the whole gland having a certain laxness about it, from the way in which the lobes are hung together by areolar tissue. There is no proper capsule to the pancreas : the areolar tissue which invests it does so very unequally in different parts, and is strictly continuous on the one hand with that which attaches it to neighbouring parts, and on the other with that which penetrates between its lobules to the internal parts of the orpan. On the anterior surface this areolar tissue is so deficient that the structure of the gland is in no way concealed by it. Internal structure. On cutting into the pancreas, we see that it is the same through- out its mass as it is on its surface ; that it is * Introdt ad Philosoph. Natur., p. 556. solid and homogeneous ; that one part exactly resembles another ; that there is the same aggregation of the acini into lobules and of lobules into lobes, and the same nesting in capsules of areolar tissue by the elasticity of which the acini and lobules are made to pro- trude from the cut surface, whereby it be- comes granulated and nodular ; and this irregularity of surface is almost the only difference between the appearance of a section and that of the external surface as just now described. A good idea of the absolute and relative size of the different elements may be obtained by a section : the lobes may be said to have an average diameter of ^ of an inch ; the lobules T L, or \ of the lobes ; the acini y 1 ^ of an inch* ; but all the measurements are liable to the greatest variety. The areolar tissue, less abundant than in the salivary glands, consists almost entirely of the white fibrous element, and the areolae are very lax and large : it is most abundant near the centre of the gland around the duct, and about the head of the pancreas, where it forms a firm and intimate union between the gland substance and the duodenum. The duct of the pancreas, which has been called the canal of Wirsung, from its dis- covery by that anatomist in the year 1642, passes from left to right throughout the en- tire length of the organ, beginning within a few lines of its splenic extremity, by the union, at an acute angle, of two or more minute ramusculi, and emptying itself into the duodenum at or near the junction of its vertical and inferior-horizontal portions. Its course is somewhat sinuous ; it lies, on the average, about equidistant between the upper and lower margin, but nearer the pos- terior than the anterior surface. It is alto- gether concealed by the gland structure, even its point of entrance into the wall of the intestine. After originating in the manner described, it receives continuously in its course small branches which enter it at right angles, and which, unlike the main duct, are perfectly straight and not sinuous, and mostly single and unbranched, each coming from its own appropriate lobe, without receiving any accessory branches from neighbouring ones ; so that they look, as Cruveilhier has said, like the legs of a centipede, of the which the main duct forms the body. By these tributary ducts the calibre of the main canal is gradu- ally increased till it reaches a diameter, at the right extremity of the gland, of ith of an inch. It is of an opaque white, and there- fore easily distinguishable from the gland substance. Its walls are thin and elastic, but dense and firm. In the tenuity of its walls it stands in strong contrast with the ducts of the salivary glands ; but, like them, it is con- tinuous, by its external loose fibrous coat, with the areolar tissue of the gland. Just before entering the intestine the duct com- * This last measurement is taken from the rabbit, as it is difficult to isolate the acini of the human pancreas, or to take their measurement in situ, without isolation ; but the measurement ill the human subject very nearly approaches it. PANCREAS. rnonly receives a large tributary branch, often nearly as large as itself, coming from the head of the pancreas. This occasionally remains permanently distinct, and opens into the in- testine by a separate orifice (see Jig. 55. B, c), a condition always present, according to Fig. 55. A Diagram of the principal Varieties to which the ter- mination of the pancreatic duct and its relation to the bile-duct is liable in the human subject, arranged in their order of frequency. A. The normal arrangement, showing a, the bile- duct, and b, the pancreatic, terminating by one orifice. B. Showing the separate termination of c, the ac- cessory duct from the head, or lesser pancreas. c. The main duct, b, terminating by an orifice distinct from the choledoch. D. Two parallel and equal ducts b, c, are here seen joining the bile-duct at its orifice. This is a case of " double pancreatic duct." Meckel, in the early foetus, so that this irre- gularity is essentially nothing but an exten- sion of a foetal condition into adult life. When this duplicity of orifice exists, the separate duct from the head of the pancreas has its own little papilla, proportionately smaller than the normal one, and separated from it about fths of an inch or an inch. It is usually lower down, though sometimes higher up, than the main orifice. It has been ob- served by Cruveilhier, that when there is one duct opening by a distinct orifice appropriated to itself alone, there is always another open- ing into the duodenum in the normal way in common with the ductus choledochus. Ac- cording to other authorities*, however, the pancreatic duct and the bile duct will some- times open on the mucous membrane of the duodenum by two entirely distinct orifices, when the former is single and there is no secondary one, as seen in Jig. 55. c. Oc- casionally the pancreatic duct is double throughout its whole length, the two running side by side and communicating, just before * Scemmerring, Corp. Human. Fabrica. their junction, with the ductus choledochus, as shown in Jig. 55. D. Scemmerring asserts that there are sometimes three diicts dis- tinct throughout and opening separately. All the varieties of method of termination of the pancreatic duct have been collected and ana- lysed by Tiedemann*; and he has come to the interesting conclusion that they all have their analogues in the different arrangements found in the various species of the lower animals. The method of termination of the pancre- atic duct, and its relation to the ductus cho- ledochus, is rather curious. The duct, unlike the ducts of the salivary glands, which have a long course after leaving the gland before they terminate, passes at once from the gland to the intestine at a point where the former is closely applied to the latter, so that it is quite covered up and has no peritoneal in- vestment. At this situation it comes into contact, at an acute angle, with the ductus choledochus, which has descended to this point either in a groove of the pancreas, between it and the intestine, or in a complete channel through the gland substance. The pancreatic is placed to the left of the choledoch duct ; and, maintaining this relation, the two perforate obliquely the duodenum about the middle of its second portion and at the left side of its posterior wall. Side by side they perforate in succession the muscular, the fibrous, and the mucous coats, which they elevate into a ridge when injected or when a probe is passed into them, and after an oblique course of about eight lines, they open into the bottom of a little papilla situated in a transverse fold near the junction of the middle with the third portion of the duodenum. f In their transit through the walls of the intestine they are separated by a valve-like process, composed of the tissues that constitute their walls, which gradually gets thinner and thinner till it terminates at the base of the little olive-shaped ampulla about two lines in depth, into which the cavity of the papilla is dilated ; and since the mucous membrane lining this ampulla is of the same structure as that lining the intestine, and unlike that lining the ducts, these latter must be said to open by two distinct orifices at the base of the papilla, and not by one at its apex, as is usually described ; in fact the lining membrane of the cavity of the papilla is part of the general mucous surface of the * Sur les differences que le canal excre'teur du pancreas pre'sente dans 1'homme et dans les mam- miferes. Journal Compl. des Sciences Me'dicales, torn. iv. p. 370. t The point of immergence of the pancreatic duct is variously stated by various authors. Soan- merriny gives it as from three to twelve fingers breadth" o below the pylorus ; Meckel, from three to four inches, but possibly amounting to ten ; Quain, three to four inches ; Cruveilhier, at the lower part of the second portion of the duodenum ; Ue Graaf, quatuor digitis transversis sub pyloro ; Gavard, five fingers breadth from the pylorus ; and so on. That given in the text, which coincides with Cruveil- hier, is the normal one ; the extremes are very ex- ceptional. G 3 86 PANCREAS. duodenum.* Towards its orifice the duct more or less enlarges itself ; but at its very aperture, on the contrary, it undergoes a contraction. The appearance of a valve guarding the orifice depends merely on the partition which separates its mouth from that of the choledoch duct. Occasionally near the orifice, occasionally higher up, there is a valve-like process "projecting from its inner surface ; but this is not constant either in its situation or its existence. From the narrowness of the duodenal ori- fice of the pancreatic duct, from the movable and yielding nature of the eminence upon which it opens, and from the oblique course of the duct through the walls of the duo- denum, it follows that the pancreatic fluid and the bile may pass freely into the duode- num, but cannot regurgitate from it into the ducts. " On this subject," says Cruveilhierf , " I have made several experiments. I have forcibly injected both water and air into the duodenum, included between two ligatures, but nothing escaped; on the other hand, I have injected the same fluids from the duct into the duodenum, which I was thus able to distend at pleasure. But then, on com- pressing the bowel with great force, I have never been able to cause the slightest re- flux into the ducts." The spur-like process formed by the lining membrane reflected upon itself at the junction of the ductus choledochus and the pancreatic duct, ex- tending down to the duodenal orifice, does not prevent the fluid of one canal from passing into the other. Thus the pancreatic fluid might regurgitate into the ductus choledochus, and, on the other hand, the bile might enter the pancreatic duct, if these canals were not habitually full. Moreover, this spur-shaped process between the two canals cannot arrest the flow either of the bile or pancreatic fluid, by being applied to the orifice of the one or the other duct. Fig. 56. is a diagrammatic representation of the manner in which the ducts traverse the walls of the duodenum and terminate in the papilla, and of their relation to one another, and to the coats of the intestine. Vessels. The arteries of the pancreas, which, for the size of the gland, are large and numerous, are, like those of other conglomerate glands, contributed from many sources, and are derived from the branches of the coeliac axis, and from the superior mesenteric. The principal are thepancreatico-duodenalis of the hepatic and the pancreatic branches of the splenic artery, one of which, the pancrea- tica magna, sometimes runs nearly the whole * Scemmerring considers the apposition of the two ducts in the wall of the intestine a junction, and the partition between them merely a valve : indeed, it is to that portion of the bile duct, so joined by the pancreatic, that he restricts the ap- pellation ductus choledochus. " Ductus choledoch us, id est ductus hepaticits, cysticus, et pancreaticus in unum confuti." (Corp. Hum. Fab.) A definition as unphysiologieal as it is inconsistent with critical anatomy. f Descriptive Anatomy. length of the gland, accompanying the duct The branches from the Fig. 56. from left to right. Diagram of the normal method of termination of the pancreatic and bile-ducts in the human subject, showing their oblique transit through the successive elements of the wall of the intestine, their gradual approximation to one another, their final union at the base of the ampulla or cavity of the papilla, and the spur-like process separating them, a is the bile-duct ; b, the pancreatic ; c, d, and e, the muscular and mucous coats of the intestine. superior mesenteric are mostly derived from that small twig which, given off just at the lower border of the pancreas, anastomoses with the pancreatico-duodenalis. The veins empty themselves into the superior mesenteric and splenic. The lymphatic vessels have not, that I know of, been demonstrated, nor have I been able to detect them myself: they are supposed to enter the lumbar glands in the neighbourhood. The nerves are derived from the solar plexus, and enter the gland at different parts, accom- panying the branches from the arteries of the coeliac axis. II. MICROSCOPICAL ANATOMY. Gland substance. The subject of the micro- scopical anatomy of the pancreas is one of great difficulty, and, until I came to examine it for myself, I had no idea how great. The deli- cacy and tenuity of the structures, even when seen most advantageously ; the destruction of all their natural relations, and consequent im- pairment of the value of observations by cut- ting, tearing, or compression ; the rapid change that takes place in the microscopical elements themselves, in whatever medium they are placed for examination, by deliquescence, so- lution, or endosmosis; the conflicting character of different appearances ; and the discrepancy of many of them with the interpretations PANCREAS. given by the most trustworthy authorities, all conspire to invest the practical investigation of the subject with an amount of difficulty and doubt greater, I think, than that which would beset almost any other path of micro- scopical research. And I may add to this, what would naturally be its accompaniment, a deficiency on the part of the authors that I have consulted, in that very kind of informa- tion that the practical difficulties of original research make one crave in others. The only observations on which I think reliance is to be placed for the solution of the difficulties that the examination of a structure so in- volved and delicate as the one under consi- deration presents, are observations made with the microscope on the parts, fresh, in situ, unaffected by re-agents, and undisturbed by such manipulation as shall interfere with the normal relations of their minute anatomy ; and such observations I cannot find. Miiller's descriptions and drawings on this subject, in his admirable monograph, " De Glandularum Secernentium Structura Penitiori," are either taken from the parts unmagnified, or magnified with such low powers as make them valueless for the solution of the special difficulties of the case. The same observation applies to the accounts of the minute structure of the pan- creas contained in the ordinary works on de- scriptive anatomy, from their being descrip- tions of the minute structure as seen by the naked eye, or as made out by a coarse kind of disintegration, or by mercurial injections. The most satisfactory microscopical ex- aminations of the pancreas may be made, I think, from those of the Rodents ; for in them the gland being spread out in its proper me- sentery in an arborescent or seaweed-like form, it is in some parts so thin as to trans- mit sufficient light for its examination without any compression or dissection whatever ; in- deed, along the edges and in some of the smallest lobules, the ultimate follicles are dis- tributed in a single layer only. This arrange- ment makes a careful and satisfactory scrutiny much easier ; and I shall, therefore, in this part of my paper, draw principally from the appearances of the gland in these animals, as the rat, rabbit, mouse ; at the same time the close approximation in ultimate structure to the human pancreas will make my observa- tions apply as well to the gland in man as in these lower mammalia. On putting a minute lobule of the pancreas of a rat or mouse under the microscope, we see a number of follicles grouped together, of various forms and sizes, constituting, by their grouping, the acini of the gland, or ultimate grannies visible to the naked eye ; and when the acinus is constituted by a small number of follicles, and isolated, the whole of it may be brought under the field of the microscope at once (as in fig. 57.). These follicles are formed by the involution of the limitary mem- brane of the gland, and they contain the se- creting epithelium, and within that (at least under some circumstances) a central cavity. These elements of the follicle the basement 87 membrane, the epithelium, and the cavity I shall consider in succession. Fig. 57. Minute lobule or acinus of the pancreas of a Mouse, showing the two forms or stages of the epithelium, and the varied forms and sizes of the ultimate fol- licles (^magnified 180 diameters'). a. The basement or limitary membrane. It is to the modification and arrangement of this fundamental tissue that the pancreas (in common with all other glands, either folli- cular or tubular, simple or compound) owes its shape and appearance as a conglomerate gland, and its position in the gland series as a *' compound gland with canals of the ramified type having follicular extremities."* From this membrane, as from a starting-point, the distribution and anatomy of all the other ele- ments of the gland structure proceed. The branched character of the ducts, the parti- cular manner in which the follicles are grouped, the racemose or panniculated cha- racter of their clusters, the isolation of the epithelium within them, the amount and ar- rangement of the areolar tissue without them, and the form of the capillary network, all result from the particular way in which this basement membrane is laid down. To this simple truth anatomists have been a long time in coming. Malpighi first, in 16G5, an- nounced the fact that the compound glands were mere multiplications or repetitions of the simple ones, and that all glands consisted of tubes with blind dilated or undilated extre- mities, which received the secretion from the blood and poured it into the excretory duct. This view, after having some doubt thrown upon it by the researches of Ruysch and the experiments of Haller, has been entirely con- firmed and greatly elaborated by the labours of Miiller, who, in his great work on the inti- * Miiller, Physiol. by Baly, vol. i. p. -114. 88 PANCREAS. mate structure of glands, has contributed more than any other author to our knowledge of this particular section of general anatomy. In the human pancreas the follicles are so closely packed that their individual shape cannot well be seen ; but in the rodents the arborescent arrangement of the gland exhibits them well ; and they arc seen, when isolated, to be ovoidal or nearly spherical*, although, in the central part of the lobule, they become variously polygonal from mutual compression. The outline, however, even where they are not compressed, is not that of smooth sphe- rules, but presents slight convexities cor- responding to the epithelium within them ; but the endosmosis of water, by detaching the epithelium from the basement membrane, and at the same time distending the follicle, causes these convexities to disappear. There is great difference in the size of the follicles, some being as much as j-g-g-th of an inch in diameter, some as small as ^ji-g-th ; and the extremes in size will often be contiguous, a very small one packed among many large. The average size of a pancreatic follicle is about Tj^o-th of an inch. The number of fol- licles in a single group varies still more, being from half a dozen to a hundred or even more. In the rodents these groups are often entirely separate from one another on every side ; but in most of the mammalia their isolation is not so complete, and they are more or less massed and fused together. Fig. 58. loose and unattached appearance, the simi- larity of their granular contents to that of the secretion when free, and the want of definite- ness of outline in many of them, which seem dissolving in their own contents, the cell-wall having disappeared, and the cluster of con- tained granules merely marking its situation. (See Jig. 57.) In neither of the stages can I detect nuclei. From the great opacity of the more advanced cells, and their grouping towards the centre of the follicle, they give a portion of pancreas, viewed with a low power, a mottled appearance, a dark spot marking the centre of each follicle, and the number of dark spots showing the number of follicles, which, in some parts, from their close packing, could not be otherwise counted When the follicles are ruptured, the epi- thelium escapes, and the two forms may be seen floating freely about. Fig. 58. repre- sents some of the more advanced cells ; they are magnified 400 diameters, and are seen to be filled with the particular granular matter which imparts to them their darkness and opacity, and which differs only from the free granular matter floating about in the secretion in being localized and confined by the vesicular envelope of the cell. What might be called the granular or molecular base of the pan- creatic fluid, is evidently the contents of these mature cells, liberated by the rupture or solu- tion of the cell-wall. The cells that have attained this appear- ance, although they may be grouped together, as seen in fig., 57. are never adherent to one another. The less advanced cells, however Fig. 59. Isolated cells of mature secreting epithelium from the pancreas of the Rat, showing their opaque granular contents. (Magnified 400 diameters.) $. The epithelium is of the glandular type, spheroidal or polygonal in shape, varying in diameter from T^^th to -Tnn>th of an inch, and presenting two distinct appearances, in- dicating, I think, two stages of development an early stage, and one of more complete maturity. The cells of the early stage are smaller, more spherical, homogeneous in structure, and most abundant at the peri- pherae of the follicles or in immediate contact with the basement membrane. The more advanced cells are larger, of more varied shape, full of granular contents, and loosely aggregated towards the centre of each follicle. I consider the form first described to be the early stage because the cells are so small, are in contact with or near the cell-generating surface, and are free from secretion con- tents. The others, I imagine to be the more advanced stage, from their greater size, their * In Cruveilhier's Anatomy, p. 533. note, it is said that the ultimate follicles of the pancreas are cylindrical, while those of the salivary glands are slightly dilated. ( ?) Epithelium liberated by rupture of the follicles, show- ing the method of detachment, and the mutual lateral adhesion, of the cells. From the Mouse. (Mag- nified 200 diameters.) or those in contact with the membrane of the follicle, are often so closely adherent, that when they escape from their containing fol- licle they form little crescentic masses, as seen in fig. 59., the convexity coinciding with the follicle-membrane, the concavity with the central space, and the adherent surfaces of the cells presenting the appearance of radiat- ing lines, passing from convexity to concavity at right angles to them : this close package of the epithelium gives it a columnar appearance ; and, indeed, some of the little crescentic groups referred to closely resemble the scraps of sheaths of columnar epithelium shed from an intestinal villus during digestion. Sometimes, instead of the follicles being filled with distinct epithelium cells, they ap- pear to contain a number of variously-sized globules, of a smooth, homogeneous, and highly refracting appearance, surrounded by a me- dium of much less refracting power, and PANCREAS. 89 finely granular. These globules are of various and even surface. At any rate, it is a rare shapes, according as they are isolated or com- thing to see the appearance clearly, and when it is visible, it requires accurate focussing for Fig. 60. its satisfactory display ; for if either the nearer or more distant surface of the follicle is in focus instead of the centre, all appear- ance of cavity vanishes, and the follicle seems to be full of epithelium. Perhaps it is in part owing to this, and in part to the fact that the condition accompanies a particular and transitory stage of the secretion, that it is Fig. 61. Appearance of homogeneous globules of various sizes and shapes, occasionally seen in the follicle of the pancreas. From the Eat. {Magnified 200 dia- meters.') pressed by neighbouring ones. They range in size from 1 J to -^^ of an inch, and are evi- dently not contained in any cell-membrane. (Seejtfg. 60.) The appearance, in my opinion, results from a spontaneous solution of the epithelium in the follicles, and a separation of the different elements of the secretion ; but what are the particular circumstances that de- termine it I do not know; the longer the object is kept under the microscope, the more marked is the appearance, and the larger the globules, from their running one into the other : it is possible that endosmosis may have something to do with it, for I do not remember ever to have seen the appearance in specimens promptly examined immediately after death.* y. Occasionally there is an appearance of a central cavity in each follicle, the epithelium lining it in a single columnar-looking layer, and leaving a central space unoccupied. The central space thus left is very small, not ex- ceeding in diameter that of the thickness of the epithelial layer lining the follicle ; it is only now and then that this appearance can be detected, and even then it requires careful focussing to see it satisfactorily : it may either arise from the epithelium being shed in suc- cessive generations of layers, one passing from the follicle as the succeeding crop is produced, or it may be explained by the mere liquefaction of the central and older cells, ' which, escaping in a fluid form from the fol- licle, leave the peripheral cells with a definite * Since writing the above, I have had satisfac- tory evidence that the appearance is owing to en- dosmosis. I have seen the globules form under the microscope from their first trace to their attainment of a size equal to that shown in the figure. Some- times the eudosmotic current is so strong as to cause visible movement in the contents of the follicles ; the globules are the eudosmosed fluid, the intervening material the granular contents of the follicle ; in fact, the secretion. I have thought it worth while to retain the figure and description, as it is an appearance that might very easily give rise to error. A group of follicles from the pancreas of a Rat, viewed so as to bring their central cavity into focus. {Magnified 150 diameters.") not more frequently visible. I have repre- sented it in fig. 61., as seen in a group of fol- licles from the pancreas of a rat : it displays the proportional thickness of the central ca- vity and the epithelial lining, and shows one or two follicles, where, from being out of focus, the cavity is not visible and the follicle appears solid throughout. It was sketched immediately after death. Duct. The duct of the pancreas, like that of other conglomerate glands, consists of three coats : a middle, elastic, dense, fibrous, and white ; an external, loose, and areolar ; and an internal epithelial. Between the middle and internal there is probably a basement mem- brane, described, indeed, by some authors, but which I have been unable to detect. The middle coat consists of a firm, dense, and matted stratum of fibrous tissue, mainly longitudinal in direction, but closely inter- woven and netted together, very much re- sembling white fibrous tissue in appearance, but evidently not consisting of this entirely, as the striation is not removed by acetic acid. A certain amount of clarification, however, is produced by adding the acid, and the fibres that remain visible afterwards appear to con- sist of a particular form of yellow fibrous tissue, extremely fine, so as to lose the cha- racteristic appearance of double outline and even calibre. These fibres are exclusively longitudinal and parallel, except towards the outer surface of the duct where they inter- lace. Besides these, acetic acid displays, irregularly and sparingly scattered, some trans- verse and some longitudinal, the nuclei of some unstriped muscular fibres. These fibres I have never been able to isolate or see satis- factorily, for the density and opacity of the fibrous tissue previous to the addition of acetic acid renders them invisible : they lie near the inner surface, and, from the paucity of the nuclei, must be very few. The external coat is merely the loose areolar web which connects the duct to the 90 PANCREAS. gland substance, and is continuous with that which pervades the whole gland : it differs not, therefore, from that which has been already described. Fig. 62. Tesselated appearance of columnar epithelium lining the pancreatic duct. (Magnified 200 diameters.) The epithelium is columnar, arranged ap- parently in a single stratum, and presenting a beautiful honeycomb appearance of closely- packed hexagons and pentagons, when looked at on its free surface. Further up, however, near the extremities of the ultimate ducts, the epithelium changes its character, and becomes more globular, as is shown in Jig. 63, which represents a portion of the epithelial lining of a duct, about ^-^ of an inch dia- meter, from the human subject. A certain ap- proximation is here seen to the form of se- creting epithelium, with which, however, it strongly contrasts in its clearness and free- dom from granular contents. Fig. 63. Portion of epithelium lining a small duct ^th of an inch in diameter. From a Rabbit. (Magnified 300 diameters.) There is every probability of the existence of a basement membrane here as in other sub-epithelial situations, and it is probably continuous with that which alone constitutes the walls of the ultimate ducts : for the fibrous and muscular elements gradually diminish as the ducts get finer, until in the smallest that are seen all fibrous appearance has vanished, and a homogeneous membrane alone remains. According to Henle the homogeneous wall of the smallest ducts consists of fibres fused and run together in a plane ; a supposition that would imply the non-existence of this mem- brane in the larger ducts, where they are not so fused. Capillaries. The arrangement of the ca- pillaries remarkably resembles that of fat. They form a close and pretty even-meshed net work, open on all sides, among the meshes of which the follicles lie, just as the vesicles do in the case of fat ; so that the closeness of the plexus is a measure of the size of the follicles. Their general appearance is well seen in the accompanying figure (Jig. 6-t.). Fig. 64. Arrangement of the capillaries of the pancreas. Frnm a minute lobule of the pancreas of a young Rabbit. (Magnified 80 diameters.) III. COMPARATIVE ANATOMY. Invertebrata, Certain organs connected with the alimentary canal have in some of the higher invertebrata received the name of pan- creas ; but they have done so rather from their position and interred function than from any certain evidence of their use, or from their anatomical structure. In Gasteropoda we find the first indications of the organ, and it presents in them the form of a single, long, blind, glandular sac, communicating with the ' beginning of the intestine ; such a pancreas may be seen in the different species of Aplysia and Doris, Tritonia and Scyllasa. Cephalopoda. The Tetrabranchiate Cepha- lopods possess, attached to the upper part of the intestine, a laminated sac, which receives the canal into which the two main hepatic ducts unite, and which diverts the bile by a peculiar development of one of its laminae, from flowing into the gizzard. Professor Owen considers that the follicular structure of this and the other folds of membrane sufficiently indicate its glandular character, PANCREAS. 91 and regards the entire laminated pouch as a more developed form of pancreas than the simple caecum, which we have just described as representing that gland in some of the Gasteropods. Vertebrata. Fishes. At the commence- ment of the intestinal canal, close to the pylorus, are found, in most osseous fishes, certain caeca or blind tubes, budding out from the wall of the canal, which from their position have received the name of py/oric appendages, and have been regarded by most anatomists as the analogue of the pancreas in higher animals. In their most simple form that of a single, or two or three short buddings of the intestinal wall, not differing from it in the structures that form them, the analogy would hardly suggest itself, but by gradual steps we are conducted from this simple re- presentative of the organ, through a series of forms of increasing complexity, to a structure bearing some analogy to a conglomerate gland, and at any rate deserving to be considered a special glandular appendage to the alimentary canal ; the casca becoming more and more numerous as we ascend in the scale, and the whole organ more and more concentrated. Thus in the Sandlance (Ammodytes lancea), and Polypterus there is but one pyloric cae- cum; in most of the Labyrinthibranchs, in many species of Amphiprion, in the Angler (Lophins piscatorius), Turbot (Pleuronectes maximus), and Mormyrus there are two ; in the Perch(Percaj/?Mwzafe7zs),the percoid Popes, the Asprodes and Diploprions, three ; in the Miller's Thumb {Coitus gobio), and Father Lasher (Cottus scorpius), from four to nine ; in the Gurnard (Trigla}, from five to nine ; in Scorpasna ancl Holocentrum, six and up- wards ; in the Pilchard (Cupea pilcardus), and Lump-fish (Cyclopterus lumpus), there are fifty, and upwards of fifty in the Tunny {Scomber thynnus) ; in the Cod (Gadus mor- rhua'), there are upwards of 120 : and in the Sturgeon (Aeipenser sturio) and Paddle-fish they cannot be counted.* But the increased complexity, and divergence from the simple caecal form, is not produced only by the greater number of the appendages. As they increase in number, they more and more coalesce at their bases, so that many caeca open by few orifices, and thus the character of the gland is gradually changed, becomes clustered and branched, and passes from the tubular to the racemose type. Thus, in the Pilchard, fifty tubes communicate with the intestine by thirty orifices ; in the Lump-fish the same number by six ; in the Tunny, by five ; in the Sword-fish (Xip/iias g/adius^ there are but two orifices ; and in the Stur- geon, the whole mass of caeca, by continually uniting and re-uniting, come at last to empty themselves into a single tube, equivalent, in fact, to a short and wide pancreatic duct. The reasons which have induced anato- mists to regard this organ as the analogue of the pancreas are these. * Owen's Lectures on Comparative Anatomy. In the first place, the situation ; it is placed at the pyloric extremity of the intestine ; and besides this general similarity in situation, there is this special one, that the hepatic duct has the same relation to it as it has to the pancreas in higher animals. If there is but one orifice, as in the Sturgeon, the hepatic duct opens at its base; if many, at the base of one of them . Secondly. If they were merely multiplica- tions of surfaces to which food was to be ex- posed, we should find food getting into them; but this is never the case. I have not been able to detect any alimentary materials in even the largest of them ; their function there- fore must be that of pouring forth some special secretion. Again, it is not the way, the particular method, in which surface is multiplied ; that is done by modifications of the lining mem- brane of the intestine, the mucous structures, alone by folds, villi, crypts and not by extension of the whole intestinal wall, mus- cular and sub-mucous, as welL Lastly, the filling up of all the intervening stages from simple tube to conglomerate pan- creas goes a great way to prove the essential identity of the extreme forms. But what is very remarkable with regard to these appendages is their entire absence in many classes of fish. In all the Abdominal Malacopterygii, except the Salmonidaa and Clupeidae, they are wanting ; in most of the Labroids, Gabioids, Cyprinoids, and Lucioids, they are absent ; in the Apodous Malacop- terygii, in the Lophobranchs and Plectognats there is no trace of them; nor in the genera Antennarius, Malthaeus, and Batrachus In some cases they appear to be wanting in con- sequence of their place being supplied by a more elaborate mucous surface, as in the highly developed stomach of the Anarrhichas, and the glandular palate and long intestine of the Ciir\f(Ci/priniis); in others, their absence seems to be but a part of a general simplicity of the alimentary apparatus, as, for instance, in the Dermopterous fish. In the Eel, where there are no caeca, the mucous membrane at the pylorus suddenly becomes thick, vascular, and spongy, and continues so for about an inch ; and on pressure an abundant secretion may be squeezed out of its wall, of an appearance exactly identical with that found in the pyloric appendages where they are present. It is difficult to seize on the law of their existence ; we may, however, say that they are, for the most part, wanting in fish that live on vegetable substances, although there are many similarly circumstanced that are carnivorous and very voracious. Their de- velopment, or their relative size, their number and complication, are probably in proportion to the activity of digestion and rapidity of growth ; the Salmonidae, the Clypeidae and Scomberida?, seem to indicate this : in these last these pyloric caeca exhibit a remarkable complexity. In the Turbot (Pleuronectcs maximus) these caeca are seen in their most rudimentary 92 PANCREAS. form; they are two in number, ample, conical, and recurved, projecting back from the duo- denum at its very commencement, so as to give it a barbed or arrow-head appearance, as seen in the drawing (fig. 65.). The stomach in this fish is very small, and the duodenum Pyloric cceca of the Turbot. a, oesophagus ; b, stomach ; c, intestine. (Drawn one-third the natural diameter.) very large, and the food probably passes into the intestine with but little delay. The caeca in this case must be considered an exception to the rule I have above laid down, that they are never filled with the contents of the alimentary canal ; for in the specimen I examined they were completely stuffed with taeniae, with which also the intestine was filled. Fig. 66. Pyloric cceca of the Sprat (Clupcea sprattus). a, oesophagus ; b, stomach ; c, intestine. (Na- tural size.) In the Sprat, the pyloric casca are nine in number, long, slender, and simple ^ee fig. 66.) : the upper five unite together at their bases, and open into the duodenum, close to the pylorus by one orifice ; the last four open separately, each by its own orifice, in linear series along the duodenum. In the Gadidce, as in the whiting (fig. 67.), the caeca are arranged in the form of a ring, Fig. 67. Alimentary canal of the Whiting (Merlangus vul- garis), showing the pile of caeca around the pylorus. (One half the natural diameter.) constituting a frill around the intestine, and consits of four bunches, each containing about thirty caeca. These unite and re-unite till they terminate, each bunch, in a single duct ; so that there are finally four orifices, so placed as to fall on two converging sides of a triangle, of which the orifice of the hepatic duct would form the apex. As each bunch contains thirty caeca, there are a hundred and twenty Fig. 68. One of the four bunches of pyloric appendages of the Whiting, isolated; showing their union and reunion till at h'nijlli they end in a single tube. PANCREAS. 93 in all. The appearance of the frill of pyloric caeca is shown in fig. 67, and one of the bunches separate in fig. 68. In the Salmon this apparatus of caeca is much more voluminous. It is not condensed around a particular portion of the intestine, but extends linearly, from close to the duo- denum for a distance of about eight inches along the intestinal wall ; each caecum opens There is no by its own separate orifice. coalescence or fusion, so that the inside of the intestine there are seen as many orifices as caeca; they form a double row on each side, so that altogether there are four rows, and are arranged with the utmost regularity. The amount of secreting surface of these caeca some of them are ten big round as a tobacco-pipe; they rapidly on looking on must be very inches long, great ; and as Fig. 69. Portion of the alimentary canal of the Salmon ( Salmo //), showing one double row of ctccal appetidayes and the pyloric extremity of the other. a, oesophagus; b, stomach; c, pylorus; d, small intestine; e, gall-duct. (One-third the natural diameter.) increase in length from the first three down- wards, and the third from the stomach is generally the longest. They then gradually diminish, slightly in calibre, considerably in length, to those furthest down the intestine, which are pbout three inches long. Altogether the secreting surface of these caeca must con- siderably exceed that of the rest of the ali- mentary canal, and the whole apparatus, taken together, is next to the liver, by far the largest of the viscera. Each double row con- tains about thirty, so that altogether there are sixty caeca, and as the average length of each caecum is 6 inches, the whole length of secreting surface must be 390 inches, or upwards of 32 feet. In their internal ultimate structure these caaca exhibit considerable variety ; in many the mu- cous surface is closely laminated ; in some it is covered with flattened, fused villi with crypts thickly planted between their bases. In the Herring (Jig. 70.) the structure is very peculiar : on looking vertically on the internal surface it is seen to be mapped out into hexagonal and pentagonal cells about -^ of an inch in diameter, very evenly and geo- metrically arranged, and each filled with a mass of epithelium. The septa between them appear to consist of sub- mucous fibrous tissue, and on making a section and looking at it laterally they are seen to project between the acervuli of epithelium, and rather beyond them, and to have no epithe- lial investment of themselves. The masses of epithelium are seen to be of a spheroidal form and very smooth outline, though I could not distinguish any basement membrane or capsule wall of which they might be supposed to be the contents, or any special structure determining their outline. I have thought this structure suffi- ciently peculiar to give a figure of it. A represents the appearance on looking down on the surface; B, a view of the wall in section, seen with a lower magnifying power. Many anatomists deny the true pancreatic nature of these pyloric ca?ca, and assert that many fish pos- sess, over and above them, a true glandular pancreas, analogous in struc- ture to the pancreas of higher animals. Weber first described such an organ in the carp, as interlaced with the lobules of the liver, anil, so to speak, confounded with them, but having a proper excretory canal opening into the intestine by the side of the cystic ; he also thought that he had seen traces of a pancreatic duct in the pike. Much more recently Alessan- drini described the same exircting duct, as also the volume and position of the pancreas, in the same fish. In the Siiurus glanis MM. Brandt and Ratzebourg have taken for the pan- 94- PANCREAS. creas a glandular body very like the liver in appearance, stretched as a layer between the Fig. 70. Mucus membrane of the interior of the pyloric caecum of a Herring (Clupcea harengus). A, the surface seen vertically, showing the honey- comb appearance formed by the septa separating the masses of epithelium. (Magnified 150 dia- meters.) B, a section vertical to the surface, showing the flattened spheroidal shape of the acuvuli of epithe- lium, and the amount of projection of the septa between them. (Magnified CO diameters.) folds of thegastro-hepatic omentum, enveloping the cystic canal and accompanying it as far as the intestine. These three examples of mala- cocopterygious fish have no pyloric caeca, and this glandular structure might be considered as replacing them ; but Alessandrini has also de- scribed in the sturgeon, the walls of whose in- testinal canal are particularly glandulous and in which the pancreatic cseca form an elaborate apparatus, a proper pancreas with an excretory duct opening into the intestine in the middle of a tubiform papilla about an inch from the pyloric orifice. In this last case Cuvier be- lieved the bod}' indicated as the pancreas to be a lobe of the liver. " The tubiform pa- pilla," he says, " truly exists ; indeed I have found two, besides that appertaining to the choledoch duct. In one of the examples it formed a sort of cul-de-sac ; in the other the fibre which was introduced conducted to a canal which took a direction towards the liver. I have clearly seen an excretory duct in a very large silurus, piercing the intestine of the side of the choledoch ; but that canal was, in my opinion, hepatic, for the glandular substance taken as the pancreas was evidently continuous with the right lobe of the liver, and formed, as it were, a middle lobe ; its appearance was in other respects the same, except that its colour was rather clearer in consequence of its substance being less thick at that part. The duct discovered in the pike certainly exists, as far as my re- searches go ; but that, again, is an hepatic canal, for I have not seen any body distinct from the liver from which it takes its origin, or which could be considered as a pancreas. The same must be said of the carp, where Meckel could discover neither a pancreas nor pancreatic duct, in spite of the indications of Weber." Still more recently Stanuius has enumerated many fish in which a parenchy- matous pancreas may be found ; but the de- scription added to his enumeration is so meagre and general that nothing can be veri- fied upon it. Reptilia. In the reptiles we make a great approach to the structure of the pancreas of higher animals both in general form and struc- tural appearance. It exists in them all, and generally maintains that intimate relation to the end of the stomach and commencement of the intestine which we see so constant in birds and mammalia. In the Batrachia, the pancreas is situated in a proper mesentery or meso-gastrium of its own, extending between the lesser cur- vatur of the stomach and the duodenum, and, according to Cuvier, is more developed in terrestrial batrachians than in aquatic, in those that take their nourishment out of the water than in those that hunt and seize it in the water. In the Frog (Jig. 71.) the pancreas is shaped not unlike that of the human sub- ject, but its broad end is in the opposite position ; it is about three quarters of an inch long, weighs -27 of a grain, and is of a yellowish white colour and soft consistence ; it is in close apposition with the duodenum all the way Fig. 71. Pancreas of the Frog, shown by throwing up the stomach, and exhibiting ilie under surface of the mesogastrium. a, oesophagus ; b, stomach ; c, pylorus ; d, duo- denum ; e, small intestines ; /, liver ; p, pancreas ; s, spleen. (Natural size.) along. From near the large end it sends up a process clothing and concealing the gall duct as far as the gall bladder, the neck of which it invests. The whole of the gall duct, till the point of its immergence into the in- testine, is thus concealed in the substance of the gland, and it might at first sight be mis- taken for the pancreatic duct ; but, by care- fully nicking it and introducing a fine hair, the hair may be passed up to the liver. The most careful dissection could not reveal a proper duct ; probably small ducts from different PANCREAS. 95 parts of the gland open into the biliary duct as it passes through. The organ maintains the same relations in the Toad ; in the com- mon toad it is yellow, straight, and elon- gated. In the Tritons it is perceived with difficulty ; Cuvier describes it as appearing like a semi-transparent riband sending one bifurcation to the spleen, and another to the duodenum at the point of insertion of the biliary canals. In the Siren it resembles in miniature, as far as external appearance goes, the pancreas of the sturgeon, and joins the intestine by many parallel canals considerably in front of the cystic. In Ophidian reptiles, the pancreas varies greatly both in volume and form ; sometimes it is elongated, often globular and pyramidal, sometimes divided into two triangular lobes, ami this variety of form obtains even in con- generic species ; thus, in the Cescilia albl- vcntris it is thick, and pyramidal, and in the CcEcilia interrupta, lumbricoides, and den- ffiffi, it is straight, elongated and slightly forked. It is always placed to the right of the commencement of the intestinal canal and head of the stomach. Its substance is red with a tint of yellow, and soft, more rarely firm and consistent, and often divided into distinct lobes. In this respect it does not at all resemble the salivary glands of these ani- mals, but those only of mammifera. Its inti- mate union with the spleen is very remarkable in the true serpents, whilst in the genus Aiiguis and Ccccilka the contact and adhesion at this point does not exist. In the Saurian reptilia the pancreas is often applied against the pyloric portion of the stomach and the commencement of the duo- denum ; or it may be said to have two branches parallel to the stomachal sac, one of which accompanies the biliary canal, and the other adheres to the spleen, and these reuniting terminate at a point more or less approaching the pylorus ; it is almost always contiguous to the choledoch canal, which often traverses it before arriving at the intestine. According to Cuvier, its volume is greater in saurians living on vegetable food ; and its smallness in those that are carnivorous he believes to be com- pensated, as in fish, by the agency of the mucous and intestinal secretion of the abun- dant glandular apparatus with which their alimentary canal is furnished. In the Lacer- tida;, and Iguanulce, the pancreas is very much developed. Chelonia. " In many respects," says Cuvier, " the animals of this order are in the same conditions as birds. The jaws are similarly armed, the salivary glands are but little de- veloped, and as the volume and importance of the pancreas in birds has appeared to us to be in inverse ratio to the means of masti- cation and insalivation, we might antecedently conclude that the chelonia would also pos- sess a considerable pancreas." At the same time he adds, that the superior masticatory power of the horny jaws of the chelonia over the bills of birds, and their taking their prey generally in the water, considerably impairs the closeness of the analogy. In the com- mon turtle, the pancreas (jig. 72.) firmly ad- Fig. 72. Pancreas of the Turtle, with the duodenal curvature thrown up, showing its loose and branched cha- racter, its embrace of the spleen, its long caudate process accompanying the duodenum, and its duct entering the intestine higher up. s, spleen ; m, branch of superior mesenteric artery ; c, gall-bladder. heres to and embraces the spleen ; from that point it radiates towards the duodenum, being thick, amassed, and irregularly arborescent above and to the right, and continued in a long and tapering tail to the left: it is closely attached to the duodenum along its whole extent, a distance of about fifteen inches. The duct, nearly as large as in the human subject, passes to the right, and enters obliquely the choledoch duct, as that canal is perforating the thick intestinal wall, in a way very analogous to that already described in the human sub- ject. The gland substance has a very peculiar appearance ; it is dense, opaque, nearly white, and along its edges the lobules are scattered in the clear gelatinous-looking cellular tissue in which the gland is embedded, and appear to be quite distinct from each other ; but on dissecting them out from this gelatinous bed, they are seen to be attached by little pedicles in some of gland substance, in some apparently merely of the duct of the lobule and its vessels to the rest of the gland. It is the most arborescent and ramified pancreas I have seen, next to the rodents, but not so flattened, nor spread out so much in one plane. When looked at as an opaque object with a low power (one inch focal distance), the mapping out of the follicles is very prettily seen; but the same circumstance that lends them their 96 PANCREAS. white opacity, their fulness, that is, of a densely opaque granular material prevents their being seen to advantage as transparent objects, or with a high power ; they are so opaque that nothing of their structure can be distinguished. When ruptured by pressure, their contents escape, and are seen to consist of two very distinct materials ; one, the afore- said fine granular matter ; the other of clear, spherical, uninuclear cells of about ^V^ of an inch diameter. One would imagine ante- cedently that these would hold the relation to each other of secreting epithelium and elaborated secretion ; but the cells are so clear, so free from granular contents, and there is such a complete absence of any intermediate appearance, that I am at a loss in what way to interpret them.* The duct opens by a single orifice, and in a way admirably adapted for preventing the regress of the secretion, or the entry of the contents of the alimentary canal into it. If the aperture in the centre of the papilla by which the bile duct terminates is opened up, it is found to lead into a lacuna, or cul-de-sac, of about half an inch in length, embedded obliquely in the walls of the in- testine. At the bottom of this lacuna is a second papilla, the real termination of the pancreatic duct. Now, if the movableness of the external papilla and the smallness of its aperture were not sufficient to prevent the ingress of the contents of the alimentary canal, yet the very force that might drive in some of these contents through the outer papilla, would press the walls of the lacuna firmly against the orifice of the internal one, and so effectually close it ; nothing could be more efficient than this form of double orifice. The duct in all reptilia always enters the duodenum, generally separate, sometimes in conjunction with the choledoch, and is almost always simple. Aves. The pancreas of birds is propor- tionally larger than in any other animals ; and when we remember their deficiency in sail vary apparatus, its great development would at once suggest a function, in some degree, at least, supplementary to those organs. An- other circumstance peculiar to birds and in- dicative of the importance of the part that the gland plays, is that the ducts are generally many, two and three, and that they open by separate orifices, and often at a considerable distance from one another ; so that the secre- tion may be poured forth on different and widely separate portions of the alimentary con- tents at one and the same time, a circum- stance that must greatly increase and expedite its action. Birds, we know, seize and swallow their food generally without any mastication, and therefore it is not until it gets to the gizzard * The follicles appear to contain only the granular material, and in a minute duct I saw a number of the nucleated cells. It is possible that they may be a form of epithelium restricted to the terminal ducts, by whose rupture and compression they escaped, as the granular matter did from the fol- licles. that it is subjected to any mechanical force capable of breaking it up. This, therefore, takes place immediately before entering the duodenum, and this throws the function of mastication close down to the pancreas, so that from its situation, as well as in other respects, it should have an insalivating function. It is always enclosed between the two arms of the duodenal flexure (fig. 73.); the duo- Fig. 73. Pancreas of tfte common Goose (Anas aiiser), show- ing its relation to the duodenum, its dupkx form, and its ducts. (Natural size.) clenal portion of the gland being, so to speak' alone developed. It is retained in this posi- tion by the gastro-hepatic and gastro-colic omenta, which sometimes simply attach it to the border of the intestine, and sometimes allow it to be free and floating. There is considerable variety in its shape, but it is always more or less elongated and slender : sometimes it is undivided and single, in some species deeply cleft, in others consisting of two portions, or a double pancreas, quite dis- tinct, each having its own duct ; sometimes it is divided into three as in the pigeon. But PANCREAS. 97 these arrangements are liable to considerable variety, and perfectly independent of the in- timate structure and function of the gland, for in different individuals of the same species, the arrangement of the ducts is generally the same, however the segmentation of the gland may vary. The gland substance is firm, much more dense than in other orders, and not di- vided so distinctly into lobes and lobules : it has a finely granular and mottled appearance, in colour pink, or a little yellowish, or brownish. The pancreas seldom communicates with the intestine in birds by a single canal, the ducts are generally either two or three in number, and each continues independent and separate to its orifice. They do not communicate either with one another or with the biliary canal : although, however, exceptions are very rare, Cuvier has met with an instance in the stork, in which the single pancreatic and hepatic ducts united and opened by a common orifice. The following table, altered from Cuvier, shows the number of the pancreatic ducts in several orders of birds, and their relative si- tuation with regard to the hepatic and cystic ducts ; it also shows the relation of these last to one another. That canal which is first indi- cated has its insertion the nearest to the py- lorus ; P. stands for pancreatic, H. for hepatic, and C. for cystic. We see from this table, that, as a rule, the 1. RAPTORES. Brown Vulture 1 P. H. 2 P. 3 P. C. Duv. Common Eagle H. P C. DUV. : Golden Eagle H. C. P. Cuvier. Aquila Ossifraga - 1 P. H. 2 P. 3 P. C. Perrault. Owl - 1 P. 2 P. 3 P. H. C. Cuvier. II. INSESSORES. Night-jar H. P. C. Cuvier. Crow - IP. 2 P. H. C. 3 P. Cuvier. III. SCANSORES. Picus (Genus) 1 P. 2 P. 3 P. C. Meckel. Green Woodpecker 1 P. 2 P. 3 P. C. H. Cuvier. Parrot 1 H. 1 P. 2 P. 2H. Cuvier. Blue Macaw 1 H. {*"'} Duv. IV. RASORES. Crax (Genus) 1 P. 2 P. 1 H. 2 H. Perrault. Crax Globicera 1 P. 2 P. C. 1 H. 2H. Perrault. Common Cock 1 P. 2 P. 3 P. H. C. Duv. Quail - P. H. C. Cuvier. Pigeon 1 H. 1 P. 2 P. 2 H. Duv. Bustard 1 P. 2 P. H. C. Perrault. Ib. 1 P. 2 P. 3 P. H. C. Meckel. Cassary P. C. H. Perrault. llhea Americana - H. P. C. Meckel. Ostrich H. P. Perrault. V. GRALLATORES. Stork P. H. C. Cuvier. Bittern H. P. C. Duv. Heron 1 P. H. 2 P. 3 P. C. Cuvier. Grus pavonica 1 P. H. 2 P. C. Duv. Grus virgo 1 P. 2 P. H. C. Perrault. Curlew H. C. Duv. Ib. 1 P. 2 P. H. C. Cuvier. Gold-headed trum- Duv. peter 1 P. H. C. Flamingo 1 P, 2 P. 3 P. C. H. Cuvier. Ib. 1 P. C. H. Meckel. Parra Jacana H. 1 P. 2 P. Cuvier. VI. NATATORES. Grebe 1 P. 2 P. 3 P. H. C. Meckel. Great Diver C. H. P. Duv. Apterodytes 1 P. 2 P. H. 3 P. Cuvier. Gull - 1 P, 2 P. H. 1 C. 2 C. Meckel. Petrel 1 P. 2 P. H. C. 3 P. Meckel, Swan ] P. 2 P. H. C. Cuvier. Duck 1 P. 2 P. H. C. Duv, Sll/ip. H 98 PANCREAS. pancreatic secretion is the first poured into the intestines, and the cystic bile the last : and always when there are three pancreatic ducts, the secretion reaches the intestine early by one of them, and the others have their upenings close to the bile ducts, either before or between them. It is not safe, however, to draw any physiological conclusions from these relative positions, even supposing them to be constant ; for the ducts are so close to one another, that the mixture of the fluids must take place immediately, and their action on the food be simultaneous. In one instance, however, this is not the case ; in the ostrich the bile duct opens close to the pylorus, while the pancreatic is three feet removed from it; this is the greatest separation of the two ducts of any with which I am acquainted in the animal kingdom. It would present, if ostriches were commoner birds, great facilities for ex- periment, and implies an action in both the secretions entirely independent and auto- cratic. Mammalia. The chief differences between the pancreas in other mammalia and man re- late merely to its colour, its consistence, its more or less marked division into lobes, its form, its volume, its union into a single mass, or its separation into two distinct parts, lastly, its position and relations with different por- tions of the peritoneum. Its form is generally more or less that of a narrow band, divisible into two portions ; one, the duodenal, following the curvature of the duodenum, and placed vertically or obliquely ; the other, gastro- splcnic, extending transversely, and therefore opposite the other, from the duodenum to the spleen, against which it always abuts; the latter is always developed, the former is often incon- siderable or suppressed, and must be con- sidered merely as an accessory portion. The various forms and arrangements of the pancreas do not appear to have anything to do with its essential structure or function, or the parti- cular exigencies of the animal ; they seem to depend entirely on the relations of the neigh- bouring organs, the presence or absence of an abundant mesentery, the free movement of the duodenum, &c., and to be influenced by con- siderations of package. In the Onrang the form very much resem- bles that of man ; in most other Quadruniana Fig. 74. Pancreas o/"f/is (6 KTUQ, Gr. ; das Schaambein or Sclwossbein, Germ .). Of these three, the ilium forms the upper expanded portion, and the pubes and ischium the lower perforated portion ; the former being placed before, and the latter behind the opening. In the perfect bone, however, these three are completely soldered together by bony union in the central constricted portion, where each con- tributes to form a deep cavity, externally, for the reception of the head of the thigh bone. From this cavity the three portions radiate ; the ilium upwards, the ischium downwards, and the pubes forwards, each contributing to support the thighbone by its centra! extremity. The innominate bone may, however, be most briefly described as one bone, consisting of two surfaces, external and internal; bounded by four borders, superior, inferior, anterior, and posterior, The superior border, formed entirely by the ilium, is the most regular and the most ex- tended. It forms an arch, directed from be- hind forwards and outwards, and curved laterally so as to present, on looking at it from above, the shape of an italic f; the smaller concavity being posterior and directed outwards ; and the larger, anterior and di- rected inwards, contributing to form the ge- neral concavity of the internal surface. This border is thickened in a somewhat irregular manner, forming what is called the crest of the ilium (a,c,b), upon which maybe traced an internal and an external lip, and a rough broad central line. These ridges are caused by the attachment of the abdominal muscles. The external lip is called, by some authors, the su- perior curved line of the ilium. The crest is very much thickened and irregular at the pos- terior extremity, where it terminates in a back- ward projection, the posterior superior spinous process (l>). It is also thickened into an out- ward projection a litlle in front of the centre (c), and also in a less degree at the anterior extremity, where it projects forwards, forming the anterior superior spinous process (a). The anterior border consists of an upper vertical portion formed by the ilium, and a lower oblique portion formed by the pubis. It commences above at the anterior superior spine, an inch below which it presents a similar PELVIS. 115 projection called the anterior inferior spinous pro- cess (d), the two being separated by a smoothly edged notch (). Below the inferior spine is another indentation, wider and less deeply marked, and forming part of the overhanging edge of the cavity for the thigh bone, and in which a muscle lies. To this succeeds be- low, another rounded, less strongly marked prominence, in which the ilium and pubes are united, called the ilio-pcctineal eminence (e). From this point commences the ob- lique or inward direction of this border, which is for about two inches smooth and rounded for a muscle to glide over, and then presents a fourth well marked, acute, forward projection called the spine of the pubis (/), which is continued by a rough strongly marked ridge, the crest of the pubi* (/, g), to the ter- mination of this border in an abrupt right angle, the angle of the pubis (g). All these eminences result from the implantation of the tendons of muscles of the leg or abdomen. The inferior border, composed partly by the pubis, but principally by the ischium, here commences. It is arranged first in a rough, indented, plane, oval, and vertical surface, which in the living bone is united by fibro- cartilage to innominate bone of the opposite side, and forms the symphysis of the pubis ; avfityvfit, to grow together (h). The posterior border of this articular surface is often raised into a ridge, projecting backwards, especially in old persons. Cruveilhier mentions one, observed in a woman who had borne many children, where this was a perfect crest. Be- low this point the border assumes a direction tending first downwards and outwards, and then, somewhat sharply, curving upwards and backwards. Just below the symphysis it pre- sents a sharp, rough, irregular ridge, with a considerable outward eversion, affording attach- ment to strong fasciae and muscles of the leg and perineum, and to the root of the penis. At the most depending part it gradually widens into a very rough, large, elongated, and rounded tuberosity, 3 inches long by 1^ broad at its posterior extremity, the tuberosity of the ischium (). This tuberosity has a general inclination outward ; and along its inner margin, which projects lower than the outer, is a raised ridge for the attachment, of the great sciatic liga- ment. Upon it are implanted the large pos- terior leg muscles, and in the sedentary posi- tion the trunk is supported by it. Hence the name of this portion of bone (from irr^fiv KaQn/ttrog quod sustineat sedentes), and also its German appellation (Sitzbein or Sitz- stiick). The posterior border commences above this tuberosity. Its direction is first vertical, and then irregularly horizontal. It is formed by the ischium and ilium. Above the tuberosity of the ischium is a rounded groove, in the fresh state covered by cartilage, and called the small sciatic or obturator notch (k), over which a muscle glides. Then occurs a sharp pro- minent process, turned considerably inwards, giving attachment to a strong ligament and some muscles, and called the spine of the ischium (/). Above this the border is thin, rounded' and vertical, becoming gradually much thicker and finally curving sharply backwards and downwards. It thus forms a large notch, the great sciatic, formed principally by the ilium (?). To this succeeds a tapering, elongated, and depending prominence called the posterior in- ferior spinous jjrocess of the ilium (n\ serving for the attachment of ligaments. The border then presents an insignificant rounded notch, with a thin edge; and finally terminates in the superior border at the postovor superior iliac spine, already described. Fig. 76. External view of the innominate bone. The external or femoral surface of the os innominatum {fig. 76.) at its upper or iliac portion is directed outwards, backwards, and slightly downwards ; at its central part out- wards ; while its pubic and ischiatic or lower part is directed forwards, downwards, and out- wards. The iliac portion is broad and fan- shaped above, whence this upper portion is called the ala or wing of the ilium. It is convex at its anterior, and concave at its posterior por- tion, following the/curve of the crest before mentioned. The concavity is termed by many writers the external iliac fossa. The convexity is increased by a ridge of thick bone, which passes vertically downwards from the thick- ened portion of the crest to the cavity for the thigh bone. At the posterior part of this surface, close to the termination of the crest, is an elongated rough impression of a some- what triangular shape, having its base at the superior and inferior posterior spines, and tapering off gradually along the crest for about three inches, which marks the origin of a great muscle thegluteus maximus.and which may be called the glutcal impression ( I ). In the middle of this surface is a slightly marked I 2? 116 ELVIS. line of an irregular curvature, commencing posteriorly at the centre of the great sciatic notch, and, passing upwards and forwards, terminates in the crest a little posterior to the anterior superior spine. This is the superior curved line (2) (posterior of Cruveilhier). Below this is another line of a like character, the inferior curved line (3) (the anterior of Cruveilhier); which, commencing an inch be- low the'other in the sciatic notch, gradually diverges from it, and terminates anteriorly at the inferior spinous process of the ilium. The surface presents numerous small circular open- ings for the admission of the nutritious vessels of the bone, all of which have a direction downwards towards the articular surface?. About an inch and a half below the inferior curved line is a large articular cavity for the reception of the head of the thigh bone, which is named the acetabulum, or cotyloid cavity, KOTV\TJ, a cup ; and tlSoc, like (i). This is a perfectly hemispherical excavation of about two inches diameter in the full grown male, and cir- cumscribed by a rough irregularly raised brim or circular border, to which is attached the circumferential fibro-cartilage or cotyloid liga- ment of the hip-joint. When the bone is pro- perly placed, as in the articulated skeleton, the axis of this cavity is directed outwards and a little downwards for the better adaptation of the femur ; and to this end the posterior part of its circumference is much thicker and more prominently elevated than the anterior. Su- periorly it is still more evidently prolonged outwards at the point where the before men- tioned thick vertical ridge of the ilium springs from it. Inferiorly the border is interrupted for the space of an inch by the cotyloid notch (5), to the edges of which are attached the transverse and interarticular ligaments of the hip-joint, and which is continued into the cen- tre of the cavity by a rough depression or fossa, for the reception of some lubricating glands and the interarticular ligament connected with the femur. The bottom of the notch is on the same plane as that of the depression, and affords an entrance for the vessels and nerves supplying the joints. The remainder of the cavity is a smooth and even surface, uniformly concave and circular, which is covered in the fresh state by a cartilage of a semilunar shape. The broadest part of this surface is above where the superior border projects. The pos- terior extremity is prolonged into a lip which a little overhangs the notch posteriorly, and terminates exactly opposite to the broadest part of the overhanging superior margin. The anterior extremity is the narrowest, being notched slightly by the groove below the an- terior inferior iliac spine on the anterior bor- der of the bone before mentioned. At the bottom of the cotyloid fossa may be traced two lines arranged in the shape of a T, the lower limb of which divides the cotyloid notch into nearly equal portions. These mark the foetal division of the bone into three por- tions, of which the ilium contributes the two superior fifths, the ischium the infeiior pos- terior two fifths, and the pubis the n main ing anterior inferior fifth, to the formation of the cotyloid cavity. Externally, the brim of the ace- tabulum is convex, rounded, rough, and marked above and behind by the attachment of the capsular ligament of the hip-joint ( 6). It is here perforated by numerous foramina for the admission of nutritive vessels. It is much better pronounced above and behind, where it presents a broad, thick, convex sur- face, than in front where it is shallow, thin, and slightly depressed. The points where this difference is indicated are, the inferior spine of the ilium above, and the cotyloid notch below. Springing from the cctyloid cavity are two branches of bone ; one from the inferior part, thick, massive, directed downwards and back- wards called the descending ramus or body of the ischium(p] ; and the other, from the anterior part, slighter in structure, and directed obli- quely downwards, forwards, and inwards in the same plane with the ilium, called the horizontal ramus or body of the pubis (ry). These are each prolonged at their further extremities into flattened tapering processes, which, alter- ing the original direction of their respective portions of bone, one ascends obliquely in- wards, and the other descends obliquely out- wards, to unite with each other midway, at a point marked by a slight transverse line. They are named respectively the ascending ramus of the ischium (r), and the descending ramus of the pubis ()* Together they form the interior border of the innominate bone, and complete the formation of a large oval opening, situated immediately below, and a little internal to the cotyloid cavity, having its long axis directed obliquely downwards and outwards, and called the obturator or thyroid foramen (o) Svpeoc, a shield ; and tleog, like. The edges of this opening are thin, bevilled off, and rough, for the attachment of a fascial ligament which closes the opening, and are formed entirely by the ischium and pubes. The external edge, instead of meeting the internal superiorly, is continued inwards in front of it, along the superior ramus to the spine of the pnbes before described, forming a prominent rib of bone of a triangular shape (17), its base abut- ting on the cotyloid cavity. This rib is con- vex vertically, and concave laterally. Between it and the internal edge of the thyroid fora- men is left a groove, called the sub-pubic or obturator groove (/), for the passage of a nerve and vessels, and which has a direction down- wards and inwards. The junction of the horizontal and descending rami of the pubis is called the angle of the pubis, and it is hol- * There has been much confusion in the applica- tion of names to these bones, the term body of the pubis is applied by some to the angle, and by others to the cotyloid portion only. The term body is confined also by some to the acetabular part of the ischium. The expressions horizontal and descending rami of the pubis and ascending ramus of the ischiumwere applied before a true knowledge of the pelvic obliquity was obtained. The former would probably be well superseded by the words superior and inferior, and those applied to the ischial rami' by anterior and posterior, or greater and lesser. PELVIS. 117 lowed, rough, and broad in front, for the attachment of some muscles of the leg. There are numerous nutritious openings on this sur- face of theischium and pubis, which are chiefly directed towards the cotyloid cavity. Fig. 77. Internal view of the os innominatum, The infernal or pelvic surface presents for examination a superior or iliac portion di- rected forwards, upwards, and inwards, and an inferior ischio-pubic portion directed in- wards and backwards. The iliac portion is rough at the posterior third, and is marked by a thick, massy, irre- gular prominence, just below the posterior extremity of the crest, which is continued to the posterior superior spine, and serves for the attachment of powerful ligaments which connect this bone to the sacrum. This may be called, for brevity, the iliac tuberosity (1). An inch and a half below this is an angu- lar or semilunar articular surface, the sacral, or .auricular (. e) passes from a well-marked prominence on the anterior surface of the iliac tuberosity, downwards and inwards, to the superior lateral part of the pos- terior surface of the sacrum, principally to the two upper pieces, external to the foramina; the fibres spreading out in interlacing bundles to- wards the broader surface of implantation on the sacrum, becoming longer as they become more superficial, and leaving meshes for the interposition of masses of loose fat, and the passage of numerous small veins. The erector spinas muscles arise from the surface of this ligament, and cover it. To obtain a good view of these fibres, a transverse section along the brim of the true pelvis should be carried backward through the sacrum, as shown in the figures. This will show the manner in which the tuberosities of the ilium hang over the sacrum, suspended, as it were, between them by these ligaments. It will be more par- ticularly explained when treating on the me- chanics of the pelvis. The superficial posterior sacro-iliac ligament (Jig. S 1 ., next page, ) has been termed oblique, from the direction of its fibres ; or long, from the extent of them. It is attached above to the posterior superior spine of the ilium, and passes downwards and ob- liquely a little inwards to be implanted^into the fourth transverse tubercle of the sacrum ex- ternal to the hole. To the sides of this liga- ment, which is almost subcutaneous, are at- tached the fascia lumborum and great gluteus muscle. This ligament is described by Cru- veilhier to be attached to the third sacral vertebra. In all the cases I have seen, it is attached to the fourth transverse tubercle, which is the most prominent tubercular pro- jection in the dried bone. Bichat erroneously calls it " sacro'spinous" Attached to the same sacral tubercle, and passing horizontally outwards to be im- planted into the posterior surface of the in- ferior posterior spine of the ilium, a point exactly corresponding to the termination of the horizontal limb of the sacro-iliac articular surface, is another well-marked ligament (fig. 81. b), which, being separated by a dis- tinct cellular interval from the deep ligaments and distinguished by the more deeply seated position and horizontal direction of its fibres from the oblique ligament (), and from the PELVIS. great sacro-sciatic ligament (r), I think merits / .1 ./*.._' ... ^v ../mi*/ tfMn\fVhl"tfll posterior sacra-iliac * ligament. This ligament has been hitherto apparently confounded with the creat sacro-sciatic, which is attached to its lower border by a thin fibrous extension. Fig. 81. Posterior view of the ligaments of the pelvis. a, oblique posterior sacro-iliac ligament ; b, infe- rior posterior superficial sacro-iliac ligament; c, great sacro-sciatic ligament ; d, lesser sacro-sciatic ligament ; e, membranous expansion over the pyri- formis muscle. The ligaments which may be considered as accessory to this articulation are three in number the ilio-lumbar ligament above, and the greater and lesser sacro-sciatic ligaments below. The ilio-lumbar ligament (Jig. 80. c) is a triangular fascicular ligament, thickest at the edges, and passing from the tip of the last lumbar transverse process, to which its apex is attached, horizontally outwards, and a little backwards to the posterior fifth of the inner lip of the crest of the ilium, along which its fibres spread as far forward as the inner projecting point of the posterior curve. To the outer side and behind this ligament is attached the quadratus lumborum muscle with the tendon of the transversalis abdo- minis, and to its front the psoas magnus muscle. Meckel describes this ligament as sometimes reaching as high as the transverse process of the fourth lumbar vertebra. He also describes a second ligament lower than the preceding, but arising from the iliac crest a little behind it. They are called by him, respectively, the upper and lower anterior pelvic ligfiment.?, the latter corresponding to the sacro-vertebral ligament before described. The great sacro-sciatic ligament (ligamentum pelvis posticnm magnum, fig. 81. c) is attached behind, to the posterior inferior spine of the ilium bv a membranous expansion (e) ; to the superficial posterior sacro-iliac ligaments with which its fibres are blended; to the posterior surface and borders of the two last pieces of the sacrum; and to the posterior sacro- coccy- gean ligament and borders of the two or three upper coccygeal bones. From this broad at- tachment its fibres pass downwards, forwards, and outwards to be implanted into the whole length of the raised inner border of the great tuberosity of the ischium. The fibres of this ligament are arranged in fasciculi, which cross each other in an X-!ike manner, so as to present, at the extremities, an expanded appearance, and in the centre a thick con- tracted rounded outline. The fibres which are placed superiorly in one extremity of insertion cross at the contracted part to become inferior at the other extremity, while those which are internal cross in the opposite direction to become external. Its superior border, consequently, is directed outwards and forwards, and its inferior border inwards, and both present a curvilinear outline. At its insertion into the sciatic tuberosity, the fibres of the lower border present a falciform margin having the concavity directed upwards along the inner edge of the tuberosity, where it is united to the fascia covering the obturator in- ternus muscle. Its superficial or external fibres are continued over the tuberosity in- feriorly into the tendons of the biceps flexor cruris, and semi-tendinosus muscles. Near the posterior extremity, this ligament is almost invariably perforated by a small hole, through which passes the coccygeal branch of the ischiadic artery. To the whole length of its external or posterior surface is attached the great glutens muscle, which causes it when dissected to be very rough and flocculent. At the posterior half of its inner surface it is blended intimately with the lesser sacro-sciatic ligament, anterior to which it is smooth, and forms part of the boundary of the ischio- rectal fossa. The lesser or internal sacro-sciatic ligament (ligamentum pelvis posticum parvum, Jig. 81. d) lies internal to the last, in common with which it is attached posteriorly to the side of the two last pieces of the sacrum and of the two upper pieces of the coccyx. At its an- terior extremity it is contracted into a pointed insertion into the spine of the ischium. The direction of this ligament is horizontally for- wards and out wards, and its shape is triangular, so that its anterior contracted portion diverges from the great sacro-sciatic ligament, leaving a triangular opening between them through which pass the obturator muscle out of, and the pudic vessels and nerves into the pelvis This ligament, thus passing from the sacrum across to the ischium, converts the sacro- sciatic notch into a triangular or oval foramen through which pass the pyvamidalis muscle, the gluteal, ischiadic and pudic vessels, and the superior gluteal and great and lesser sciatic and pudic nerves out of the pelvis. With its PELVIS. 125 anterior or internal surface are blended the fibres of the ischio-coccygeus muscle, which exclude it from the ischio-rectal fossa, and render it rough when dissected. Soemmering describes the lower part of the powerful lumbar fascia as a ligament connect- ing the ilia to each other posteriorly and to the lower spines of the sacrum. This fascia does, doubtless, act powerfully in clasping the ilia upon the sacrum between them. He calls it the lateral sacro-iliac ligament, or the posterior lateral iliac ligament. The important part which these three ac- cessory ligaments play in the mechanism of the pelvis will be hereafter shown. The movements of the sacro-iliac joint are very limited indeed, its principal characteristic being compactness and strength, with just sufficient sliding motion downwards and back- wards to break the shock of concussion pass- ing from the lower extremities to the trunk. This is said by some to be increased in preg- nancy and by parturition. The pubic symphysis (Jig. 80. 2) is an azygos joint uniting the innominate bones by their pubic portions in front. The osseous surfaces composing it are oval, with the long diameter directed downwards and backwards, and ge- nerally an inch and a half long, by three quarters broad. The planes of these sur- faces not being directly opposed to each other, leave a larger interval of separation in front than behind. This interval is filled by a fibro- cartilaginous disc, which is correspondingly thicker in front, where the fibrous components are so numerous and strong as to constitute almost an interosseous ligament, and pass from one bone to the other in an oblique and concentric direction. Towards the central and posterior part this disc is generally mainly cartilaginous in structure, and is often, in females, separated in the middle by a chink forming two smooth, plane, oval contiguous articular surfaces, of various dimensions, some- times irregularly laminar, at others with a de- licate investing membrane. In parturient wo- men these surfaces often extend over nearly the whole of the articulation, and are well marked in a figure given by Dr. Hunter, in the second volume of Medical Observations and Inquiries. In males, this separation is seldom present. The whole of the disc may, however, by maceration, generally be separated into two plates (fig. 82. a, a), of a denser and more cartilaginous structure than the rest, each strongly adherent to the bone by mammil- liform fibrous processes (b), which pass into corresponding depressions in the osseous sur- faces (c), and are connected to each other on opposite sides, by continuation of their fibres, arranged in oblique and concentric layers, which interlace obliquely with each other, (r/) Dr. W. Hunter remarks, with Sanclifort and Albinus,that the two cartilaginous plates (a,a), covering the opposed surfaces of theossa pubis, are usually connected by a structure rather liga- mentous than cartilaginous ; and in a memoir on the pubic symphysis, gives an engraving of this arrangement. In several instances I have seen the fibrous processes which connect the plates with the bone very well marked, leav- Fig.82. Symphysis pubis after maceration. a, cartilaginous plates of Dr. Hunter; b, mam- millary processes on their osseous surface ; c, cor- responding osseous depressions to receive them ; d, inter-laminar concentric fibre-cartilaginous tissue divided vertically in the centre. ing on the bone, after maceration, deep conical pits. The above figure was taken from a ma- cerated preparation of this joint. According to the observations of Tenon, these processes are directed into the bone downward and backward, as well as outward, and tend to prevent displacement of the cartilage in that direction. The inter-laminar fibro- cartilagin- ous tissue is very elastic and yielding, swelling out on the cut surface when lateral pressure is made on the bone, somewhat in the manner of the intervertebral discs. It often evinces a tendency to split in a lamellar direction after maceration. Around the circumference the concentric fibres become much more numerous and strong, and are continued into the peri- pheral ligaments. These are an anterior, pos- terior, a superior, and an inferior ligament. The anterior pubic ligament (Jig. 80. rf) is a thick layer, passing between the anterior sur- faces of the bones, strengthened by and blended with the oblique fibres of the aponeurosis of the external oblique muscle continued to the opposite pubic bone in front of the joint. The posterior pubic ligament is the most feeble. It is composed of transverse fibres, somewhat scattered, and is remarkable in being raised by the posterior border of the pubic fibro-car- tilage into a vertical ridge, in old persons often very evident to the touch. It gives attachment to the superior true ligaments of the bladder, and the anterior fibres of the levator ani muscle. The superior pubic ligament (c) is formed by a thick, smooth layer of fibres often raised by a central ridge like the posterior, passing between the crests of the pubes, the super- ficial fibres extending over the greater part of the crests, and giving origin to the recti ab- dom males and pyramidales abdominal muscles, and linea alba. 126 PELVIS. The inferior or sub-pubic ligament, (%- mentum arcuatum, f) is the most powerful, passing from one descending ramus of the pubis to the other in an arched form. Its place of attachment to the pubis is often a well-marked surface, triangular, with the base upward, and half an inch in depth, cor- responding in this respect to the outline of the section of" this ligament. This ligament and the anterior are the most intimately con- nected with the fibro-cartilage of the joint. It unites below with the two layers of the deep perineal fascia or triangular ligament, be- tsveen which it gives origin to the vertical compressors urethra, and forms the superior boundary of the pubic arch, the apex of which it rounds off and smoothens. The movements of the pubic symphysis are confined to a slightly yielding sliding motion giving elasticity to the resistance of the pelvic ring. The obturator or thyroid membrane fg) is a fascial aponeurosis rather than a ligament, which closes in the oval foramen of that name. It is composed of layers of fibres, intermin- gling in a circular direction, and generally congregated more in some places than others. These are attached to the rough narrow bor- der of the descending branch of the ischiura externally, but at the internal half of its cir- cumference it is attached to the posterior sur- face of the ascending branch of the ischium and descending branch of the pubis, overlap- ping in this situation the borders of these bones posteriorly. Superiorly, it is inter- rupted by passing over from one edge of the sub-pubic notch to the other, so as to form the lower boundary of a foramen for the pas- sage of the obturator nerves and vessels. Opposite the cotyloid notch its fibres are continued into the capsular ligament invest- ing the hip joint. By its anterior surface, it gives attachment to the obturator external muscle, and, by its posterior surface, to the internal muscle of the same name. It is some- times deficient in one or more places. GENERAL APPEARANCE OF THE ARTICU- LATED PELVIS. When the bones of the pelvis are articulated together, its whole ap- pearance is that of a section of a cylinder or bent tube, having an anterior, posterior, and two lateral, and a superior and inferior aspects. Its anterior aspect (fig. 80.) is bounded on each side by a line passing from the anterior superior iliac spine, along the anterior border of the cotyloid cavity to the ischiadic tube- rosity on each side. It presents the pubic gymphysis directed downwards and forwards in the median line, and the obturator fora- mina directed forwards, outwards, and down- wards on each side. As first noticed by Cuvier, this oblique direction of the sym- physis pubis is peculiar to the human species, that of animals being parallel with the axis of the body. In addition to these parts, already described, are two large notches formed by the approximation of the inno- minate bones. Of these the superior one, which may be called the ventral notch, is formed by the vertical and horizontal portions of the anterior border of the innominate bones on each side with the peculiarities before men- tioned in its description. In the natural posi- tion of the pelvis this notch exposes to the view most of the internal surfaces of the pelvis to be described from the superior aspect. The inferior notch is formed by the oblique ascent towards the symphysis pubis of the branches of the ischium and pubis, forming what is termed the sub-pubic arch. Its apex is limited by the arched sub-pubic ligament, and there, in the male, it is generally about an inch wide, and at the base, between the ischiadic tuberosities, about three inches wide. The edges of this arch are in both sexes projected forwards, more or less, so as to present an in- clined surface to the plane of the arch. This eversion as well as the measurements are, however, considerably greater in the female pel VMS, hereafter to be considered. The lateral aspects of the pelvis present the anterior half of the external surface of the ilia abore ; the cotyloid cavities directed outwards, forward and downwards, in the middle ; and the descending branch and hinder part of the tuberosity of the ischia below, the latter being directed outwards and backwards. The posterior aspect presents the posterior surface of the sacrum and coccyx in the cen- tre, the most prominent point, in the erect position of the body, being the divided spine of the fourth sacral vertebra. On each side, next in succession, occur the overhanging and projecting tuberosities of the ilia, constituting two prominences next in importance, conceal- ing the sacro-iliac articulations, and caus- ing the lateral parts of the three upper sacral bones to appear as a deep groove on each side for the reception and origin of the powerful erector muscles of 'the back. Be- tween these points also the last lumbar ver- tebra appears sunk between the two iliac crests, so that its upper surface is on a level with their most elevated central portion. Below the sacrum, the coccyx projects downwards and forwards in a salient median point, which separates and completes the inner boundary of the sciatic notches on each side, converted into foramina by the greater and lesser sacro- sciatic ligaments. The distance of the edges of the sacrum and coccyx from the spines and tuberosities of the ischia, and consequently the size of the openings, is less in the male than in the female ; but the depth of the notches vertically is greater in the former. Above these are seen the posterior half of the external iliac surface, or external iliac fossa, surmounted by the rising crest. The superior aspect (fig* 80.) reveals to view the whole of the internal surface of the pelvis, which presents two well contrasted portions, divided by a rounded edge or border, of which the superior is wide, expanded, and deficient in front, and is called the large, or false pelvis ; and the inferior, narrower, more complete, and more compact, is called the smalt, or true pelvis ; while the border which separates them PELVIS. 127 is commonly expressed as the brim, or su- perior outlet of the true pelvis. The false pelvis is formed laterally by the con- cave surface of the internal iliac fossae directed upwards, forwards, and inwards ; and poste- riorly by the lateral masses of the base of the sacrum, directed upwards and forwards. In the middle is also seen, in the articulated pelvis, the anterior surface of the body of the last lumbar vertebra, filling up, with the pelvi- lumbar ligaments, the notch otherwise left be- tween the ilia behind. The superior border of the false pelvis is formed by the ilio -lumbar ligaments (vt hich exclude the iliac tuberosities), and the anterior three-fourths of the iliac crest, the most prominent point of which, in the proper position of the pelvis, is the centre of the posterior curve. It is terminated sud- denly, in front, by the anterior superior iliac spine, where the ventral notch commences: by the deficiency of osseous structure at this part. The brim of the pelvis is a heart- shaped opening, formed posteriorly by the body of the first sacral vertebra which overhangs the cavity of the true pelvis, so as to form a pro- jection called the promontory of the sacrum (/), corresponding to the indentation in the emble- matical heart-shape. On each side of this, the rounded arched anterior borders of the lateral masses of the sacral base continue the brim across the sacro-iliac joint, to the thick rounded ridge on the inner surface of the ilium, which is prolonged behind the ilio-pectineal eminence to the horizontal branch of the pubis where the brim becomes identified with the pectineal line. Finally, the brim is completed anteriorly by the shelving border of the body of the pubis, immediately behind the crest, and by the rounded superior part of the pubic symphysis. The part of the brim of the pelvis which is formed by the two portions of the innominate bone is sometimes called the linea Uio-pectinea, or, by some, the linea innominata. Sometimes the brim is called the inlet of the true pelvis. The cavity of the true pelvis is formed laterally by the plane sloping inner surfaces of the lower part of the ilia, opposite the cotyloid cavities, and of the descending branches of the ischia, the latter being termed by obstetricians the planes of the ischia ; in front, by the posterior surfaces of the branches and symphysis of the pubis, and by the as- cending branches of the ilia ; and behind, by the whole concave anterior surface of the sacrum and coccyx, the former being some- times called the hollow of Hie sacrum. From the oblique position of the pelvis, the posterior wall, which is the deepest, also reaches the highest, and the lateral walls the lowest ; the sub-pubic arch cutting out the anterior wall and leaving only the short symphysis pubis to represent it. The interval between the sa- crum and ossa innominata behind, forming the sacro-sciatic notch, is completed and bounded by the sacro-sciatic ligaments, the inner sur- faces of which are seen in this view. The inner surface of the coccyx is also seen to have an aspect directed upwards and for- wards, and the spines of the ischia to project considerably inwards, so as to present two opposite points, the distance between which may sometimes be of great importance in parturition. This projection is much greater in the male than the female, and will be al- luded to in the relative measurements of the pelvis. The cavity of the pelvis contracts uniformly downwards at the sides by reason of the inclination of the innominate bones ; but, from the vertical curvature of the sacrum, the antero-posterior diameter is much greater in the middle than at the superior or inferior outlets, which are hence termed straits. The presence of the obturator foramina antero- laterally, and of the sacro-sciatic foramina postero-laterally, must also be remarked as constituting four openings, diagonally op- posed to each other, capable, from the yield- ing nature of the structures filling them, of enlarging these diameters under sufficient pressure The great projection, forwards, of the coccyx and lower end of the sacrum may be considered as compensated for by the de- ficiency of the anterior wall in the sub-pubic arch directly opposite to them, gradually widening downwards as they advance. Both the forward direction of the coccyx, and the width of the pubic arch, are peculiar to the human species, and have reference to the erect posture. The inferior aspect of the pelvis presents to view the inferior strait, or outlet of the true pelvis; which, on account of its more limited extent than the superior outlet, reveals no- tiling of the interior save the overhanging promontory of the sacrum. It is remarkable in presenting three bony prominences, viz., the two tuberosities of the ischia laterally, and the coccyx posteriorly, separated by three notches, placed opposite to each prominence respectively, viz., the sacro-sciatic, postero- laterally, and the sub-pubic notch anteriorly. The sacro-sciatic notches being closed by the great sacro-sciatic ligament, the completely formed opening thus assumes a lozenge shape, of which the lower part of the pubic symphysis and the tip of the coccyx form the extremi- ties of the long diameter ; the tuberosities of the ischia those of the short diameter ; the oblique united rami of the ischia and pubes the antero-lateral, and the great sacro-sciatic ligaments the postero-lateral sides. Of these boundaries it is to be especially remarked, that the coccyx and those parts of the liga- ments which are attached to it, are not fixed like all the previously described boundaries of the pelvis, but movable, on the sacro- coccygeal articulation, and consequently, the diameters of this outlet dependent upon them, viz., the antero-posterior and the oblique or diagonal, are increased or diminished by the movements of this joint backwards or for- wards. The only fixed diameter of the in- ferior outlet of the pelvis is the transverse one between the ischial tuberosities. Of the prominent osseous points here seen, the lateral ischial tuberosities descend much lower than the symphysis pubis and coccyx, on ac- 128 PELVIS. count of the wavy outline and oblique direc- tion of the innominate bones. It is upon these tuberosities only, consequently, that the trunk rests in the sitting posture, and not upon a tripod formed by them and the coccyx, as has been erroneously supposed by some older writers. The boundaries of the inferior outlet, from the same cause, do not, like those of the superior, lie all in one plane or level, but are bent, as it were, at the ischial tuber- osities, into two planes ; an anterior, termin- ated by, and nenrlyin a line with, the symphysis pubis, looking downwards and a little for- wards ; and a posterior, terminated by and in- cluding the coccyx, directed downwards and backwards, parallel with the superior pelvic plane, but varying with the extension of the coccyx downw'ards. The plane of this outlet, however, is usually considered to be marked by a straight line joining the lower border of the symphysis pubis and the tip of the coccyx ; and its general direction to coincide with a line drawn perpendicular to this plane down- wards and backwards. Differences of the jie/vis in the sexes. Of all the bones in the human skeleton, those of the pelvis offer the most distinct characters between the male and female sex. In the female (fig. 83.), the bones are lighter, shorter, and broader, less evidently marked by tuberosities and indentations re- sulting from the attachments of the tendinous structures, and have in a less degree the peculiarities, before described, of the articu- lations, as well as those resulting from their peculiar mechanism. The iliac crest is less arched, and presents less distinctly the .S'-like curve, the iliac wings are thinner and more expanded, and the internal iliac fossce larger, mare shallow, and directed more anteriorly, and the iliac ridge extending between the cotyloid and sacro-iliac joints is less massy, less suddenly arched, and longer. The ischia do not converge so much towards the inferior outlet, and with the tuberosities are less massy, wider apart, and shorter, and the spines are less marked, and directed less inwards, and the transverse diameter of the inferior strait is greater. The ascending branches and the descending branches of the pubcs are thinner, narrower, and more oblique, turn their inner borders more forwards, and at the same time afford a more rounded expansion to the pubic arch, at the expense of the obturator foramina, which are thereby rendered smaller and more triangular in the female. The symphysis of the puhis is not so deep, and the fibro-cartilage is wider, thicker, and more vertical in position ; the united angles are more flattened posteriorly, and the horizontal branch is longer, thinner, and directed more transversely outwards, rendering the distance between the symphysis and the cotyloid cavity, and consequently the projection of the hips greater, and an increased transverse diameter of the brim. The sacrum is wider and less arched trans- versely, and its promontory does not so much overhang the pelvic cavity, and thus the su- perior outlet has less of the heart shape, being in females more properly termed oval. This difference of shape is also contributed to by the less lateral obliquity of the superior branch of the pubes. Whether the sacrum is less arched trans- versely in the female, I endeavoured to ascertain by observations taken from eighteen subjects, of which half were male and half female. A strip of lead ith of an inch thick was made to assume the form of the transverse curve of the sacrum, by being pressed across the anterior surface just below the promontory, and the breadth from one sacro-iliac joint to the other care- fully marked off. From this, a line was drawn on paper, following the curvature re- tained by the lead, the extremities of which line were joined by a straight line, forming a chord to the sacral arc. The distance of the centre of this chord from the centre of the sacral curve was then measured. In the nine males, the height of the arch thus obtained varied from six to nine lines ; in the nine females, five to nine lines, the greatest num- ber of the males being seven lines, and the greatest number of the females being six lines. In the single case of the female where the measurement was nine lines, the subject was old. When we consider, that in the great majority of instances the breadth of the sacrum measured along the curve was greater by i to ^ an inch in the female, these results will yield a still greater relative depth to the transverse sacral curve of the male. Besides this transverse arch, the vertical curvature of the sacrum is relatively much less in the female. This is more apparent in the direction of the three upper sacral pieces, which are generally little curved, and often almost plane in the female, while, in the male, the curve is most apparent in the centre and more uniformly distributed over the whole sacral surface. Upon this point, how- ever, much difference of opinion prevails amongst anatomists ; Meckel and Ward agree- ing with the opinion here enunciated, while Cloquet and Cruveilhier maintain that the curvature of the sacrum in the female is deeper and more regular. The experiments of Mr. Ward, however, correspond more entirely with my own observations on this point. Mr. Ward observes, in addition, that the male sacrum often approaches the form of the female, but the female rarely to that of the male. In old women, however, I have often seen a great vertical curvature of the sacrum. The coccyx is more moveable, more fre- quently in several jointed pieces, less pro- jected forwards, and less frequently ankylosed to the sacrum in the female. The sacro-scialic notches in the female are wider and not so deep as in the male ; the dis- tance from the ischiadic spine and tuberosity to the sacrum and coccyx being greater, and the sacro-sciatic ligaments longer and more slender. The peculiarities above mentioned give to the female pelvis a wider, shallower, more PELVIS. 129 open, and less massy appearance than that of the male, and give rise to a still more im- portant distinction derived from the measure- ments from one point to another, and from the relative diameters of the cavity and out- lets of the pelvis. Another distinction will be presently found in the relative angles which the sacrum and whole pelvis form with the axis of the spinal column, and this again will influence the relative direction of the a^vcs of the cavity and outlets. Fig. 83. Anterior view of the female pelvis, with lines of measurement. a b, conjugate diameter of brim ; c d, diagonal ditto ; ef, transverse ditto ; g h, transverse diameter of inferior outlet. The measurements of the pelvis. The most evident distinctions between the adult pelves of the sexes are derived from their com- parative dimensions, and result from the im- portant bearing they have upon the me- chanism of parturition in the female. For this purpose, an average is taken from the measurements of many well-formed pelves, and one with the average results is adopted as the standard pelvis. The measurements referring to the width of the pelvis are commonly spoken of as the diameters of the pelvis. They are taken at the brim, in the cavity, and at the inferior outlet, and are usually an anterior-posterior or conjugate, two diagonal or oblique, and a transverse, At the brim of the pelvis, the antero-pos- terior or conjugate diameter is the distance between the upper part of the posterior sur- face of the symphysis pubis and the pro- montory of the sacrum (a, b, Jig. 83.) ; the oblique, between the point of the brim nearest the pectineal eminence and the sacro-iliac joint of the opposite side (c, d) ; and the transverse diameter is the distance between the ilia at a point halfway between the sacro- iliac joint and pectineal eminence (e,f). In the cavity, the antero-posterior diameter extends between the centre of the pubic sym- physis, and the body of the third piece of the sacrum ; while the oblique and transverse correspond to those of the upper outlet, on the same plane. At the inferior strait, the antero-postorior extends from the lower extremit of the symphysis pubis to the tip of the coccyx ; and the transverse, from the middle of the inner border of one ischiadic tuberosity to the other (g, //). An oblique diameter at the inferior outlet is not one commonly given by writers, although possessed of some import- ance in certain cases of deformity. In the table on the next page, there is the average of six measurements taken on the recent subject, before the shrinking of the ligaments, from the centre or junction of the ischio-pubic rami to the centre of the great sciatic liga- ment opposite. The antero-posterior diameter of this strait is capable of much increase by the mobility of the coccyx, which will also affect, in some measure, the oblique diameters, in an opposite degree, from the stretching of the great sciatic ligaments, a point which I think has scarcely been sufficiently noticed by accoucheurs. Besides these, the distances between many other points may be of great importance to the accoucheur. Such are those pointed out by Naegele, to be presently noticed ; the distances between the spines of the ischia, so much greater in the female ; and another, which I have not hitherto seen definitively given, viz. the distance between the lower edge of the symphysis pubis and the sacral promontory, a measurement of considerable importance in the use of pelvimeters, to ascertain the conjugate diameter of the brim. This may be called the lower or inclined conjugate diameter, and it will be found to be, in most instances, half an inch more than the direct or superior conjugate diameter, being, in fact, the longest side of a triangle, having the conjugate diameter, and the breadth of the pubic symphysis for the other sides. The measurement of the circumference of the brim of the pelvis, and the proportion con- tributed to it by the sacrum, ilia and pubes respectively, announce a manifest difference between the pelves of the two sexes. In glancing over the appended table, it will be seen that the male pelvis exceeds the female in most of its vertical dimensions, while the female pelvis is larger in the horizontal di- ameters. The depth of the true pelvis, however, measured at the sacro-coccygeal column, is greater in the female, on account of the greater size of the sacrum in that sex, and also because of the less total vertical curvature. The depth from the pectineal eminence to the lowest point of the ischiadic tuberosities laterally, and at the pubic sym- physis anteriorly, show, on the contrary, a great superiority in the male ; as also does the total depth of the whole pelvis, from the highest point of the ilium to the most de- pending part of the ischium, while the width between the iliac spines and crests are much greater in the female. The horizontal dia- meters of the pelvis may be said to depend upon the ilio-pubic element, while the depth or vertical measurement depends solely on the ilio-ischion clement, so that, in the female, the former may be considered to prevail, and in the male, the hitter dement. This is re- 130 PELVIS. markable, as constituting the different pelvic properties of certain classes of animals. It will also be observed that the transverse diameter of the brim is the greatest in the dry bones, but this is so diminished by the presence of the iliac and psoas muscles and fascia, that, in the living female subject, the oblique is generally the best adapted to receive the long diameter of the foetal skull. The soft structures diminish the antero- posterior diameters of the brim by about a quarter of an inch, and the transverse, by half an inch ; the diameters of the cavity being lessened about a quarter of an inch ; a fact which it is necessary to bear in mind in estimating the width in the living subject. The measure- ments in the third double column were taken from fourteen male and eighteen female sub- jects in the dissecting room of King's College, London, and are compared in the first column with the contrasted measure- ments of the male and female pelvis given by Meckel, and quoted by most English writers on the subject ; and in the second column with those given by John James Watt, in his work on the pelvis. Uleckel. Watt. Diameters, Male. Female. Male. Female. Male. Female. in. lines. in. lines. in. lines. in. lines. in. lines. in. lines. Of the brim Transverse 4 6 5 4 6 5 6 4 7 5 2 Oblique 4 5 4 5 4 2 5 4 7 5 Antero -posterior 4 4 4 a 4 4 9 a 4 4 5 a Of the cavity Transverse 4 4 8 Oblique 5 5 4 Antero-posterior 5 4 8 - . 4 8 4 8 Of the inferior strait Transverse (inter-sciatic) 3 4 5 b 3 2 4 O b 3 5 4 4 b Oblique - - . _ 3 2 4 Antero-posterior 3 3 4 4 3 4 6 3 5 4 c Measurements. Between the anterior superior iliac spines 7 8 8 G d 9 11 " 8 8 10 d Between the centres of iliac crests 8 3 9 4 c Depth of true pelvis Between the upper and lower border of symphysis pubis 1 10 1 6 2 1 7 Between the ilio-pectineal eminence and ischial tu- berosity - ... 4 10 3 6 4 5 3 8 Between the sacral promontory and tip of coccyx 4 10 5 4 6 C 5 in. to I 6 in. Depth of whole pelvis Between the iliac crest and ischial tuberosity 8 7 7 5 f Between the anterior superior iliac spine and ischial tuberosity 6 5 6 ,, posterior superior iliac spine and ischial tuberosity 6 5 5 Between the lower border of pubic syrnphysis and sacral promontory - 4 7 spines of ischia - 3 5 4 3 ,, sacro-iliac joints (greatest breadth of sacrum) 4 3 4 8 s a 4 inches (Burns, Ramsbotham, Lee, Cloquet, Velpeau, and Baudelocque). Boivin). 4'3 inches (Rigby). b 4 inches (Burns, Lee, and Cloquet). 4^ inches (Monro and Murphy). c Increased to 5 inches or more by the mobility of the coccyx. d 10 inches (Burns). 9'G inches (Cloquet). c 10 inches (Cloquet). 11 inches (Burns). f 7 inches (Cloquet). s 4 to 4'j inches (Cloquet). 4J inches (Monro and The circumferential measurement of the brim in well-formed males gave in my own mea- surements 2 inches to each of the ilia, 3 inches to each of the pubes, and 4.J to the sacrum, which, allowing -J- inch to each of the sacro- iliac cartilages and \ inch to the pubic, gives a total circumference of 15^ inches. In the well-made female the ilia were found to be each 2|, the pubes each 3^, and the sacrum 5 inches, giving, with the same allowance for the sacro-iliac cartilages and -*- inch for the pubic, a total of 17^ inches. Thus the superior size of the brim in the female seems to depend more upon the ilia than upon the pubes, although the direct distance between the ilio-pectineal eminence and the sacro-iliac joint differs little in the sexes, because of the greater curve made by the female ilia. The circumferential extent of the borders, at the plane of the inferior outlet in a female pelvis of average diameters, and dried with thesacro- sciatic ligaments attached, was 14 inches. In the fresh state it generally amounts to lo, as the ligaments shrink by drying, and would be extended to 16 inches, or more, by the ex- tension of the coccyx and the elasticity of the ligamentous portions.* * The circumferential measurement appears to be one not generally estimated as much as its utility in detecting variations of size depending upon shape would seem to call for, in the female pelvis. A reference to the subjoined table of variations of dia- fii Burns gives also, in the female pelvis, the following distances: PELVIS. 131 2. 3. 5. 1. Between the symphysis pubis and inferior iliac spine, nearly - - 4 in. sacro-iliac joint and the pubic crest of same side - - 4i sacral promontory and the obturator notch - - 3 sacral promontory and the acetabula 31 acetabula anteriorly - 4^ C. posterior ridge of ilium and the su- perior and inferior anterior spines - 5 7. ,i centre of iliac crest and the brim of the pelvis, direct - 3i One of these measurements was repeated by Velpeau, Stoltz, and Naegele, viz. from the sacral promontory to the centre of the cotyloid cavity, or sacro-cotyloid. Naegele in 54 and Stoltz in 40 female pelves, found the mean distance to be 3 ponces, 3 to 4 lignes (pied du Roi). Dr. Murphy, considering that the true salient point or promontory lies on a level above the real pelvic brim, at the sacro- lumbar fibro-cartilage, gives also three more measurements made in the " inclined plane of the promontory," one antcro-postcrior, be- tween the fibro-cartilage and the upper border of the symphysis, which he places at 4 inches, and two lateral, from the same point to the pectineal eminences, which are on an average about 3^ inches, but which are seldom equal, because of the great tendency to deviation of this promontory from the median line. The latter seem to coincide almost with those given by Dr. Burns between nearly the same points, and the former with the conjugate diameter of the brim. External measurements of the female pelvis, made on the living subject, have also been given, Chough from few data, as follows : 1. External antero-posterior diameter, 7 to 8 inches. 2. External transverse, between iliac crests, 13 to 1 6 inches. 3. From great trochanter to the opposite sacro-iliac joint, 10 to 12 inches. 4. Depth of pelvis from top of sacrum to coccyx, 4 to 5 inches. From the first of these, according to Bau- delocque and Velpeau, 3 inches must be de- ducted for the thickness of the parietes, and from the second 4 inches. Boivin and Lacha- pelle doubted the utility of these measure- metcr, will show how frequently the diameters are compensatory to each other; as this compensation may occur m diameters not usually measured, the circumferential extent seems in many cases to be required. Dr. Churchill gives the circumference pt the brim as varying from 13 to IU inches in the female, ranch less than I have generally found it on the fresh subject after the soft parts were removed ments generally, because of the great varia- bility in the thickness of the pelvic walls; and Dr. Davis has more recently found the thick- ness of the base of the sacrum to vary from 2 to 3 inches in 17 dead subjects. The measurements of Naegele and Otto, with a view to determine the presence of obliquely deformed pelves, are of great im- portance in the practice of midwifery, and may be best given in this place. Out of forty- two female pelves of medium size, the best formed they could obtain, these observers found the following measurements : 1. From the sciatic tuberosity ") of one side to the posterior I superior iliac spiue of the f other side - J 2 From the anterior superior iliac spine of one side to the posterior superior of the other side 3. From the spine of the last lumbar vertebra to the an- terior superior iliac spine on both sides 4. From the great trochanter "1 of one side to the posterior I superior iliac spine of the [ other - J 5. From the middle of the in- 'I ferior border of the sym- physis pubis to the pos- > tcrior superior iliac spine f on both sides - J Mea- sure- ment. Greatest Diffe- rence. in. lines 6 G 7 3 G 8 8 5 6 4 lines. 4 to 5 Danyan, pursuing Naegele's system, found the great rarity of perfectly regular female pel- ves. Out of eighty female pelves he found fifty- nine differ, in the first measurement, from 1 to 6 lines. In the second measurement he found a difference, in fifty-eight pelves, of 1 to 1 1 lines ; in the third, fifty-one differed from 1 to 7 lines ; in the fourth, sixty-two from 1 to 9 lines; and in the fifth measurement, fortv- eight pelves had a difference of from 1 to' 9 lines. The table on the next pnge shows the great variety in the diameters of female pelves which may be considered as normal pelves. In males Dupuytren found the distance between the tuberosities of the ischia, in twenty-three subjects, to vary from 2 to 3i inches; and Velpeau, in forty subjects, to vary from If to 4 inches. In fourteen subjects I have found the least distance to be 3 inches, and the greatest 4 inches in the male, and measuring from the exact centres of the inner margin of the tuberosities. These observations on the male are of some im- portance with a view to the operation of lithotomy, when the stone is of great size. INCLINATION OF THE PELVIS. By making, in a well-formed subject, a direct vertical section of the spinal column, and drawing a line through the centres of the bodies of die axis and last lumbar vertebra, and by com- paring with the transverse plane of such a K 2 132 PELVIS. line those of the superior and inferior outlets pelvis to the vertebral column is obtained, of the pelvis, the general inclination of the The line so drawn will generally be found to VARIATIONS in the Diameters of healthy Female Pelves. Dr. Murphy (in 18 Cases). Taken by the writer in the King's College dissecting rooms (in 18 Case*). Extremes. Most fre- quent. Extremes. Most fre- quent. Inclined plane of promontory inches. inches. inches. inches. Antero-posterior - *3| or 3i{ to 51 4\ to 4J To left pectineal erninence- *3 or 3| to 4| 31 to 31 To right pectineal eminence *2| or 3>j to 51 3| Brim Autero-posterior *3| or 3| to 5J 4 3% to 4f 4* Transverse t3|or4ito5| 51 5 to 5| 5 /-,,. f Left - Clique { Kight l *4\ or 4f to 51 *4| or 4f to 5{ 4| 1 5 and 5 j 4f to 51 5 and 5^ Between promontory and lower edge of symphysis pubis - - 3^ to 51 4;{ and 5 Cavity Autero-posterior *4ior4lto5| 5 41 to 5f 4i| and 5\ Transverse *3|or4}to5i 5 - . Between ischiadic spines - - 3} to 41 4} Outlet Antero-posterior $31 to 41 4 to 4 r > 31 to 4} 4 Transverse 3toj5 4| to 4^ 3ij to 4| 4 5 Oblique (6 cases only) - 31 to 4^ 4 Angle of pubic arch - 45 to 100 70 to 90 * Like male pelvis, diameters small. -|- Smallest pelvis, transverse diameter of cavity 4|, of outlet 4|. j Belong to the same pelves respectively (compensating diameters). pass also through the bodies of the first dorsal and second lumbar vertebrae across their centres. The curved line of the vertebras, in most well-formed subjects, cuts the straight line at these two points, in passing from the cervical to the dorsal, and from the latter to the lumbar curve. The plane of the pelvic brim has been termed by Naegele and the brothers G. and E. Weber the superior plane of the pelvis, and that of the inferior outlet the inferior plane, These observers measured the angle formed by these planes with the ground-level in the standing position, i. e. with the horizon, or with a plane drawn horizontally, at right angles, to the above-mentioned transverse vertical plane, which, in the erect posture, was found to be perpendicular to the base of support. The angle which the superior plane of the pelvis forms with the transverse vertical plane or with the horizon is termed by them the angle of inclination of the pelvis, or the pelvi- vertebral angle (fig. 84. page 134.), (ae, ec). It is remarkable that, in man only, are the boundaries of the superior outlet in one plane, i. c. in man only is the direction of the superior pubic rannts in the same plane with that of the cott/lo-sacral rib of the ilium. In all other ani- mals, as far as my own observations go, the pubis is bent backward or forward, so as to make an angle with the ilium, and the pe/vi- vcrtcbral angle is thus resolved into two angles, a vcrtebro-Uiac and an ilio-pitbic. The angle of the superior plane was found by the Webers on the dead body, by fixing the connected spinal column and pelvis of a recent well-made subject, in plaster of Paris, to preserve the natural position, then making through the whole a direct vertical section, ami afterwards measuring off the angles. On making a transverse vertical section through the centres of the heads of the femurs and cotyloid cavities, they also found that, when the body is in the erect position and the pelvis at the proper angle, the coty- loid notch and depression, and the fibres of the liganientum teres, have an almost directly vertical direction, and fall exactly in the trans- verse vertical plane of the vertebras (see jig. 87. page 140., in which the line a a' lies in the plane of the transverse vertical section). It will be further seen, by inspecting the figure, that this plane, being continued downwards, crosses the obturator foramina, and falls very nearly in the line of suture of the ischio-pubic rami. And this will be found to be the case, with a plumb line dropped from the sacral pro- montory, which is cut by the above plane in the erect position of the pelvis. A detached pelvis may be placed in the erect living po- sition, consequently, by keeping the poste- rior part of the notch the most depending point of the cotyloid brim, and its inclinations will then accord with those taken in connec- tion with the spine. In the consideration of these pelvic angles it must be borne in mind that the direction of the curve of the three last lumbar vertebras, below the point where the great dorsal con- cavity terminates, is such that, if prolonged upwards, the axial line would pass out at the junction of the manubrium with the body of the sternum. This makes the pclvi-lumbar angle much less in man than the whole pelvi- vertebral ; a circumstance to be borne in mind in comparing them with those of animals. In fact, the transvertical section just mentioned passes through the body of the third lumbar vertebra considerably posterior to its centre in most cases (sec a, b,Jig. Si. page 134.). PELVIS. 133 By an inverse method, proceeding on Roederer's plan from the horizontal plane (j%. 84. , vertical plane - ) With horizon 106 51' 16 51' 101 to 102 10 to 11* By the inspection of the above table the greater inclination of the pelvis to the spine in the male will become evident, constituting another distinguishing characteristic of the sexes. . The older observers estimated the pelvic anMes too low, as in the incorrect drawings of^Albinus, Levret, and Cloquet, where the superior an^le is given as 35 with the horizon, and, by Oslander, at 30. Cams gives the superior at 55, and the inferior at 11 with the horizon. Angles of the anterior and posterior pelvic walls with the transverse vertical plane. The pelvic inclination, in the opinion of Cm veilhier, depends upon the angle which the sacrum forms with the spinal column, giving more or less of obliquity to the innominate bones on each side. This angle (/g. 84. next pa^e, a e i, and fig. 112. I. fa g, page 173), winch may be called the sacra-vertebral angle t I have, in as many opportunities as have oc- curred to me, endeavoured to ascertain and establish, with a view of comparing it with the pelvi-vertebral angle in the two sexes. To do this I made a vertical section of the pelvis (with as many vertebrae as possible attached to it), from behind forward in the median line, which showed clearly the angle made by the sacrum. Then, by intersecting the line of the transverse vertical plane of the spinal column drawn as before mentioned, by a line drawn in the mean direction of the three first sacral vertebra through the centre of their bodies, an<"les closely approximative to the sacro- vertebral ansle in the living subject were ob- tained, showing the following results : * Weber, however, found the angle of the inferior plane with the horizon to be but little less marked than that of the male; making it 4'5 more than the ancle of Naegele here given. Naegele remarks that the inferior angle is much more variable than the superior in ordinary cases. K .J 134. PELVIS. In twenty-five males, Nine were from 1 1 6 to 1 1 2, five from 1 15 to 117, nine from 1-20 to 125, and two only 130. In twenty-five females, Nine were- from 120 to 125, eight from 128 to 130, five from 133 to 140, two were 145, and one, an aged subject, 118 only. Fig. 81. A' 9- Diagram (sl'ujhily altered from Naegele) of a well- formed female pelvis^ showing the angles of inclina- tion and axes. From these we may deduce 117 as the average sacro-vertebral angle in the male, and 130 as the same angle in the jema/e. This remarkable average difference of 13 shows the much greater suddenness of the altera- tion of direction in the spinal column at its sacral extremity in the male subject, and is much greater than the difference of 5 to 6 in the pelvic inclination of the sexes com- pared in the tables of Weber and Nacgele. But, in order more clearly to ascertain it the pelvic inclination invariably depended upon the variations of the sacro-vertebral angle, I compared the sacro-vertebral and pelvKverte- bral angles in nine male and nine female sub- jects. In the former, I found the difference between these angles to vary from 5 to 35, and, in the latter, to vary from 5 to 25. In one instance only, in a male, the sacro-verte- bral was as large as the pelvi-vertebral angle. From these observations, which were very carefully taken, it would seem that the total pelvic inclination does not exactly depend upon the sacro-vertebral angle ; and that, in males, where the average pelvic obliquity is a little greater, the average sacro-vertebral angle is much and disproportionately less. These results contradict, also, the assumption somewhat indefinitely stated by Blumenbach and others, on the authority of Bonaccioli, of Ferrara, that the sacrum inclines more backward, and that the sacro-vertebral angle is more promi- nent in the female than in the male. If the long diameter of the pubic symphysis be continued in its direction downwards and backwards, it will, in a well-formed female pelvis, cut the transverse vertical plane of the spine, also prolonged, at an angle of 50 to 55 (j%. 84. &), which will be found to beabout the complementary or opposite angle to the sacro-vertebral angle in the female. This shows the general parallelism of the anterior or pubic wall of the pelvis, with the upper part of the posterior or sacral wall, although, on account of the rapid thinning of the latter as it descends, its pelvic surface seems to diverge from the pubis. Naegele found the anterior pelvic wall to be often at right angles to the plane of the inlet, but the posterior generally somewhat more than a right angle. The great obliquity of the symphysis pubis to the transverse vertical plane of the vertebrae is one of the great characteristics of the human pelvis, as will be seen hereafter in the consider- ation of the comparative anatomy of the pelvis. The angle formed by the symphysis pubis with the horizon is given by Cuvier from 75 to 95. This is much too large ; from 35 to 40 is the true angle of the symphysis with the horizon in the human subject. Ilio-ischial angle. While the pubis in the human subject is continued in the same right line with the mean direction of the ilium, which coincides with the cotylo-sacral rib of that bone, the ischium is inclined backwards, forming an angle of 1 10 to 1 15 with the same rib of bone (sec^/zg. .! 12. \.a c