In the very interesting paper of Foulis, the conclusion is arrived at, that while the ova are derived from the germinal epithelium, the cells of the follicle originate from the ordinary connective tissue cells of the stroma. Foulis regards the zona pellucida as a product of the ovum and not of the follicle. To both of these views I shall return, and hope to be able to shew that Foulis has not traced back the formation of the follicle through a sufficient number of the earlier stages. It thus comes about that though I fully recognise the accuracy of his figures, I am unable to admit his conclusions. Kölliker's statementsare again very different from those of Foulis. He finds certain cords of cells in the hilus of the ovary, which he believes to be derived from the Wolffian body, and has satisfied himself that they are continuous with Pflüger's egg-tubes, and that they supply the follicular epithelium. To the general accuracy of Kölliker's statements with reference to the relations of these cords in the hilus of the ovary I can fully testify, but am of opinion that he is entirely mistaken as to their giving rise to the follicular epithelium, or having anything to do with the ova. I hope to be able to give a fuller account of their origin than he or other observers have done.
My investigations on the mammalian ovary have been made almost entirely on the rabbit—the type of which it is most easy to procure a continuous series of successive stages; but in a general way my conclusions have been controlled and confirmed by observations on the cat, the dog, and the sheep. My observations commence with an embryo of eighteen days. A transverse section, slightly magnified, through the ovary at this stage, is represented onPl.26, fig. 35, and a more highly magnified portion of the same in fig. 35A. The ovary is a cylindrical ridge on the inner side of the Wolffian body, composed of a superficial epithelium, the germinal epithelium (g.e.), and of a tissue internal to this, which forms the main mass of it. In the latter two constituents have to be distinguished—(1) an epithelial-like tissue (t), coloured brown, which forms the most important element, and (2) vascular and stroma elements in this.
The germinal epithelium is a layer about 0.03 - 0.04mm.in thickness. It is (videfig. 35A,g.e.) composed of two or three layers of cells, with granular nuclei, of which the outermost layer is more columnar than the remainder, and has elongated rather than rounded nuclei. Its cells, though they vary slightly in size, are all provided with a fair amount of protoplasm, and cannot be divided (as in the case of the germinal epithelium of Birds, Elasmobranchii,&c.), into primitive ova, and normal epithelial cells. Very occasionally, however, a specially large cell, which, perhaps, deserves the appellation primitive ovum, may be seen. From the subjacent tissue the germinal epithelium is in most parts separated by a membrane-like structure(fluid coagulum); but this is sometimes absent, and it is then very difficult to determine with exactness the inner border of the epithelium. The tissue (t), which forms the greater mass of the ovary at this stage, is formed of solid columns or trabeculæ of epithelial-like cells, which present a very striking resemblance in size and character to the cells of the germinal epithelium. The protoplasm of these cells stains slightly more deeply with osmic acid than does that of the cells of the germinal epithelium, so that it is rather easier to note a difference between the two tissues in osmic acid than in picric acid specimens. This tissue approaches very closely, and is in many parts in actual contact with the germinal epithelium. Between the columns of it are numerous vascular channels (shewn diagrammatically in my figures) and a few normal stroma cells. This remarkable tissue continues visible through the whole course of the development of the ovary, till comparatively late in life, and during all the earlier stages might easily be supposed to be about to play some part in the development of the ova, or even to be part of the germinal epithelium. It really, however, has nothing to do with the development of the ova, as is easily demonstrated when the true ova begin to be formed. In the later stages, as will be mentioned in the description of those stages, it is separated from the germinal epithelium by a layer of stroma; though at the two sides of the ovary it is, even in later stages, sometimes in contact with the germinal epithelium.
In most parts this tissue is definitely confined within the limits of the ovary, and does not extend into the mesentery by which the ovary is attached. It may, however, be tracedat the anterior endof the ovary into connection with the walls of the Malpighian bodies, which lie on the inner side of the Wolffian body (videfig. 35B), and I have no doubt that it grows out from the walls of these bodies into the ovary. In the male it appears to me to assist in forming, together with cells derived from the germinal epithelium, the seminiferous tubules, the development of which is already fairly advanced by this stage. I shall speak of it in the sequel as tubuliferous tissue. The points of interest in connection with it concern the male sex, which I hope to deal with in a future paper, but I have nohesitation in identifying it with the segmental cords (segmentalstränge) discovered by Braun in Reptilia, and described at length in his valuable memoir on their urogenital system[405]. According to Braun the segmental cords in Reptilia are buds from the outer walls of the Malpighian bodies. The bud from each Malpighian body grows into the genital ridge before the period of sexual differentiation, and sends out processes backwards and forwards, which unite with the buds from the other Malpighian bodies. There is thus formed a kind of trabecular work of tissue in the stroma of the ovary, which in the Lacertilia comes into connection with the germinal epithelium in both sexes, but in Ophidia in the male only. In the female, in all cases, it gradually atrophies and finally vanishes, but in the male there pass into it the primitive ova, and it eventually forms, with the enclosed primitive ova, the tubuli seminiferi. From my own observations in Reptilia I can fully confirm Braun's statements as to the entrance of the primitive ova into this tissue in the male, and the conversion of it into the tubuli seminiferi. The chief difference between Reptilia and Mammalia, in reference to this tissue, appears to be that in Mammalia it arises only from a few of the Malpighian bodies at the anterior extremity of the ovary, but in Reptilia from all the Malpighian bodies adjoining the genital ridge. More extended observations on Mammalia will perhaps shew that even this difference does not hold good.
It is hardly to be supposed that this tissue, which is so conspicuous in all young ovaries, has not been noticed before; but the notices of it are not so numerous as I should have anticipated. His[406]states that the parenchyma of the sexual glands undoubtedly arises from the Wolffian canals, and adds that while the cortical layer (Hulle) represents the earlier covering of a part of the Wolffian body, the stroma of the hilus, with its vessels, arises from a Malpighian body. In spite of these statements of His, I still doubt very much whether he has really observed either the tissue I allude to or its mode of development. In any case he gives no recognisable description or figure of it.
Waldeyer[407]notices this tissue in the dog, cat, and calf. The following is a free translation of what he says, (p. 141):—“In a full grown but young dog, with numerous ripe follicles, there were present in the vascular zone of the ovary numerous branched elongated small columns (Schläuche) of epithelial cells, between which ran blood-vessels. They were only separated from the egg columns of the cortical layer by a row of large follicles. There can be no doubt that we have here remains of the sexual part of the Wolffian body—the canals of the parovarium—which in the female sex have developed themselves to an extraordinary extent into the stroma of the sexual gland, and perhaps are even to be regarded ashomologues of the seminiferous tubules(the italics are my own). I have almost always found the above condition in the dog, only in old animals these seminiferous canals seem gradually to atrophy. Similar columns are present in the cat, only they do not appear to grow so far into the stroma.”Identical structures are also described in the calf.
Romiti gives a very similar description to Waldeyer of these bodies in the dog[408]. Born also describes this tissue in young and embryonic ovaries of the horse as theKeimlager[409]. The columns described by Kölliker[410]and believed by him to furnish the follicular epithelium, are undoubtedly my tubuliferous tissue, and, as Kölliker himself points out, are formed of the same tissue as that described by Waldeyer.
Egli gives a very clear and accurate description of this tissue, though he apparently denies its relation with the Wolffian body.
My own interpretation of the tissue accords with that of Waldeyer. In addition to the rabbit, I have observed it in the dog, cat, and sheep. In all these forms I find that close to the attachment of the ovary, and sometimes well within it, a fair number of distinct canals with a large lumen are present, which are probably to be distinguished from the solid epithelial columns. Such large canals are not as a rule present in the rabbit. In thedog solid columns are present in the embryo, but later they appear frequently to acquire a tubular form, and a lumen. Probably there are great variations in the development of the tissue, since in the cat (not as Waldeyer did in the dog) I have found it most developed.
In the very young embryonic ovary of the cat the columns are very small and much branched. In later embryonic stages they are frequently elongated, sometimes convoluted, and are very similar to the embryonic tubuli seminiferi. In the young stages these columns are so similar to the egg tubes (which agree more closely with Pflüger's type in the cat than in other forms I have worked at) that to any one who had not studied the development of the tissue an embryo cat's ovary at certain stages would be a very puzzling object. I have, however, met with nothing in the cat or any other form which supports Kölliker's views.
My next stage is that of a twenty-two days' embryo. Of this stage I have given two figures corresponding to those of the earlier stage (figs. 36 and 36A).
From these figures it is at once obvious that the germinal epithelium has very much increased in bulk. It has a thickness 0.1 - 0.09mm.as compared to 0.03mm.in the earlier stage. Its inner outline is somewhat irregular, and it is imperfectly divided into lobes, which form the commencement of structures nearly equivalent to the nests of the Elasmobranch ovary. The lobesare notseparated from each other by connective tissue prolongations; the epithelium being at this stage perfectly free from any ingrowths of stroma. The cells constituting the germinal epithelium have much the same character as in the previous stage. They form an outer row of columnar cells internal to which the cells are more rounded. Amongst them a few large cells with granular nuclei, which are clearly primitive ova, may now be seen, but by far the majority of the cells are fairly uniform in size, and measure from 0.01 - 0.02mm.in diameter, and their nuclei from 0.004 - 0.006mm.The nuclei of the columnar outer cells measure about 0.008mm.They are what would ordinarily be called granular, though high powers shew that they have the usual nuclear network. There is no special nucleolus. The rapid growth of the germinal epithelium is dueto the division of its cells, and great masses of these may frequently be seen to be undergoing division at the same time. Of the tissue of the ovary internal to the germinal epithelium, it may be noticed that the tubuliferous tissue derived from the Malpighian bodies is no longer in contact with the germinal epithelium, but that a layer of vascular stroma is to a great extent interposed between the two. The vascular stroma of the hilus has, moreover, greatly increased in quantity.
My next stage is that of a twenty-six days' embryo, but the characters of the ovary at this stage so closely correspond with those of the succeeding one at twenty-eight days that, for the sake of brevity, I pass over this stage in silence.
Figs. 37 and 37Aare representative sections of the ovary of the twenty-eighth day corresponding with those of the earlier stages.
Great changes have become apparent in the constitution of the germinal epithelium. The vascular stroma of the ovary has grown into the germinal epithelium precisely as in Elasmobranchii. It appears to me clear that the change in the relations between the stroma and epithelium is not due to a mutual growth, but entirely to the stroma, so that, as in the case of Elasmobranchii, the result of the ingrowth is that the germinal epithelium is honeycombed by vascular stroma. The vascular growths generally take the paths of the lines which separated the nests in an earlier condition, and cause these nests to become the egg tubes of Pflüger. It is obvious in figure 37 that the vascular ingrowths are so arranged as imperfectly to divide the germinal epithelium into two layers separated by a space with connective tissue and blood-vessels. The outer part is relatively thin, and formed of a superficial row of columnar cells, and one or two rows of more rounded cells; the inner layer is much thicker, and formed of large masses of rounded cells. The two layers are connected together by numerous trabeculæ, the stroma between which eventually gives rise to the connective tissue capsule, or tunica albuginea, of the adult ovary.
The germinal epithelium is now about 0.19 to 0.22mm.in thickness. Its cells have undergone considerable changes. A fair number of them (fig. 37A,p.o.), especially in the outer layer of the epithelium, have become larger than the cells aroundthem, from which they are distinguished, not only by their size, but by their granular nucleus and abundant protoplasm. They are in fact undoubted primitive ova with all the characters which primitive ova present in Elasmobranchii, Aves,&c.In a fairly typical primitive ovum of this stage the body measures 0.02mm.and the nucleus 0.014mm.In the inner part of the germinal epithelium there are very few or no cells which can be distinguished by their size as primitive ova, and the cells themselves are of a fairly uniform size, though in this respect there is perhaps a greater variation than might be gathered from fig. 37A. The cells are on the average about 0.016mm.in diameter, and their nuclei about 0.008 to 0.001mm., considerably larger, in fact, than in the earlier stage. The nuclei are moreover more granular, and make in this respect an approach to the character of the nuclei of primitive ova.
The germinal epithelium is still rapidly increasing by the division of its cells, and in fig. 37Athere are shewn two or three nuclei in the act of dividing. I have represented fairly accurately the appearance they present when examined with a moderately high magnifying power. With reference to the stroma of the ovary, internal to the germinal epithelium, it is only necessary to refer to fig. 37 to observe that the tubuliferous tissue (t) forms a relatively smaller part of the stroma than in the previous stage, and is also further removed from the germinal epithelium.
My next stage is that of a young rabbit two days after birth, but to economise space I pass on at once to the following stage five days after birth. This stage is in many respects a critical one for the ovary, and therefore of great interest. Figure 38 represents a transverse section through the ovary (on rather a smaller scale than the previous figures) and shews the general relations of the tissues.
The germinal epithelium is very much thicker than before—about 0.38mm.as compared with 0.22mm.It is divided into three obvious layers: (1) an outer epithelial layer which corresponds with the pseudo-epithelial layer of the Elasmobranch ovary, average thickness 0.03mm.(2) A middle layer of small nests, which corresponds with the middle vascular layer of the previous stage; average thickness 0.1mm.(3) An inner layer of larger nests; average thickness 0.23mm.
The general appearance of the germinal epithelium at this stage certainly appears to me to lend support to my view that the whole of it simply constitutes a thickened epithelium interpenetrated with ingrowths of stroma.
The cells of the germinal epithelium, which form the various layers, have undergone important modifications. In the first place a large number of the nuclei—at any rate of those cells which are about to become ova—have undergone a change identical with that which takes place in the conversion of the primitive into the permanent ova in Elasmobranchii. The greater part of the contents of the nucleus becomes clear. The remaining contents arrange themselves as a deeply staining granular mass on one side of the membrane, and later on as a somewhat stellate figure: the two stages forming what were spoken of as the granular and stellate varieties of nucleus. To avoid further circumlocution I shall speak of the nucleus undergoing the granular and the stellate modifications. At a still later period the granular contents form a beautiful network in the nucleus.
The pseudo-epithelium (fig. 38A) is formed of several tiers of cells, the outermost of which are very columnar and have less protoplasm than in an earlier stage. In the lower tiers of cells there are many primitive ova with granular nuclei, and others in which the nuclei have undergone the granular modification. The primitive ova are almost all of the same size as in the earlier stage. The pseudo-epithelium is separated from the middle layer by a more or less complete stratum of connective tissue, which, however, is traversed by trabeculæ connecting the two layers of the epithelium. In the middle layer there are comparatively few modified nuclei, and the cells still retain for the most part their earlier characters. The diameter of the cells is about 0.012mm., and that of the nucleus about 0.008mm.In the innermost layer (fig. 38B), which is not sharply separated from the middle layer, the majority of the cells, which in the previous stage were ordinary cells of the epithelium, have commenced to acquire modified nuclei. This change, which first became apparent to a small extent in the young two days after birth, is very conspicuous at this stage. In some of the cells the nucleus is modified in the granular manner, in others in thestellate, and in a certain number the nucleus has assumed a reticular structure characteristic of the young permanent ovum.
In addition, however, to the cells which are becoming converted into ova, a not inconsiderable number may be observed, if carefully looked for, which are for the most part smaller than the others, generally somewhat oval, and in which the nucleus retains its primitive characters. A fair number of such cells are represented in fig. 38B. In the larger ones the nucleus will perhaps eventually become modified; but the smaller cells clearly correspond with the interstitial cells of the Elasmobranch germinal epithelium, and are destined to become converted into the epithelium of the Graafian follicle. In some few instances indeed (at this stage very few), in the deeper part of the germinal epithelium, these cells commence to arrange themselves round the just formed permanent ova as a follicular epithelium. An instance of this kind is shewn in fig. 38B,o. The cells with modified nuclei, which are becoming permanent ova, usually present one point of contrast to the homologous cells in Elasmobranchii, in that they are quite distinct from each other, and not fused into a polynuclear mass. They have around them a dark contour line, which I can only interpret as the commencement of the membrane (zona radiata?), which afterwards becomes distinct, and which would thus seem, as Foulis has already insisted, to be of the nature of a vitelline membrane.
In a certain number of instances the protoplasm of the cells which are becoming permanent ova appears, however, actually to fuse, and polynuclear masses identical with those in Elasmobranchii are thus formed (cf.E. van Beneden[411]). These masses become slightly more numerous in the succeeding stages. Indications of a fusion of this kind are shewn in fig. 38B. That the polynuclear masses really arise from a fusion of primitively distinct cells is clear from the description of the previous stages. The ova in the deeper layers, with modified granular nuclei, measure about 0.016 - 0.02mm., and their nuclei from 0.01 - 0.012mm.
With reference to the tissue of the hilus of the ovary, it may be noticed that the tubuliferous tissue (t) is relativelyreduced in quantity. Its cells retain precisely their previous characters.
The chief difference between the stage of five days and that of two days after birth consists in the fact that during the earlier stage comparatively few modified nuclei were present, but the nuclei then presented the character of the nuclei of primitive ova.
I have ovaries both of the dog and cat of an equivalent stage, and in both of these the cells of the nests or egg tubes may be divided into two categories, destined respectively to become ova and follicle cells. Nothing which has come under my notice tends to shew that the tubuliferous tissue is in any way concerned in supplying the latter form of cell.
In a stage, seven days after birth, the same layers in the germinal epithelium may be noticed as in the last described stage. The outermost layer or pseudo-epithelium contains numerous developing ova, for the most part with modified nuclei. It is separated by a well marked layer of connective tissue from the middle layer of the germinal epithelium. The outer part of the middle layer contains more connective tissue and smaller nests than in the earlier stage, and most of the cells of this layer contain modified nuclei. In a few nests the protoplasm of the developing ova forms a continuous mass, not divided into distinct cells, but in the majority of instances the outline of each ovum can be distinctly traced. In addition to the cells destined to become ova, there are present in these nests other cells, which will clearly form the follicular epithelium. A typical nest from the middle layer is represented onPl.26, fig. 39A.
The nests or masses of ova in the innermost layer are for the most part still very large, but, in addition to the nests, a few isolated ova, enclosed in follicles, are to be seen.
A fairly typical nest, selected to shew the formation of the follicle, is represented onPl.26, fig. 39B.
The nest contains (1) fully formed permanent ova, completely or wholly enclosed in a follicle. (2) Smaller ova, not enclosed in a follicle. (3) Smallish cells with modified nuclei of doubtful destination. (4) Small cells obviously about to form follicular epithelium.
The inspection of a single such nest is to my mind a satisfactoryproof that the follicular epithelium takes its origin from the germinal epithelium and not from the stroma or tubuliferous tissue. The several categories of elements observable in such a nest deserve a careful description.
(1)The large ova in their follicles.—These ova have precisely the character of the young ova in Elasmobranchii. They are provided with a granular body invested by a delicate, though distinct membrane. Their nucleus is large and clear, but traversed by the network so fully described for Elasmobranchii. The cells of their follicular epithelium have obviously the same character as many other small cells of the nest. Two points about them deserve notice—(a) that many of them are fairly columnar. This is characteristic only of the first formed follicles. In the later formed follicles the cells are always flat and spindle-shaped in section. In this difference between the early and late formed follicles Mammals agree with Elasmobranchii. (b) The cells of the follicle are much more columnar towards the inner side than towards the outer. This point also is common to Mammals and Elasmobranchii.
Round the completed follicle a very delicate membrana propria folliculi appears to be present[412].
The larger ova, with follicular epithelium, measure about 0.04mm., and their nucleus about 0.02mm., the smaller ones about 0.022mm., and their nucleus about 0.014mm.
(2)Medium sized ova.—They are still without a trace of a follicular epithelium, and present no special peculiarities.
(3)The smaller cells with modified nuclei.—I have great doubt as to what is the eventual fate of these cells. There appear to be three possibilities.
(a) That they become cells of the follicular epithelium; (b) that they develop into ova; (c) that they are absorbed as a kind of food by the developing ova. I am inclined to think that some of these cells may have each of the above-mentioned destinations.
(4)The cells which form the follicle.—The only point to be noticed about these is that they are smaller than the indifferentcells of the germinal epithelium, from which they no doubt originate by division. This fact has already been noticed by Waldeyer.
The isolated follicles at this stage are formed by ingrowths of connective tissue cutting off fully formed follicles from a nest. They only occur at the very innermost border of the germinal epithelium. This is in accordance with what has so often been noticed about the mammalian ovary,viz.that the more advanced ova are to be met with in passing from without inwards.
By the stage seven days after birth the ovary has reached a sufficiently advanced stage to answer the more important question I set myself to solve, nevertheless, partly to reconcile the apparent discrepancy between my account and that of Dr Foulis, and partly to bring my description up to a better known condition of the ovary, I shall make a few remarks about some of the succeeding stages.
In a young rabbit about four weeks old the ovary is a very beautiful object for the study of the nuclei,&c.
The pseudo-epithelium is now formed of a single layer of columnar cells, with comparatively scanty protoplasm. In it there are present a not inconsiderable number of developing ova.
A layer of connective tissue—the albuginea—is now present below the pseudo-epithelium, which contains a few small nests with very young permanent ova. The layer of medium sized nests internal to the albuginea forms a very pretty object in well stained sections, hardened in Kleinenberg's picric acid. The ova in it have all assumed the permanent form, and are provided with beautiful reticulate nuclei, with, as a rule, one more especially developed nucleolus, and smaller granular bodies. Their diameter varies from about 0.028 to 0.04mm.and that of their nucleus from 0.016 to 0.02mm.The majority of these ova are not provided with a follicular investment, but amongst them are numerous small cells, clearly derived from the germinal epithelium, which are destined to form the follicle (videfig. 40AandB). In a few cases the follicles are completed, and are then formed of very flattened spindle-shaped (in section) cells. In the majority of cases all the ova of each nest are quite distinct, and each provided with a delicate vitelline membrane (fig. 40A)In other instances, which, so far as I can judge, are more common than in the previous stages, the protoplasm of two or more ova is fused together.
Examples of this are represented inPl.26, fig. 40A. In some of these the nuclei in the undivided protoplasm are all of about the same size and distinctness, and probably the protoplasm eventually becomes divided up into as many ova as nuclei; in other cases, however, one or two nuclei clearly preponderate over the others, and the smaller nuclei are indistinct and hazy in outline. In these latter cases I have satisfied myself as completely as in the case of Elasmobranchii, that only one or two ova (according to the number of distinct nuclei) will develop out of the polynuclear mass, and that the other nuclei atrophy, and the material of which they were composed serves as the nutriment for the ova which complete their development. This does not, of course, imply that the ova so formed have a value other than that of a single cell, any more than the development of a single embryo out of the many in one egg capsule implies that the embryo so developing is a compound organism.
In the innermost layer of the germinal epithelium the outlines of the original large nests are still visible, but many of the follicles have been cut off by ingrowths of stroma. In the still intact nests the formation of the follicles out of the cells of the germinal epithelium may be followed with great advantage. The cells of the follicle, though less columnar than was the case at an earlier period, are more so than in the case of follicles formed in the succeeding stages. The previous inequality in the cells of the follicles is no longer present.
The tubuliferous tissue in the zona vasculosa appears to me to have rather increased in quantity than the reverse; and is formed of numerous solid columns or oval masses of cells, separated by strands of connective tissue, with typical spindle nuclei.
It is partially intelligible to me how Dr Foulis might from an examination of the stages similar to this, conclude that the follicle cells were derived from the stroma; but even at this stage the position of the cells which will form the follicular epithelium, their passage by a series of gradations into obviouscells of the germinal epithelium and the peculiarities of their nuclei, so different from those of the stroma cells, supply a sufficient series of characters to remove all doubt as to the derivation of the follicle cells. Apart from these more obvious points, an examination of the follicle cells from the surface, and not in section, demonstrates that the general resemblance in shape of follicle cells to the stroma cells is quite delusory. They are in fact flat, circular, or oval, plates not really spindle-shaped, but only apparently so in section. While I thus fundamentally differ from Foulis as to the nature of the follicle cells, I am on this point in complete accordance with Waldeyer, and my own results with reference to the follicle cannot be better stated than in his own words (pp.43, 44).
At six weeks after birth the ovary of the rabbit corresponds very much more with the stages in the development of the ovary, which Foulis has more especially studied, for the formation of the follicular epithelium, than during the earlier stages. His figure (Quart. Journ. Mic. Sci.,Vol.XVI.,Pl.17, fig. 6) of the ovary of a seven and a half months' human fœtus is about the corresponding age. Different animals vary greatly in respect to the relative development of the ovary. For example, the ovary of a lamb at birth about corresponds with that of a rabbit six weeks after birth. The points which may be noticed about the ovary at this age are first that the surface of the ovary begins to be somewhat folded. The appearances of these folds in section have given rise, as has already been pointed out by Foulis, to the erroneous view that the germinal epithelium (pseudo-epithelium) became involuted in the form of tubular open pits. The folds appear to me to have no connection with the formation of ova, but to be of the same nature as the somewhat similar folds in Elasmobranchii. A follicular epithelium is present around the majority of the ova of the middle layer, and around all those of the inner layer of the germinal epithelium. The nests are, moreover, much more cut up by connective tissue ingrowths than in the previous stages.
The follicle cells of the middle layers are very flat, and spindle-shaped in section, and though they stain more deeply than the stroma cells, and have other not easily characterised peculiarities, they nevertheless do undoubtedly closely resemblethe stroma cells when viewed (as is ordinarily the case) in optical section.
In the innermost layer many of the follicles with the enclosed ova have advanced considerably in development and are formed of columnar cells. The somewhat heterodox view of these cells propounded by Foulis I cannot quite agree to. He says (Quart. J. Mic. Sci.,Vol.XVI., p. 210):“The protoplasm which surrounds the vesicular nuclei acts as a sort of cement substance, holding them together in the form of a capsular membrane round the young ovum. This capsular membrane is the first appearance of the membrana granulosa.”I must admit that I find nothing similar to this, nor have I met with any special peculiarities (as Foulis would seem to indicate) in the cells of the germinal epithelium or other cells of the ovary.
Figure 41 is a representation of an advanced follicle of a six weeks' rabbit, containing two ova, which is obviously in the act of dividing into two. Follicles of this kind with more than one ovum are not very uncommon. It appears to me probable that follicles, such as that I have figured, were originally formed of a single mass of protoplasm with two nuclei; but that instead of one of the nuclei atrophying, both of them eventually developed and the protoplasm subsequently divided into two masses. In other cases it is quite possible that follicles with two ova should rather be regarded as two follicles not separated by a septum of stroma.
On the later stages of development of the ovary I have no complete series of observations. The yolk spherules I find to be first developed in a peripheral layer of the vitellus. I have not been able definitely to decide the relation of the zona radiata to the first formed vitelline membrane. Externally to the zona radiata there may generally be observed a somewhat granular structure, against which the follicle cells abut, and I cannot agree with Waldeyer (loc cit., p. 40) that this structure is continuous with the cells of the discus, or with the zona radiata. Is it the remains of the first formed vitelline membrane? I have obtained some evidence in favour of this view, but have not been successful in making observations to satisfy me on the point, and must leave open the question whether my vitelline membrane becomes the zona radiata or whether the zona is not alater and independent formation, but am inclined myself to adopt the latter view. The first formed membrane, whether or no it becomes the zona radiata, is very similar to the vitelline membrane of Elasmobranchii and arises at a corresponding stage.
Summary of observations on the mammalian ovary.—The general results of my observations on the mammalian ovary are the following:—
(1) The ovary in an eighteen days' embryo consists of a cylindrical ridge attached along the inner side of the Wolffian body, which is formed of two parts; (a) an external epithelium—two or three cells deep (the germinal epithelium); (b) a hilus or part forming in the adult the vascular zone, at this stage composed of branched masses of epithelial tissue (tubuliferous tissue) derived from the walls of the anterior Malpighian bodies, and numerous blood-vessels, and some stroma cells.
(2) The germinal epithelium gradually becomes thicker, and after a certain stage (twenty-three days) there grow into it numerous stroma ingrowths, accompanied by blood-vessels. The germinal epithelium thus becomes honeycombed by strands of stroma. Part of the stroma eventually forms a layer close below the surface, which becomes in the adult the tunica albuginea. The part of the germinal epithelium external to this layer becomes reduced to a single row of cells, and forms what has been spoken of in this paper as the pseudo-epithelium of the ovary. The greater part of the germinal epithelium is situated internal to the tunica albuginea, and this part is at first divided up by strands of stroma into smaller divisions externally, and larger ones internally. These masses of germinal epithelium (probably sections of branched trabeculæ) may be spoken of as nests. In the course of the development of the ova they are broken up by stroma ingrowths, and each follicle with its enclosed ovum is eventually isolated by a layer of stroma.
(3) The cells of the germinal epithelium give rise both to the permanent ova and to the cells of the follicular epithelium. For a long time, however, the cells remain indifferent, so that the stages, like those in Elasmobranchii, Osseous Fish, Birds, Reptiles,&c., with numerous primitive ova embedded amongst the small cells of the germinal epithelium, are not found.
(4) The conversion of the cells of the germinal epithelium into permanent ova commences in an embryo of about twenty-two days. All the cells of the germinal epithelium appear to be capable of becoming ova: the following are the stages in the process, which are almost identical with those in Elasmobranchii:—
(a) The nucleus of the cells loses its more or less distinct network, and becomes very granular, with a few specially large granules (nucleoli). The protoplasm around it becomes clear and abundant—primitive ovum stage. It may be noted that the largest primitive ova are very often situated in the pseudo-epithelium. (b) A segregation takes place in the contents of the nucleus within the membrane, and the granular contents pass to one side, where they form an irregular mass, while the remaining space within the membrane is perfectly clear. The granular mass gradually develops itself into a beautiful reticulum, with two or three highly refracting nucleoli, one of which eventually becomes the largest and forms the germinal spotpar excellence. At the same time the body of the ovum becomes slightly granular. While the above changes, more especially those in the nucleus, have been taking place, the protoplasm of two or more ova may fuse together, and polynuclear masses be so formed. In some cases the whole of such a polynuclear mass gives rise to only a single ovum, owing to the atrophy of all the nuclei but one, in others it gives rise by subsequent division to two or more ova, each with a single germinal vesicle.
(5) All the cells of a nest do not undergo the above changes, but some of them become smaller (by division) than the indifferent cells of the germinal epithelium, arrange themselves round the ova, and form the follicular epithelium.
(6) The first membrane formed round the ovum arises in some cases even before the appearance of the follicular epithelium, and is of the nature of a vitelline membrane. It seems probable, although not definitely established by observation, that the zona radiata is formed internally to the vitelline membrane, and that the latter remains as a membrane, somewhat irregular on its outer border, against which the ends of the follicle cells abut.
General Observations on the Structure and Development of the Ovary.
In selecting Mammalia and Elasmobranchii as my two types for investigation, I had in view the consideration that what held good for such dissimilar forms might probably be accepted as true for all Vertebrata with the exception of Amphioxus.
The structure of the ovary.—From my study of these two types, I have been led to a view of the structure of the ovary, which differs to a not inconsiderable extent from that usually entertained. For both types the conclusion has been arrived at that the whole egg-containing part of the ovary is reallythe thickened germinal epithelium, and that it differs from the original thickened patch or layer of germinal epithelium, mainly in the fact that it is broken up into a kind of meshwork by growths of vascular stroma. If the above view be accepted for Elasmobranchii and Mammalia, it will hardly be disputed for the ovaries of Reptilia and Aves. In the case also of Osseous Fish and Amphibia, this view of the ovary appears to be very tenable, but the central core of stroma present in the other types is nearly or quite absent, and the ovary is entirely formed of the germinal epithelium with the usual strands of vascular stroma[413]. It is obvious that according to the above view Pflüger's egg-tubes are merely trabeculæ of germinal epithelium, and have no such importance as has been attributed to them. They are present in a more or less modified form in all types of ovaries. Even in the adult Amphibian ovary, columns of cells of the germinal epithelium, some indifferent, others already converted into ova, are present, and, as has been pointed out by Hertwig[414], represent Pflüger's egg-tubes.
The formation of the permanent ova.—The passage of primitive ova into permanent ova is the part of my investigation to which the greatest attention was paid, and the results arrived at for Mammalia and Elasmobranchii are almost identical. Althoughthere are no investigations as to the changes undergone by the nucleus in other types, still it appears to me safe to conclude that the results arrived at hold good for Vertebrates generally[415]. As has already been pointed out the transformation which the so-called primitive ova undergo is sufficient to shew thatthey are not to be regarded as ova but merely as embryonic sexual cells. A feature in the transformation, which appears to be fairly constant in Scyllium, and not uncommon in the rabbit, is the fusion of the protoplasm of several ova into a syncytium, the subsequent increase in the number of nuclei in the syncytium, the atrophy and absorption of a portion of the nuclei, and the development of the remainder into the germinal vesicles of ova; the vitellus of each ovum being formed by a portion of the protoplasm of the syncytium.
As to the occurrence of similar phenomena in the Vertebrata generally, it has already been pointed out that Ed. van Beneden has described the polynuclear masses in Mammalia, though he does not appear to me to have given a complete account of their history. Götte[416]describes a fusion of primitive ova in Amphibia, but he believes that the nuclei fuse as well as the bodies of the ova, so that one ovum (according to his view no longer a cell) is formed by the fusion of several primitive ova with their nuclei. I have observed nothing which tends to support Götte's view about the fusion of the nuclei, and regard it as very improbable. As regards the interpretation to be placed upon the nests formed of fused primitive ova, Ed. van Beneden maintains that they are to be compared with the upper ends of the egg tubes of Insects, Nematodes, Trematodes,&c.There is no doubt a certain analogy between the two, in that in both cases certain nuclei of a polynuclear mass increase in size, and with the protoplasm around them become segmented off from the remainder of the mass as ova, but the analogy cannot be pressed. The primitive ova, or even the general germinal epithelium, rather than these nests, must be regarded as giving origin to the ova, and the nests should be looked on, in my opinion, as connectedmore with the nutrition than with the origin of the ova. In favour of this view is the fact that as a rule comparatively few ova are developed from the many nuclei of a nest; while against the comparison with the egg tubes of the Invertebrata it is to be borne in mind that many ova appear to develop independently of the nests.
In support of my view about the nests there may be cited many analogous instances from the Invertebrata. In none of them, however, are the phenomena exactly identical with those in Vertebrata. In the ovary of many Hydrozoa (e.g. Tubularia mesembryanthemum), out of a large number of ova which develop up to a certain point, a comparatively very small number survive, and these regularly feed upon the other ova. During this process the boundary between a large ovum and the smaller ova is indistinct: in the outermost layer of a large ovum a number of small ova are embedded, the outlines of the majority of which have become obscure, although they can still be distinguished. Just beyond the edge of a large ovum the small ova have begun to undergo retrogressive changes; while at a little distance from the ovum they are quite normal. An analogous phenomenon has been very fully described by Weismann[417]in the case of Leptodera, the ovary of which consists of a germogene, in which the ova develop in groups of four. Each group of four occupies a separate chamber of the ovary, but in summer only one of the four eggs (the third from the germogene) develops into an ovum, the other three are used as pabulum. In the case of the winter eggs the process is carried still further, in that the contents of the alternate chambers, instead of developing into ova, are entirely converted, by a series of remarkable changes, into nutritive reservoirs. Fundamentally similar occurrences to the above are also well known in Insects. Phenomena of this nature are obviously in no way opposed to the view of the ovum being a single cell.
With reference to the origin of the primitive ova, it appears to me that their mode of development in Mammals proves beyond a doubt that they are modified cells of the germinal epithelium. In Elasmobranchii their very early appearance, and the difficultyof finding transitional forms between them and ordinary cells of the germinal epithelium, caused me at one time to seek (unsuccessfully) for a different origin for them. Any such attempts appear to me, however, out of the question in the case of Mammals.
The egg membranes.—The homologies of the egg membranes in the Vertebrata are still involved in some obscurity. In Elasmobranchii there are undoubtedly two membranes present. (1) An outer and first formed membrane—the albuminous membrane of Gegenbaur—which, in opposition to previous observers, I have been led to regard as a vitelline membrane. (2) An inner radiately striated membrane, formed as a differentiation of the surface of the yolk at a later period. Both these membranes usually atrophy before the ovum leaves the follicle. In Reptilia[418]precisely the same arrangement is found as in Elasmobranchii, except that as a rule the zona radiata is relatively more important. The vitelline membrane external to this (or as it is usually named the chorion) is, as a rule, thin in Reptilia; but in Crocodilia is thick (Gegenbaur), and approaches the condition found in Scyllium and other Squalidæ. It appears, as in Elasmobranchii, to be formed before the zona radiata. A special internal differentiation of the zona radiata is apparently found (Eimer) in many Reptilia. No satisfactory observations appear to be recorded with reference to the behaviour of the two reptilian membranes as the egg approaches maturity. In Birds[419]the same two membranes are again found. The first formed and outer one is, according to Gegenbaur and E. van Beneden, a vitelline membrane; and from the analogy of Elasmobranchii I feel inclined to accept their view. The inner one is the zona radiata, which disappears comparatively early, leaving the ovum enclosed only by the vitelline membrane, when it leaves the follicle. All the large-yolked vertebrate ova appear then to agree very well with Elasmobranchii in presenting during some period of their development the two membranes above mentioned.
Osseous fish have almost always a zona radiata, which it seems best to assume to be equivalent to that in Elasmobranchii.Internal to this is a thin membrane, the equivalent, according to Eimer, of the membrane found by the same author within the zona in Reptilia. A membrane equivalent to the thick vitelline membrane of Elasmobranchii would seem to be absent in most instances, though a delicate membrane, external to the zona, has not infrequently been described; Eimer more especially asserts that such a membrane exists in the perch within the peculiar mucous covering of the egg of that fish.
In Petromyzon, a zona radiata appears to be present[420], which is divided in the adult into two layers, both of them perforated. The inner of the two perhaps corresponds with the membrane internal to the zona radiata in other types. In Amphibia the single late formed and radiately striated (Waldeyer) membrane would appear to be a zona radiata. If the suggestion on page 605 turns out to be correct the ova of Mammalia possess both a vitelline membrane and zona radiata. E. van Beneden[421]has, moreover, shewn that they are also provided at a certain period with a delicate membrane within the zona.
The reticulum of the germinal vesicle.—In the course of description of the ovary it has been necessary for me to enter with some detail into the structure of the nucleus, and I have had occasion to figure and describe a reticulum identical with that recently described by so many observers. The very interesting observations of Dr Klein in the last number of this Journal[422]have induced me to say one or two words in defence of some points in my description of the reticulum. Dr Klein says, on page 323,“I have distinctly seen that when nucleoli are present—the instances are fewer than is generally supposed; they are accumulations of the fibrils of the network.”I have no doubt that Klein is correct in asserting that nucleoli are fewer than is generally supposed; and that in many of these instances what are called nucleoli are accumulations,“natural or artificial,”of the fibrils of the network; but I cannot accept the universality of the latter statement, which appears to me most certainly not to hold good in the case of ova, in which nucleoli frequently exist in the absence of the network.
Again, I find that at the point of intersection of two or morefibrils there is, as a rule, a distinct thickening of the matter of the fibrils, and that many of the dots seen are not merely, as Dr Klein would maintain, optical sections of fibrils.
It appears to me probable that both the network and the nucleoli are composed of the same material—what Hertwig calls nuclear substance—and if Dr Klein merely wishes to assert this identity in the passage above quoted, I am at one with him.
Although a more or less distinct network is present in most nuclei (I have found it in almost all embryonic nuclei) it is not universally so. In the nuclei of primitive ova I have no doubt that it is absent, though present in the unmodified nuclei of the germinal epithelium; and it is present only in a very modified form in the nuclei of primitive ova undergoing a transformation into permanent ova. The absence of the reticulum does not, of course, mean that the substance capable of forming a reticulum is absent, but merely that it does not assume a particular arrangement.
One of the most interesting points in Klein's paper, as well as in those of Heitzmann and Eimer, is the demonstration of a connection between the reticulum of the nucleus and fibres in the body of the cell. Such a connection I have not found in ova, but may point out that it appears to exist between the sub-germinal nuclei in Elasmobranchii and the protoplasmic network in the yolk in which they lie. This point is called attention to in myMonograph on Elasmobranch Fishes, page 39[423], where it is stated that“the network in favourable cases may be observed to be in connection with the nuclei just described. Its meshes are finer in the vicinity of the nuclei, and the fibres in some cases appear almost to start from them.”The nuclei in the yolk are knobbed bodies divided by a sponge work of septa into a number of areas each with a nucleolar body.
EXPLANATION OF PLATES 24, 25, 26.
Plate 24.
List of Reference Letters.
dn.Modified nucleus of primitive ovum.do.Permanent ovum in the act of being formed.dv.Developing blood-vessels.dyk.Developing yolk.ep.Non-ovarian epithelium of ovarian ridge.fe.Follicular epithelium.gv.Germinal vesicle.lstr.Lymphatic region of stroma.nn.Nests of nuclei of ovarian region.o.Permanent ovum.ovr.Ovarian portion of ovarian ridge.po.Primitive ovum.pse.Pseudo-epithelium of ovarian ridge.str.Stroma ingrowths into ovarian epithelium.v.Blood-vessel.vstr.Vascular region of stroma adjoining ovarian ridge.vt.Vitelline membrane.x.Modified nucleus.yk.Yolk.zn.Zona radiata.
Fig. 1. Transverse section of the ovarian ridge of an embryo ofScy.canicula, belonging to stage P, shewing the ovarian region with thickened epithelium and numerous primitive ova. Zeiss C,ocul.2.Picric acid.
Fig. 2. Transverse section of the ovarian ridge of an embryo ofScyllium canicula, considerably older than stage Q. Zeiss C,ocul.2.Picric acid.Several nests, some with distinct ova, and others with the ova fused together, are present in the section (n.n.), and several examples of modified nuclei in still distinct ova are also represented. One of these is markedx. The stroma of the ovarian ridge is exceptionally scanty.
Fig. 3. Transverse section through part of the ovarian ridge, including the ovarian region of an almost ripe embryo ofScyllium canicula. Zeiss C,ocul.2.Picric acid.Nuclear nests (n.n.), developing ova (d.o.), and ova (o.), with completely formed follicular epithelium, are now present. The ovarian region is still well separated from the subjacent stroma, and does not appear to contain any cells except those of the original germinal epithelium.
Fig. 4. Section through ovarian ridge of the same embryo as fig. 3, to illustrate the relation of the stroma (str.) and ovarian region. Zeissa a,ocul.2.Picric acid.
Fig. 5. Section through the ovarian ridge of an embryo ofScyllium canicula, 10cm.long, in which the ovary was slightly less advanced than in fig. 3. To illustrate the relation of the ovarian epithelium to the subjacent vascular stroma. Zeiss A,ocul.2.Osmic acid.y.points to a small separated portion of the germinal epithelium.
Fig. 6. Section through the ovarian ridge of an embryo ofScyllium canicula, slightly older than fig. 5. To illustrate the relation of the ovarian epithelium to the subjacent vascular[TN12]stroma. Zeiss A,ocul.2.Osmic acid.
Fig. 7. More highly magnified portion of the same ovary as fig. 6. To illustrate the same points. Zeiss C,ocul.2.Osmic acid.
Fig. 8. Section through the ovarian region (close to one extremity, where it is very small) from a young female ofScy.canicula. Zeiss C,ocul.2.Picric acid.It shews the vascular ingrowths amongst the original epithelial cells of the ovarian region.
Fig. 9. Section through the ovarian region of the same embryo as fig. 8, at its point of maximum development. Zeiss A,ocul.2.Picric acid.
Fig. 10. Section through superficial part of the ovary of an embryo, shewing the pseudo-epithelium; the cells of which are provided with tails prolonged into the general tissue of the ovary. Atf.e.is seen a surface view of the follicular epithelium of an ovum. Zeiss C,ocul.2.Picric acid.
Fig. 11. Section through part of an ovary ofScyllium caniculaof stage Q, with three primitive ova, the most superficial one containing a modified nucleus.
Fig. 12. Section through part of an ovary of an example ofScyllium canicula, 8cm.long. The section passes through a nest of ova with modified nuclei, in which the outlines of the individual ova are quite distinct. Zeiss E,ocul.2.Picric acid.
Fig. 13. Section through part of ovary of the same embryo as in fig. 5. The section passes through a nest of nuclei, with at the least two developing ova, and also through one already formed permanent ovum. Zeiss E,ocul.2.Osmic acid.
Figs. 14, 15, 16, 17, 18 [Figs. 17 and 18 are onPl.25]. Sections through parts of the ovary of the same embryo as fig. 3, with nests of nuclei and a permanent ova in the act of formation. Fig. 14 is drawn with Zeiss D D,ocul.2. Figs. 15, 16, 17, with Zeiss E,ocul.2.Picric acid.
Plate 25.
List of Reference Letters.
do.Permanent ovum in the act of being formed.dyk.Developing yolk.fe.Follicular epithelium.fe´.Secondary follicular epithelium.gv.Germinal vesicle.nn.Nests of nuclei of ovarian region.o.Permanent ovum.pse.Pseudo-epithelium.str.Stroma ingrowths into ovarian epithelium.vt.Vitelline membrane.x.Modified nucleus.yk.Yolk (vitellus).zn.Zona radiata.
[Figs. 17 and 18.Videdescription of Plate 24.].
Fig. 19. Two nuclei from a nest which appear to be in the act of division. From ovary of the same embryo as fig. 3.
Fig. 20. Section through part of an ovary of the same embryo as fig. 6, containing a nest of nuclei. Zeiss F,ocul.2.Osmic acid.
Fig. 21. Ovum from the ovary of a half-grown female, containing isolated deeply stained patches of developing yolk granules. Zeiss B,ocul.2.Picric acid.
Fig. 22. Section through a small part of the ovum of an immature female ofScyllium canicula, to shew the constitution of the yolk, the follicular epithelium, and the egg membranes. Zeiss E,ocul.2.Chromic acid.
Fig. 23. Section through part of the periphery of a nearly ripe ovum ofScy.canicula. Zeiss C,ocul.2. It shews the remnant of the vitelline membrane (v.t.) separating the columnar but delicate cells of the follicular epithelium (f.e.) from the yolk (yk.). In the yolk are seen yolk-spherules in a protoplasmic network. The transverse markings in the yolk-spherules have been made oblique by the artist.
Fig. 24. Fully formed ovum containing a second nucleus (x), probably about to be employed as pabulum; from the same ovary as fig. 5. The follicular epithelium is much thicker on the side adjoining the stroma than on the upper side of the ovum. Zeiss F,ocul.2.Osmic acid.
Fig. 25. A. Ovum from the same ovary as fig. 21, containing in the yolk three peculiar bodies, similar in appearance to the two small bodies in the germinal vesicle. B. Germinal vesicle of a large ovum from the same ovary, containing a body of a strikingly similar appearance to those in the body of the ovum in A. Zeiss E,ocul.2.Picric acid.
Fig. 26. Section of the ovary of a young female ofScyllium stellare16½ centimetres in length. The ovary is exceptional, on account of the large size of the stroma ingrowths into the epithelium. Zeiss C,ocul.2.Osmic acid.
Fig. 27. Ovum ofScyllium canicula, 5mm.in diameter, treated with osmic acid. The figure illustrates the development of the yolk and a peculiar mode of proliferation of the germinal spots. Zeiss A,ocul.2.
Fig. 28. Small part of the follicular epithelium and egg membranes of a somewhat larger ovum ofScyllium canicula than fig. 22. Zeiss D D,ocul.2.
Fig. 29. The same parts as in fig. 28, from a still larger ovum. Zeiss D D,ocul.2.
Fig. 30. Ovum of Raja with follicular epithelium. Zeiss C,ocul.2.
Fig. 31. Small portion of a larger ovum of Raja than fig. 30. Zeiss D D,ocul.2.
Fig. 32. Follicular epithelium,&c., from an ovum of Raja still larger than fig. 31. Zeiss D D,ocul.2.
Fig. 33. Surface view of follicular epithelium from an ovum of Raja of about the same age as fig. 33.
Fig. 34. Vertical section through the superficial part of an ovary of an adult Raja to shew the relation of the pseudo-epithelium to the subjacent stroma. Zeiss D D,ocul.2.
Plate 26.
Complete List of Reference Letters.
do.Developing ovum.fc.Cells which will form the follicular epithelium,fe.Follicular epithelium.ge.Germinal epithelium.mg.Malpighian body.n.Nest of cells of the germinal epithelium.nd.Nuclei in the act of dividing.o.Permanent ovum.ov.Ovary.po.Primitive ovum.t.Tubuliferous tissue, derived from Malpighian bodies.
Fig. 35. Transverse section through the ovary of an embryo rabbit of eighteen days, hardened in osmic acid. The colours employed are intended to render clear the distinction between the germinal epithelium (ge.) and the tubuliferous tissue (t.), which has grown in from the Wolffian body, and which gives rise in the male to parts of the tubuli seminiferi. Zeiss A,ocul.2.
Fig. 35A. Transverse section through a small part of the ovary of an embryo from the same female as fig. 35, hardened in picric acid, shewing the relation of the germinal epithelium to the subjacent tissue. Zeiss D D,ocul.2.
Fig. 35B. Longitudinal section through part of the Wolffian body and the anterior end of the ovary of an eighteen days' embryo, to shew the derivation of tubuliferous tissue (t.) from the Malpighian bodies, close to the anterior extremity of the ovary. Zeiss A,ocul.1.
Fig. 36. Transverse section through the ovary of an embryo rabbit of twenty-two days, hardened in osmic acid. It is coloured in the same manner as fig. 35. Zeiss A,ocul.2.
Fig. 36A. Transverse section through a small part of the ovary of an embryo, from the same female as fig. 36, hardened in picric acid, shewing the relation of the germinal epithelium to the stroma of the ovary. Zeiss D D,ocul.2.
Figs. 37 and 37A. The same parts of an ovary of a twenty-eight days' embryo as figs. 36 and 36Aof a twenty-two days' embryo.
Fig. 38. Ovary of a rabbit five days after birth, coloured in the same manner as figs. 35, 36 and 37, but represented on a somewhat smaller scale.Picric acid.
Fig. 38A. Vertical section through a small part of the surface of the same ovary as fig. 38. Zeiss D D,ocul.2.
Fig. 38B. Small portion of the deeper layer of the germinal epithelium of the same ovary as fig. 38. The figure shews the commencing differentiation of the cells of the germinal epithelium into true ova and follicle cells. Zeiss D D,ocul.2.
Fig. 39A. Section through a small part of the middle region of the germinal epithelium of a rabbit seven days after birth. Zeiss D D,ocul.2.
Fig. 39B. Section through a small part of the innermost layer of the germinal epithelium of a rabbit seven days after birth, shewing the formation of Graafian follicles. Zeiss D D,ocul.2.
Figs. 40Aand 40B. Small portions of the middle region of the germinal epithelium of a rabbit four weeks after birth. Zeiss D D,ocul.2.
Fig. 41. Graafian follicle with two ova, about to divide into two follicles, from a rabbit six weeks after birth. Zeiss D D,ocul.2.
[370]From theQuarterly Journal of Microscopical Science,Vol.18, 1878.
[371]Arbeiten a. d. zool.-zoot. Institut Würzburg,Bd.I.
[372]Archiv f. micr. Anat.Vol.XI.