Paper - The embryonic development of the ovary and testis of the mammals

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Allen BM. The embryonic development of the ovary and testis of the mammals. (1904) J. Anat. 3(2): 89-154.

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This historic 1904 paper by Allen describes early gonad, ovary and testis, development in mammals. Please note that many concepts display the understanding of these to organs at that time and are incorrect for the origin and generation of the germ cells. We currently have a much better understanding.



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The Embryonic Development of the Ovary and Testis of the Mammals

By

Bennet Mills Allen.


From the Hull Zoological Laboratory, University of Chicago.


With 7 Plates and 5 Text Figures.

Introduction

This work was carried on with the aim of solving the following problems: (1) The origin and development of the seminiferous tubules and their homologues in the ovary; (2) the origin, development and homologies of the rete tubules together with their relations to the Malpighian corpuscles of the mesonephros on the one hand, and to the seminiferous tubules of the testis on the other; (3) the origin, development and homologies of the connective tissue elements and interstitial cells of the ovary and testis.


Incidental to the solution of these problems, the work has involved to a greater, or less extent a consideration of the following allied problems: (1) The development of sex cells; (2) the morphological phases of sex differentiation; (3) cell degeneration in the sex gland and rete; (4) the degeneration of the mesonephros and the development of the Wolffian and Miillerian ducts.


This work, covering as it does a very broad field, naturally touches upon many points that have already been treated by previous workers. Although much has been written upon this subject there is a singular lack of unanimity in the results attained. This is largely due to the fact that only in a very few cases has the process of development of the sex gland been followed in an extensive series of stages. Su6h work has naturally resulted in giving rise to many false and contradictory views upon these subjects.


The difficulties in the investigation of these problems are further enhanced by the fact that the sex glands are composed entirely of mesodermal tissue, in which a large part of the cells are without definite cell boundaries.


2. Material and Technique

The material employed includes numerous stages in the development of the ovary and testis of the rabbit, from the 13-day embryo to and including adult stages. The pig material includes only embryonic stages, but is more complete for the period covered than is the rabbit material.


The two forms studied - rabbit and pig - are complementary in their points of special suitability for this work. Although there are minor differences in the development of the sex glands in these two forms, yet the general process is essentially the same. In general the pig is the more instructive form for a study of the early stages of embryonic development, while the rabbit furnishes material better suited for the study of the post-embryonic stages.


The following tables indicate the stages studied, the sex of the specimen and the number of series cut in each case. The stage of development is indicated in the rabbit by the number of days and in the pig by the length of the embryo.

---Insert table here---


Note The pig embryos were measured from the cervical to the tail bend in all stages up to 5 cm. length, when the measurement was taken from the base of the tail to the top of the head. This change was made for practical reasons of precision. The 5 cm. stage would be about equivalent to the 4 cm. stage.


The material Avas, in practically all cases, fixed in Flemming's fluid and stained with Heidenhain's iron hematoxylin with a counter stain of Siiurefuchsin, Orange G. or Bordeaux red. Sections were cut to a thickness of 6f /j or in a few cases 10 /u and were mounted in series. It was, of course, necessary in all cases to section the anterior part of the mesonephros together with the sex gland itself.


An account of the earlier literature upon this subject would be useless repetition, since we have such valuable and extensive reviews as those given by Waldeyer, 70 and 02, Born, 94, Coert, 98, Winiwarter, 00, Bouin, 00, Mihalkovics, 85. In the general summary reference will be made to some of the more recent and important works bearing upon these problems; but no attempt will be made to attain completeness in a consideration of the earlier literature, a very large part of which is of merely historical value.

II. General Topography

The orientation of the various organs to be considered may be well understood from, a study of the pig embryo of 2.5 cm. length (Text Fig. 1 and Plate I, Fig. 1). The mesonephra are a pair of elongated laterally compressed bodies attached to the dorsal body wall on each side of the mesenter}'. Their long axes diverge anteriorly and converge posteriorly. They are prominent structures, extending three-fourths of the length of the abdominal cavity, being closely united to the dorso-median part of its wall by their short, broad mesenteries. Each mesonephros is flattened on its median face while the lateral face is convex. A sharp ridge extending the entire length of the medio-ventral face marks the course of the Wolffian and Miillerian ducts, the latter being ventral to the former. The genital ridge is situated on the median surface of the mesonephros and extends its entire length immediately ventral to the mesentery. It is covered by a thickened layer of epithelium continuous with the general peritoneal lining of the abdominal cavity, yet differing from it in that its component cells are columnar instead of fiattened, and are closely crowded together,


Fig. 1. Mesonephros and associated structures. Pig embryo of 3.5 cm. length. a. r., adrenal body ; e. p., epithelial plate ; m. r., mesenteric ridge ; r. c, rete ridge ; t., testis ; W. M., Wolifian and Mullerlan ducts.


Three distinct regions may be distinguished in this genital ridge, each of which occupies, roughly speaking, one-third of its length. Named in their order, they are: (1) the rete; (3) the sex gland; (;^) the mesenteric ridge.


The anterior end of the rete is a low plate of thickened epithelium in which lies the opening of the Miillerian duct. Posterior to this plate, the rete assumes the form of a slender, low ridge thot terminates at the anterior end of the sex gland.


In both male and female of this stage, the sex gland is cyliAdrical, and rounded at both ends. It projects well into the body cavity, being united to the mesonephros along its entire length by a relatively narrow mesentery.


For the posterior third of the genital ridge I suggest the term mesenteric ridge. This diminishes in height from its anterior to its posterior end, which grades off into the general peritoneal covering of the mesonephros.


A transverse section of the epithelial plate in which the ]\riillerian duct takes its origin, shows it to be similar to that investing the remainder of the genital ridge. At the dorsal edge of the plate are seen more or less solid invaginations, the rete cords, while the opening of the Miillerian duct is situated in the ventral part. It appears as a hollow invagination clothed with cells much like those of the epithelial plate, from which they are undoubtedly derived, as can be easily seen from a study of earlier stages.


The rete consists of a series of cords embedded in a loose stroma. Their proximal ends are directly continuous with the peritoneum while their distal extremities lie deep in the stroma, in some cases reaching to the Malpighian corpuscles with which they are frequently in direct contact.


The rete cords penetrate into the sex gland a short distance behind its anterior end. This point, termed the hilum, is morphologically the anterior end of the sex gland, although it appears to be situated more posteriorly in the testis, owing to a secondary flexui'e of that organ. The ovary, on the other hand, retains the primitive condition in this regard.


At this stage the ovary and testis can be readily distinguished, although they contain essentially the same structures, viz: Sex cords, albuginea and germinal epithelium. (1) The sex cords of the testis develop into the seminiferous tubules which, at this stage, appear as long contorted anastomosing and branching cords of cells. Their homologues in the ovarv are termed the medullarv cords. These have all the essential characters of the seminiferous tubules save for the fact that they are by no means so well-developed nor so extensive as those structures. (2) A zone of connective tissue separates the sex cords from the peripheral peritoneal investment of the sex glands. It is compact in the testis, while in the ovary it is loose, broad and irregular in outline, forming only an incomplete barrier between the peritoneum and medullary cords. In both ovary and testis this peripheral connective tissue zone is continuous with masses of loose connective tissue (stroma) packed in between the sex cords. I shall refer to it as the albuginea in both testis and ovary, although that term is usually applied to it in the testis alone. (3) The peritoneal layer is thin in the testis and its component cells are flattened. Quite a different condition prevails in the ovary where it is decidedly thickened and is seen to be giving off cords of cells from its inner edge. These are the so-called egg-tubes of Pflüger. They are in some instances continuous with the medullary cords, although such cases are rather rare, the two sets of structures being usually distinctly separated by the albuginea.


The posterior third of the genital ridge (mesenteric ridge) need not be considered further save to note that it hecomes more elevated in later stages and takes on a more decided mesenteric character.


In the mesonephros there soon appear processes of degeneration that "bring about decided changes. Even in the embryo 3.5 cm. in length there is seen a commencement of degeneration in certain tubules in its anterior portion. This process continues during succeeding stages, chiefly affecting the Malpighian corpuscles, but sparing from 10 to 12 of the tubules destined to form the rete efferentia of the testis, but which later degenerate in the female. To such an extent has this degeneration process been carried on in the 10 cm. embryo that the portion of the meso^iephros lying anterior to the hiluni is shrunken and the investing peritoneum thrown into wrinkles. Degeneration of the portion posterior to the hilum has just begun at this stage. In the female, the shrinkage of the anterior part of the mesonephros has caused the anterior ends of the Miillerian and Wolfiian ducts to be bent over the ovary in a dorsal direction to such a degree that sections through this region show these ducts to be cut through twice (Fig. 3). After this, the degeneration process rapidly reduces the mesonephros, until, in the 20 cm. embryo, it consists of little more than a mere mass of connective tissue containmg a few scattered glomeruli and uriniferous tubules, the vasa efferentia in the testis alone being spared.

III. Observations Upon Successive Stages of Development in the Pig

0.7 cm. Embryo. - The mesonepliros is covered by a more or less distinct peritoneal layer, which is not clearly differentiated from the stroma, except in the dorsal and lateral portions, but becomes increasingly distinct on the medio-ventral surface, where the genital ridge later takes its origin. The transition is, however, a very gradual one and the differences slight. There is a rather loose vascular mesenchyme tissue that fills in the space between the peritoneum on the one hand and the Malpighian corpuscles and mesonephric tubules on the other.


The cells of both the peritoneum and underlying mesenchyme do not have definite boundaries, appearing in this, and in later stages as well, to form a continuous protoplasmic network, to which the nuclei give character by their more or less definite arrangement. A region of the peritoneum extending from the base of the mesentery one-third the distance to the Wolffian duct is of particular importance, since it is the rudiment from which the genital ridge takes its origin (Text Eig. 2 and Plate I, Eig. 3). A point about opposite the twentieth glomerulus marks the boundary between the future sex gland and the rete. In the region of the genital ridge (Plate I, Eig. 3), as defined above, the greater part of the nuclei are of various shapes and sizes and stain rather deeply with ha3matoxylin. The nuclei of the peritoneum are closely packed together and are usually elongated by mutual pressure. They rest upon a loose felt-like basement membrane, which is formed by the interlacing of numerous slender branching protoplasmic fibrils given off by both the peritoneal and stroma cells. The peritoneal origin of the stroma is clearly indicated at many points where mutual pressure of the peritoneal cells is crowding them through the basement membrane, which has, in fact, disappeared at such spots as a result of this process. The positions and angles of inclination of the columnar nuclei give satisfactory evidence on this point. The presence of numerous mitotic figures in the peritoneum indicates a rapid multiplication of its cells. On the other hand the stroma cells divide with far less frequency.



Fig. 2. Transverse section of mesonephros and associated structures. Pig- embryo 1.4 cm. Jengtti. d. a., dorsal aorta: </., glomerulus; %. m., mesentery of the intestine ; Ji., liver; m. /., mesentei-ic fundament ; 8. {/., sex gland; u. t., uriniferous tubule; W. a.. Wolffian duct, x 26.


As might be expected from the above, the stroma cells are practically identical with the peritoneal cells from which they are originating. In general their nuclei tend to assume a more rounded shape.


Here and there in both peritoneum and stroma one finds cells quite different from those described above. These have clearly marked boundaries, lightly staining cytoplasm, a centrosphere and a centrosome. The large, round nucleus contains prominent nucleoli, usually two in number, and also a chromatin network of slender strands quite different in appearance from the rather granular irregular chromatin masses of the peritoneal and stroma nuclei. These primitive ova are so rare that one must hunt through as many as seven or eight sections in order to find one. They divide by mitosis, as a result of which division they are found to occur in small groups.


The inner boundary of the mesenchymal portion of the sex gland rudiment is formed by the capsules of the Malpighian corpuscles. The component cells of the capsules resemble those of the stroma in their lack of definite boundaries and in the character of their nuclei, being distinguished from the latter chiefly by the darker color of their cytoplasm.


The rudiment of the rete (Plate I, Fig. 2) is essentially like the sex gland rudiment save for the fact that the basement membrane of the peritoneum is somewhat less distinct in the former than in the latter and the primitive ova are not quite so numerous; these differences are probably due to the fact that the tissue of the sex gland rudiment is more dense than that of the rete rudiment.


0.8 cm. Embryo. - In this stage, the mesonephros is found to have almost doubled in size; for this reason there has been little thickening of the rete and sex gland rudiments. The number of primitive ova has greatly increased and many clear cases of mitosis are found among them. The basement membrane of the peritoneal layer has become roore clearly defined in both rete and sex gland rudiments, yet it is still broken in spots where cells are being proliferated into the underlying stroma, sometimes forming chains of two, three or four cells.


1.0 cm. Embryo. - The mesonephros has become half again as broad in this stage as in the preceding one, and has also increased in the dorso-ventral dimension.


The rete rudiment has not grown in thickness, yet the peritoneal cells are seen to be rapidly dividing by mitosis. This results in a crowding which in some places is so great as to bring about the formation of actual peritoneal invaginations which extend into the stroma and frequently come in contact with the attenuated capsules of Bowman.


Peritoneal invaginations arising in the sex gland rudiment (compare Plate II, Figs. 5 and 6) by the same process of crowding are more diffuse, and more numerous than in the rete. Another difference between these two regions of the genital ridge is found in the fact that the sex gland invaginations do not in any case reach as de^p as the capsules of Bowman, the stroma being thicker in this region than in the rete rudiment.


In many cases the stroma cells are assuming the character of connective tissue. Primitive sex cells are present in the peritoneum and in the peritoneal invaginations and stroma of both rete and sex gland rudiments. Their number has increased in the sex gland rudiment, while they have shown little or no numerical increase in the rete region.


1.25 cm. Embryo. - Although the rete rudiment has increased but little in thickness, the peritoneal invaginations of the rete region, which may now be termed rete tubules, are much further developed than in the preceding stage. The sex gland rudiment, on the other hand, has increased greatly in thickness. Its peritoneal invaginations (sex cords) have also increased in length and in number. Their nature can be best understood by referring to Plate II, Fig. 5. In this and in later stages the nuclei are still attached to the basement membrane which is in fact formed, as we have seen, from protoplasmic processes connected with them. So closely are the sex cords placed that there are very few stroma cells between them. No clear cases of such are seen in this figure. Such, however, are present and form in part the rudiment of the intertubular stroma so prominent in later stages. There is no doubt tbat this stroma from time to time receives additions from cells which pass through the investing membrana propria of the sex cords.


The sex cords are tubular invaginations of the peritoneum and their membrana propria are accompanying infoldings of the basement membrane as seen in earlier stages (Plate I, Fig. 4 and Plate II, Fig. 5). The sex cord nuclei are connected with the membrana propria by fine fibrils which apparently hold them in position.


In its earliest stage, this is a true process of invagination, but in the later stages it is only apparent because of the fact that the sex cords grow at their points of attachment to the peritoneum (centrifugal growth). Without doubt the sex cords are homodynamous with • the rete tubules.


Llf. cm. Embryo. - The nuclei of the peritoneum covering the rete are more numerous than in the preceding stage, being even more closely crowded. This has resulted in a further increase in the number of rete cords. Primitive ova may or may not occur in any given rete cord (Plate II, Fig. 6), there being apparently no regularity in this matter.


Immediately ventral to the peritonemn of both rete and sex gland there is a thickened area of stroma to which addition is constantly being made by proliferation from the peritoneum. This thickening is of no especial importance in the rete, but in the sex gland it, together with a similar but less important area dorsal to the sex gland, furnishes the connective tissue that goes to form the mesentery. The peritoneal nuclei of these mesenteric rudiments (Plate II, Fig. 7) are cylindrical, with their long axes perpendicular to the long axis of the sex gland.


The rudiment of the sex gland shows very little advance over the preceding stage in point of structure. There has been a continued growth of the sex cords, "resulting in such a thickening of the sex gland rudiment as to cause it to appear hemispherical in cross-section (Text Fig. 2). As in the previous stage, the peripheral layer of cells is not marked off from the underlying sex cords attached to it because of the fact that it is still adding to the latter by rapid proliferation.


The capillary blood-vessels and stroma cells already noted are quite evident in the interspaces between the sex cords. Here and there the walls of these capillaries show spindle-shaped nuclei which are far more attenuated than are the stroma cells. The latter are most nmnerous at the distal (inner) ends of the sex cords where they form a loose layer separated from the capsules of Bowman by a layer of attenuated, deeply-staining connective tissue cells which have their origin in the mesenteric fundaments already described. Posteriorly the sex gland gradually shades off into the mesenteric ridge, the sex cords becoming fainter and fainter and the primitive sex cells decreasing in number. The last named are found to occur ahnost at the posterior extremity of the mesonephros, where the genital ridge exists only as a strip of tissue along which the peritoneum and underlying stroma are thicker and denser than ordinary. There is an equally gradual transition- from the sex gland to the rete. In following the sex gland into the rete region, the first sign of transition from the former to the latter is noted in the nearer approach of the peritoneal invaginations to the capsules of Bowman. They become less crowded and the genital ridge decreases in height.


1.5 and 1.6 cm. Embryos. - The sex gland increases in volume to such an extent that in the 1.6 cm. stage it appears circular in cross-section. It is attached to the mesonephros by its mesentery which is now much narrower than the sex gland. The latter appears to have been constricted off from the surface of the mesonephros by lateral furrows. This, however, is not the case, because measurements show the mesentery to be as broad as the base of the sex gland of the 1.4 cm. stage. The constriction is apparent, not real. In reality the centrifugal growth of the sex cords has caused the sex gland to expand on all sides until it is now cylindrical in shape, instead of appearing as a slight elevation above the surface of the mesonephros as in preceding stages.


The sex cords are becoming longer and more contorted. Together with the growth in extent they become more clearly defined. The increase in the surface of the peritoneum caused by the expansion of the organ has not been accompanied by the formation of new cords, hence space is left in which many sex cords become arranged parallel to and immediately beneath it.


There appears the beginning of a most important process by which the sex cords become separated from the peritoneum through the development of the albuginea (Plate III, Pig. 8). The nuclei of the cords at their points of attachment to the peripheral cell layer (peritoneum) begin to elongate and in many cases to assume the appearance of connective tissue nuclei, while their cytoplasm is drawn out into slender strands that stretch across the necks of the sex cords. At the same time, a basement membrane is forming beneath the peritoneum dividing it from the elongated cells just described.


The mesentery is composed of cells derived from the dorsal and ventral mesentery fundaments, their rather irregular arrangement being disturbed by the ingrowth of a number of blood-vessels.


At this stage appears the peritoneal invagination that forms the Miillerian duct. It arises in the ventral part of a plate of thickened epithelium which forms the anterior end of the genital ridge. Exactly similar invaginations forming the most anterior rete tubules arise in the same epithelial plate immediately dorsal to this rudiment of the Miillerian duct. Prom the foregoing it seems fair to assume that the Miillerian duct is homodynamous with the rete tubules and sex cords.


1.7 cm. Embryo. - In this stage the sex cords become completely separated from the peritoneum (Plate III, Fig. 9). As we saw above, this was foreshadowed in the 1.6 cm. stage l)y the transformation of the basal nuclei of the sex cords into elongated connective tissue elements. They remain attached to the membrana propria, which in almost all cases becomes ruptured and allows them to lie free between the peritoneum and the intact inner portions of the sex cords. They now form a connective tissue layer (albuginea) separating the sex cords from the peritoneum. Here and there one can see a sex cord that still remains attached to the peritoneum, and it is not at all difficult to find portions of the membrana propria to which a number of the connective tissue cells are still attached. The l^asement membrane shown in the preceding stage to be forming at the places where these sex cords are breaking away, has become completely formed except at a few points where the sex cords are still attached. The connective tissue nuclei formed in the manner above described are very similar to the mesenteric nuclei. This fact has led many to claim that the albuginea is composed of nuclei that immigrate from the mesenteric fundaments. We cannot hold this view in the face of the facts above noted. Furtlier substantiation of the view of development in sihi is furnished by the fact that nuclei exactly like those forming the albuginea are found in the peritoneum, being no doubt formed by the same process that produced the albuginea tissue.


The two sexes cannot be clearly distinguished from one another at this stage, the process above outlined taking place in both ovary and testis. The separation of medullary cords is, however, not quite so complete in the ovary as in the testis, yet this can hardly serve to sharply distinguish the sexes at the stage now under consideration.


The rete strands are quite well developed at this period, being long and somewhat contorted (see Plate lY, Fig. 11), They usually take a course more or less nearly parallel to the peritoneum to which they still remain attached at their points of origin. In general, they grow posteriorly, those found at the posterior end of the rete regions extending into the anterior part of the sex gland. Along their course they frequently touch the capsules of Bo^vman, some of them growing straight inward from the peritoneum in such a manner that their tips come directly in contact with the IMalpighian corpuscles, thus appearing to form, in some cases, a part of their epithelial walls. This closeness of union is frequently sufficient to deceive one into considering them to take their origin from the capsules of Bowman.


At the boundary between sex gland and rete there is a transition area in which the peritoneum becomes considerablv thickened. Posterior to this it is found to send numerous sliort projections into the underlying stroma, and further back, these assume the character of closely-crowded sex cords. At this place the rete tubules are few in number and arise exclusively along a line very close to the mesentery. They can be distinguished from^sex cords only by their isolation and by their greater length.


In general the rete tubules are made up largely of cells without definite boundaries and in all other regards like those of the peritoneum from which they originate. Only occasionally does one find a primitive sex cell.


1.8 cm. Embryo. - Sexual dift'erentiation is not yet clearly established, although, in a vague way, the general distinctions mentioned in connection with the 1.7 cm. stage are to be taken as criteria.


The sex cords stand out in greater contrast to the stroma owing to the fact that the cytoplasm of the component cells is much denser than in the preceding stages (Plate IV, Fig. 13). In some places these cords show a central lumen. This is not due to any regiilar process of lumen formation, but has significance only in showing that there is in each sex cord a line of weakness or rudimentary lumen which owes its existence to the fact that these cords originated by a process of invagination of the peritoneum.


Exclusive of the primitive sex cells, there are two extreme types of nuclei common to the sex cords, intercordal stroma and albuginea. Those of the one kind are small and elongated, taking a deep diffuse stain (Plate IV, Pig. 13). The nuclei of the other type are larger, clearer and more rounded. There are all intermediate forms between these two extremes. The larger nuclei predominate in the peritoneum, the smaller variety characterize the albuginea, while the two kinds appear in about equal number in the sex cords and in the intercordal stroma. The marked similarity between the nuclei of the sex cords and stroma is not surprising when one considers the fact of their common origin. In the sex cords we shall term these the gerrainative cells in contradistinction to the primitive sex cells. Transition forms are found to unite the two distinct types of cells, thus showing that certain of the germinative cells are being transformed into primitive sex cells. The medium-sized germinative cells are probably the most primitive; these form the sex cells on the one hand and on the other the connective tissue cells. It is interesting to note that the nuclei of many genninative cells are dividing by amitosis.


The germinal epithelium of the sex gland of the 1.8 cm. embryo does not contain any primitive sex cells clearly differentiated as such.


It is significant to note that one finds here certain transition forms which link the usual type of peritoneal cells with the primitive sex cells found in the sex cords. In some cases these transition nuclei show much the same characters as regards chromatin and nucleolus as do the nuclei of the primitive sex cells, yet they differ in shape and size. It should be noted, in this connection, that the peritoneal layer is almost completely separated from the sex cords by the albuginea.


There are numbers of spherules of fat in the peritoneum covering the sex gland. These evidently indicate a process of fatty degeneration that seems to attack the cytoplasm of the cells and later to destroy a few of the nuclei, resulting in giving to the peritoneum a ragged appearance, there being large gaps where the cells have been destroyed.


2.5 cm. Embryo. - Sex differentiation is very strikingly shown in this most important stage. In the testis the albuginea has become thicker and denser than in the preceding stage. At the same time, the peritoneum has become flattened and is definitely separated from the albuginea by a distinct basal membrane. It contains no primitive sex cells. The peritoneal covering (germinal epithelium) of the ovary has become thickened and has even begun to send a few slender cords of cells into the loose underlying albuginea. These are the cords of Pflliger. They are, in many cases, loosely connected with the ovarian sex cords which we shall hereafter designate as medullary cords.


The process of fatty degeneration noted in the peritoneum of the preceding stage is still taking place in both ovary and testis, and has even extended to the sex cords in which large numbers of fat spherules appear. These occur almost exclusively in the syncytial cytoplasm of the germinative cells. They are most numerous in the portion of the sex cords furthest from the mesentery. Their fatty nature seems pretty evident from the fact that they are stained black by osmic acid and also from the spherical form that they assume.


The medullary cords are quite shrunken, being in most part clumped together in an irregular mass lying near to the mesentery. Cell degeneration occurring in them is not balanced by sufficiently rapid cell division. The primitive sex cells are surrounded by the undifferentiated cells that we have been terming germinative cells. This term, however, should henceforth be applied strictly to these cells in the seminiferous tubules alone. All resemblance to a tubular condition is lost, the medullary cords appearing in the form of masses of cells with no evidence of a very regular arrangement.


Numerous stroma cells are found to have become highly modified. The cytoplasmic portions increase in amount, becoming clearly marked off from surrounding cells, a centrosphere and centrosome appear, and the nucleus becomes rounded, while its chromatin network stains deeply. In general they assume a certain resemblance to primitive sex cells, yet the nucleus shows marked differences in its smaller size and more deeply-staining chromatin network. They differ also in the fact that their cytoplasm becomes granular and in later stages contains droplets of fat. These modified stroma elements are the interstitial cells. They are very numerous in the testis and very rare in the ovary. In both sex glands they divide by mitosis. A large portion of the stroma nuclei do not undergo this transformation into interstitial cells, but become elongated and take on the character of connective tissue. In all iDrobability these are the cells whose nuclei were smallest in the preceding stage where a difference in size and appearance of the stroma nuclei was noted. Particular stress should be laid upon the fact that the interstitial cells appear contemporaneyiusly with the process of fatty degeneration in the sex cords and that they show points of resemblance to the primitive sex cells. The latter point is particularly significant in view of the fact that in the 1.8 cm. embryo the nuclei of the stroma were to all appearances similar to those of the germinative cells of the sex cords which were in some cases developing into primitive sex cells. This would lead to some very attractive hypotheses; but one should be cautious about drawing hasty conclusions from such points of mere resemblance.


3 cm. Embryo. - There are no essential differences between the rete of ovary and testis. The rete cords are still being formed, their points of connection with the peritoneum persisting along the entire length of the rete rudiment in both sexes. Another point common to both sexes is the degeneration of certain Malpighian corpuscles of the anterior part of the mesonephros. Those lying nearest to the rete cords are especially affected, suffering a decrease in size and a consolidation of the capillaries contained in them.


Primitive sex cells are being formed in the rete cords from the syncytial cells that have retained the primitive character exhibited by the peritoneal cells, from which these cords arise. This development of undifferentiated peritoneal derivatives to form primitive sex cells is probably homologous with the process by which the germinative cells of the seminiferous tubules of the testis and the cells of the cords of Pflliger of the ovary are being transformed into primitive sex cells. The sudden impulse to renewed activity in the formation of these cells apparently affects both rete and sex gland at the same time. It is barely possible that the presence of fatty spherules so evident in the 2.5 cm. stage may be in some manner correlated with the active formation of primitive sex cells in the seminiferous tubules, cords of Pflüger and rete cords, all of which structures have been shown to be homodynamous. Such a hypothesis would, however, require more evidence for its proof than we have yet found.


The fat globules so numerous in the sex gland of the 2.5 cm. embryo have almost entirely disappeared from all save the interstitial cells of the testis, in the fat globules of these there has been an increase which may or may not be correlated with their disappearance in the seminiferous tubules. Whatever loss of cells may have taken place in the serniniferous tubules at the preceding stage has been compensated for by a process of rapid cell division. This, together with the transformation of germinative cells, has resulted in a decided increase in the number of primitive sex cells.


The peritoneum of the testis has become still further flattened, and its fatty spherules have almost wholly disappeared.


The albuginea nuclei have become more attenuated than in previous stages, yet they do not differ essentially from certain other connective tissue elements of the stroma, many of the more attenuated of which are seen to become applied to the membrana propria of the seminiferous tubules in such a manner as to form thin connective tissue sheaths.


The interstitial cells are an extremely important constituent of the testis, occupying the interspaces between the seminiferous tubules (see Plate IV, Fig. 13). In the ovary, on the other hand, they are very sparse. In the place of them one finds great masses of loose connective tissue, filling the interspaces between the other ovarian tissues.


3.5 cm. Embryo. - This stage will be noted chiefiy to record the reduction in the cytoplasm of the interstitial cells of the ovar}^ jSTot only has the cytoplasm of these sparse cells become shrunken, but the centrosome and centrosphere have almost disappeared. Both primitive sex cells and follicle cells of the medullary cords are suffering extensive degeneration. This continued process of degeneration is even more marked in the cortex and cords of Pflüger, which are now just beginning to assume importance.


4- cm. Embryo. - There is an interesting process of karyolytic degeneration that appears in the rete cords of this stage. The chromatin of the nuclei so affected gathers together in a rounded solid


104 Eml)rvonic Development of Ovary and Testis of Mammals mass which is finally set free in the cytoplasm by the rupture of the nuclear wall. It now breaks up into irregular fragments which finally become more or less rounded and eventually disappear. Here and there one finds nuclei of the seminiferous tubules and cords of Pflüger which degenerate in the same manner. This process takes place very extensively in later embryonic stages.


In both sexes the rete cords along at least three-fourths of the length of the rete region have become separated from the peritoneum by a layer of stroma. It was impossible to determine whether this process is analogous to the separation of the sex cords from the peritoneum covering the sex gland.


Attention has already been called to the degeneration of certain Malpighian corpuscles of the anterior end of the mesonephros in the 3 cm. stage. In the particular specimen' now under consideration (4 cm. embiyo) this process has continued, affecting eight of the most anterior corpuscles. The remaining ten or twelve corpuscles between the degenerate ones and the hilum of the testis are, as a whole, quite normal. Certain of the more peripheral of these intact Malpighian corpuscles send out short evaginations that come in contact with corresponding processes from the mass of rete cords and fuse with them (Plate V, Fig. 15). In this manner preparation is made for the establishment of a subsequent connection between the mesonephric tubules and the rete cords. Connection is also no doubt established without the aid of these evaginations in cases where the rete tubules press tightly against the capsules of Bowman. The number of the above described evaginations arising from each Malpighian corpuscle varies decidedly. In many cases there are none at all; in others there are as many as three.


Smaller evaginations from the capsules of Bowman were found in a 3 cm. embryo.


5.7 cm. Embryo. Ovary. - One is struck by the very close resemblance between the medullary cords and the cords of Pflüger. They are practically identical, position alone serving to distinguish them. The medullary cords (Plate III, Fig, 10) lie in the central axis of the sex gland, separated by a zone of connective tissue from the cord^ of Pflüger which project inwards from the peritoneum. Both elements are in large part composed of primitive sex cells - in fact there are but few small, deeply-staining nuclei which may be identified as those of rudimentary granulosa cells. The latter have no well-defined limits, being in every regard similar to the cells of the peritoneal layer from which the cords of Pflüger arise.


Fatty degeneration has almost ceased in the medullary cords and cords of Pflüger.


7.5 cm. Embryo. Ovary. - A few of the ^:x cells of the cords of Pflüger have undoubtedly developed into the condition of oocytes because of the fact that their chromatin threads have taken on the synapsis form described by Winiwarter, oo, in the rabbit. Corr'3Sponding synaptic stages are also found in the medullary cords, thus bringing out the close homology of the two structures.


The cords of Pflüger have become elongated and have at the same time branched and anastomosed to form a network in a manner quite like that of the seminiferous tubules in the testis. The resemblance is still further heightened by the fact that the cords of Pflüger are invested with a connective tissue layer formed by attenuated connective tissue cells of the stroma. The same is true of the medullarscords.


We might homologize these three structures by considering the seminiferous tubules and medullary cords as exactly homologous structures, while the cords of Pflüger constitute a second series of invaginations in all respects homologous with the medullary cords save as regards the time of origin.


Testis. - The structure of the testis is essentially the same as in earlier stages. There has been a progressive increase in the extent of the system of seminiferous tubules, which has been brought about by the continued growth and branching of those already laid down previous to their separation from the peritoneum (1.7 cm. embryo). The nuclei of the germinative cells are attached to the basement membrane by strands denser than the surrounding cytoplasm. This relation to tlie basement membrane is exactly similar to that of the peritoneal cells in the earliest stages (0.7 cm. embryo). The primitive sex cells are increasing in number by two processes, namely: (1) division by mitosis of those already present in earlier stages; (2) transformation of germinative cells into sex cells. All stages in this transformation process can be noted, any transverse section of the testis at this stage (Plate IV, Fig. 14) showing a complete series of transition forms. The same may be seen in embiyos earlier and later than this, namel}', from 3 cm. to 13 cm. in length. Primitive sex cells occur in the rete cords of both male and female, those of the male being apparently in the same stage of development as are those of the seminiferous tubules. This is not true in the female at this stage, owing to the fact that the primitive sex cells of the cords of Pflüger and medullary cords have developed precociously, outstripping those of the rete ovarii.


8.5 to 10 cm. Embryo. - Up to the stage when the embryo is 8.5 cm. in length, the rete cords extend but a short distance straight in from the hiliim in the case of both ovary and testis, and are similar in both sexes, the primitive sex cells of the rete ovarii becoming larger than those of the rete testis.


The embryo of 10 cm. length shows the rete testis to have rapidly developed an axial core of loose connective tissue that fills in the space central to the free tips of the radially directed seminiferous tubules. In the female, on the other hand, the rete ovarii extends no further into the ovary than in the preceding stages, remaining in contact with the anterior end of the irregular mass of medullary cords. The rete ovarii and rete testis now follow different courses of development.


13 cm. Embryo. Male. - The cords of the rete testis have in most cases undergone a process of lumen formation. This is brought about by the drawing apart of the cells from the axis of the cords. As already shown, these rete cells are attached to the ensheathirrg membrana propria, hence the lumen is formed by a very simple process by which they are made to separate along the line of greatest weakness (axis of cord). These rete tubules branch and anastomose quite like the seminiferous tubules. Their homology to the latter is still more clearly shown by the great similarity in the component cells of the two structures.


The rete tubules contain a few primitive sex cells (Plate VI, Fig. 21) exactly like those found in the seminiferous tubules. These are nothing new, as we have seen them to occur in the rete tubules of the very earliest stages. Another point of similarity is found in the character of the epithelial cells of the rete tubules which are in every regard similar to the germinative cells of the seminiferous tubules. Transition forms between epithelial cells and primitive sex cells do not exist in the former, while they are quite plentiful in the latter.


The rete tiibules send out side branches (tubuli recti) that fuse with the inner ends of the seminiferous tubules. In this manner one rete tubule may come into direct connection with a large number of seminiferous tubules. In one section, a rete tubule was seen to send out four tubuli recti connecting with as many seminiferous tubules. The point of Junction of tubulus rectus and seminiferous tubule (Plate V, Fig. 18) is easily recognized by the difference in diameter of the two elements, by the difference in arrangement of their component cells and by the presence of a lumen in the rete tubules as contrasted with the absence of such in the seminiferous tubules. In cases where connection has not been completely established between these two structures, the point of junction is marked by the persistence of the hasement membrane of tubulus rectus and seminiferous tubule. These membranes are soon absorbed and the two structures are in direct continuity with one another.


Female. - The primitive sex cells are found in all parts of the rete ovarii, yet their distribution is in no sense uniform. The intraovarian portion contains great numbers of primitive sex cells, which show a close resemblance to those found in certain regions of the cords of Pflüger. Associated with these sex cells of the rete are other and smaller cells which are practically identical with certain cells of the cords of Pflüger destined to form the granulosa of the Graafian follicles.


As stated above, the primitive sex cells are not by any means confined to the intra-ovarian portion of the rete tissue, yet their number in the portion of the rete lying within the mesonephros is found to become less and less as the distance from the ovary increases. The same principle holds true in the male.


The medullary cords are greatly reduced (Text Fig. 3), consisting of clumps of cells containing sex cells in various stages of development, the most advanced being large oocytes with a well-formed layer of granulosa cells. Such young follicles are rare j,,^, 3. Transverse section of ovary nnrl icnlafcirl and niesonei>hrJc structures of pigr em duu ifeUidLeu. ^j.yQ Length 13 cm. c, cortex ;/(. p., The irinprmo^t pnrl^ of tbp porrls; of hollow cord of Ptiflger: ni., medulla; m. Xiie mueimosi enu^ 01 ine COras 01 ^^ medullary cord; M. d.. Mullerian Pflüger are being broken up to form 'ir.^'i', woith^n'duct.^T2^!'"^*^ follicles. These follicles are young, each consisting of a large oocyte and a single layer of granulosa cells. The oocytes almost invariably contain numbers of fat globules situated in their cytoplasm and especially numerous about the centrosphere, where they appear to congregate, eventually combining to form a single large mass. This appears to be without doubt a process of degeneration, leaving clumps of granulosa cells which persist for some time after the oocytes have disappeared. Not only are these oldest sex cells being destroyed by fatty degeneration, but there is an independent process of karyolysis which destroys great numbers of younger sex cells. In addition to these two processes is that by which the fine chromatin threads in the nuclei of the oocytes at the synapsis stage of their development frequently break down into a powder-like mass of very fine granules. This is no doubt another process of degeneration.


On the side of the ovary facing the mesonephros, there are a number of invaginations of the peritoneum (Text Fig. 3). These often appear as hollow tubules that extend for some distance into the ovary. At points along their extent they are found to be solid, their cells being similar to those of the cords of Pflüger. In fact they are to be interpreted as such. Transition regions are found in which the peritoneal lining of these hollow tubules is found to contain a greater and greater percentage of primitive sex cells (Plate VII, Fig. 25) up to the condition of the solid portions of the tubules where the lumen is entirely obliterated by the enlargement of the peritonealcells to form primitive sex cells.


15 cm. Embryo. Female. - In this ^stage the above described hollow egg-tubes of Pflüger, while still most common around the hilum, are also found in the region of the cortex furthest from the mesentery. They penetrate more deeply into the tissue of the ovary than in the preceding stage, some of them extending into its very center, where they could be readily mistaken for" rete tubules by persons who might have studied these structures in ether forms, such as the cat. There is no mistaking their identity, however, because they bear no resemblance to the true rete tubules and because they were readily followed through the series to their point of union with the peritoneum. These invaginations may arise either from deep grooves or from the smoother surfaee of the peritoneum. At this stage the medullary cords are still further reduced, no young follicles being found among them.


The mass of rete tissue is now found to be constricted at its point of entrance into the ovary. Further development of the sex cells and of the intra-ovarian rete has caused the boundaries of the rete cords to become obscured.


Male. - In the male, the glomeruli connected with the rete tissue have degenerated to such an extent that the rete tubules are now found to be in almost direct contact with the mesonephric tubules. A minute description of the seminiferous tubules of this stage will serve to unify the points touched upon in the preceding pages (see Plate 'Y , Fig. 18). They are still solid, yet their tubular nature is shown by the arrangement of the dense peripheral layer of nuclei belonarinff to the fferminative cells. Each nucleus is attached to the membrana propria by a cylindrical condensation of cytoplasm, frequently so short that the nucleus appears to rest directly upon the membrana propria. The axial portion of the tubule is occupied by a loose network of protoplasm. At no time do these germinative cells have definite boundaries.


There are at this stage no transition forms between the germinative cells and primitive sex cells.


IS cm. Embryo. Female. - This stage shoAvs some interesting points in the development of the intra-ovarian portion of the rete tissue. Lying in the mesentery at the hilum, it extends but a short distance into the ovary, not reaching the inner ends of the adjoining cords of Pflüger. In this intra-ovarian portion of the rete, the oogonia have in some cases developed so far as to be surrounded by well-defmed follicles (Plate VI, Fig. 20). These young follicles have but a single layer of granulosa cells and are exactly like the follicles formed in the inner portions of the cortex, at this stage. With these rete follicles are found sex cells in all stages of development, likewise resembling corresponding sex cells in the cords of Pflüger. The resemblance is made more complete by the fact that the sex cells of the rete are undergoing the same process of degeneration as are corresponding sex cells in the cords of Pflüger.


The portion of the rete lying in the mesonephros has become distinctly separated from the intra-ovarian portion just described. Most noteworthy, however, is the fact that the few primitive sex cells found in it have not developed beyond the original condition which they exhibited in the early stages of development.


Few of the open tubes of Pflüger exist as such at this stage, most of them having become transformed into solid cords of cells such as characterize the cortex as a whole.


20 cm. Emhryo. Female. - A careful study of the inner ends of the cords of Pflüger shows that many of the oocytes of the young primitive follicle! have disappeared as a result of the process of degeneration already described. The granulosa cells are apparently not affected, but persist in solid elongated clumps, similar to the remains of the medullary cords found scattered through the axial portion of the ovary.


All the primitive sex cells and oocytes of the rete tissue have disappeared, leaving only the granulosa cells and their homologues.


25 cm. Embryo. Female. - At this stage the surface of the ovary is found to have become wrinkled and irregular. The cords of Pflüger form a thick, dense, cortical layer, within which is the medullary portion of the ovary, made up of loose connective tissue (stroma) -which extends between the cords of Pflliger in the form of strands and plates having a texture denser than that of the central mass. These strands are continuous with a sub-peritoneal layer of connective tissue that separates the cords of Pflüger from the peritoneum, thus putting an end to their further growth at the expense of the latter. Here and there a slender ingi'owth from the peritoneum is still found to pierce the connective tissue layer, yet these are of slight importance. I am not prepared to say whether they assume greater importance in later stages.


The cortex contains sex cells in all stages of development, from the very young oogonia of the peripheral region to the small follicles in its innermost edge. The remains of the medullary cords and of the intra-ovarian portion of the rete are still present in the medullary region and are fomid to be in practically the same condition as in the 20 cm. embryo. Not a sign of sex cells is to be seen in either the intra- or extra-ovarian portions of the rete.


The rete tissue is much more extensive in this stage than in the 20 cm. stage. There it was again more extensive than in the preceding (18 cm.) stage). Although these observations would seem to point to its growth after the degeneration of the sex cells, one should not lay too much stress upon this point. These seemingly conclusive facts may be conditioned l^y the great variability universally seen to exist in vestigial structures. A study of the rete cells failed to reveal extensive nuclear division in the above stages.


IV. Details of Development in the Rabbit

13-Day Embryo. - There is at this stage no obser^^able difference between the structure of the sex gland and rete rudiments. This stage corresponds with the 0.8 cm. stage of the pig. One is struck with the vagueness of the basement membrane of the peritoneum both in this and in succeeding stages of the rabbit, yet it is as tridy present as in the pig embryos where it appears with remarkable distinctness. Many of the cells of both stroma and peritoneum are found to be quite -irregular in shape, in many cases even amoeboid. Frequently they appear to be dividing by amitosis. It is very difficult to decide whether this be merely apparent or real. This point deserves special study, as it is of prime importance. A few figures of mitosis appear here and there. Primitive sex cells are present, though rare, occurring either in the peritoneal layer or beneath it. They are to be distinguished from the surrounding peritoneal and stroma cells l)y the same criteria noted in the pig embryo. Both rete and sex regions are found to contain them.


lJ^y2-Day Embryo. - At this stage the sex gland rudiment is easily distinguishable from the rete portion of the genital ridge. It is hemispherical in transverse section, having attained a marked increase in height over the preceding stage by multiplication of the cells of the peritoneum and of the stroma cells which are manifestly derived from it. Sex cords are well formed, as in the 1.4 cm. stage of the pig, being likewise continuous with the peritoneum from which they were formed by a process of invagination. Although the cords appear with a fair degree of clearness, the rabbit is by no means so favorable a subject for the determination of the manner of their formation, as is the pig.


The rete portion of the genital ridge is quite low in comparison with the rudiment of the sex gland. Here one finds certain scattered diffuse cords projecting from the peritoneum into the. underlying stroma, each invested by a membrana propria continuous with the basement membrane of the peritoneum. There are a few primitive sex cells in these rete tubules, but the predominating type of cells comprises those with small, oval, deeply-staining nuclei without cell boundaries, such as compose the peritoneum. These cells are attached to the membrana propria or basement membrane, as the case may be, by slender strands of cytoplasm, showing the same relation in this regard as do the corresponding cells in the pig embryo.


The nuclei of the stroma in both rete and sex gland rudiments are found to be irregular in shape, giving the appearance of undergoing division by amitosis.


16-Day Embryo. - The rete tubules of the region anterior to the sex gland can be readily detected. They lie in a mass triangular in transverse section. This is limited by the mesentery of the mesonephros, the capsules of Bowman and the peritoneum. In places the rete tubules can be seen growing in from the peritoneum and branching in the stroma. Each has a rudiment of a lumen which opens into the body cavity on the one hand, and on the other extends for a short distance into the interior of the tubule. These rete tubules can be found along the entire length of the rete rudiment and back beneath the sex cords of the rudimentary sex gland. In this region - the anterior end of the sex gland - it is difficult to distinguish the rete tissues from the underlying layer of connective tissue cells that separates them from the Malpighian corpuscles. The rete nuclei differ from those of overlying sex cords in that they are slightly smaller, more irregular and more deeply-stained than the latter. These rete nuclei are not to be distinguished from the stroma nuclei nor from those of the peritoneum, all of the above named being amoeboid in shape, and giving the appearance of dividing by amitosis.


A few large, well-marked, primitive sex cells are found in the rete tubules, beneath the sex gland and in those lying well within the anterior portion of the mesonephros in front of the sex gland.


Testis. - In the anterior end of the testis the sex cords are still attached to the peritoneum. They reach a considerable length, and are often seen to branch once or twice in their course. More posteriorly one finds cords in process of separation from the peritoneum. As in the 1.7 cm. pig embryo, the nuclei of the proximal ends of these separating sex cords are becoming elongated and are assuming the character of connective tissue elements. Finally they break away from the basement membrane to form the albuginea dividing the peritoneum from the sex cords. More posteriorly still, this process is found to have been completed.


Ovary. - In the ovary, the sex cords have not begun to separate from the peritoneum, although the introductory stages of such a process are seen. The sex cords are not so definite in outline as are 'those of the testis, owing to the fact that the stroma tissue separating them is not so dense as in that organ. The cells of the ovary are in all regards quite like the corresponding ones in the testis, the same small, amoeboid nuclei being found in the sex cords, rete tubules, stroma and albuginea, in addition to the primitive sex cells of rete tubules and sex cords.


The peritoneum of the ovary is much thicker than that of the testis, being three cells thick in many places. Primitive sex cells are found occasionally in the innermost portions, together with intermediate forms connecting them with the ordinary peritoneal cells. The inner edge of the peritoneum is more or less irregular in outline, showing a number of short, rounded protuberances - the rudiments of the cords of Pflüger.


17-Day Embryo. Testis. - The seminiferous tubules of the testis contain primitive sex cells and germinative cells, with many transitional forms between the two. Both kinds divide by mitosis. The stroma nuclei are now more regular in shape, and no longer give the appearance of dividing by amitosis. As in the pig embryo, an investing membrane of connective tissue is found around the seminiferous tubules. This tissue has undergone a decided 'increase.


Cases of karyolytic degeneration are seen here and there among the cells of the seminiferous tubules and rete tubules, although it is by no means common.


The rete tubules are separated from the peritoneum save only at the anterior end of the mesouephros. They lie in a direction parallel to the long axis of the sex gland, being in some places closely applied to the capsules of Bowman. Many Malpighian corpuscles have given ' out evaginations that have fused with the rete tubules in the manner described in the pig embryo. The latter are distinctly separated from one another by their clear-cut membrana propria. No primitive sex cells are, at this stage, found in the rete tubules anterior to the sex gland, but they occur here and there in those underlying the anterior part of the sex gland. , Ovary. - The description of the rete testis applies to the rete ovarii, there being no essential differences between the two.


All except a very few of the medullary cords have broken away from the peritoneum. These resemble the seminiferous tubules to a certain extent, yet they have a tendency to form spherical clumps of cells which remind one of follicles.


21-Day Embryo. Testis. - The rete tissue extends beneath the testis for over half its length. It occupies the space between the somewhat excavated inner face of that organ and the mesentery. A transverse section of the testis would show the mass of seminiferous tubules to appear as a crescent between the horns of which lie the rete tubules. These anastomose, forming a mass of tissue in which the boundaries of the component cords are largely obscured. From this unified mass slender branches (tubuli recti) pass to the seminiferous tubules, with which they unite. The nuclei of the rete cells are strikingly like those of the germinative cells, this resemblance being heightened by the fact that neither kind possess cell boundaries. Here and there in the seminiferous tubules, nuclei are found to degenerate by karyolysis. Clear transition forms are found to connect the primitive sex cells with the germinative cells (Fig. 23). This stage shows the interstitial cells to be well developed. They are characterized by having well-defined limits, granular cytoplasm, centrosphere and centrosome, and a spherical nucleus somewhat smaller than that of the primitive sex cells.


Ovary. - The chief advance over the preceding stage is found in the extension of the cords of Pflüger into the loose connective tissue of the albuginea. It will be remembered that these cords were mere rudiments in the 17-day stage ; now they are quite well-developed.


The medullary cords are but indistinctly separated from one another by rather sparse stroma cells which resemble the follicular cells of the medullary cords.


23-Day Embryo. Testis. - In the testis of this stage there is no important advance over the preceding stage.


Ovary. - The rete tubules have assumed the appearance of the medullary cords save for the fact that they contain no primitive sex cells - a fact which might have been noted in the 21-day embryo. There has been little essential change in their general character. The nuclei of the rete tubules and their homologues in the medullary cords are still very irregular in form, giving the appearance of being in process of division by amitosis. Undoubted amitosis occurs among similar nuclei in the cords of Pflüger (Plate VI, Fig. 22). These are destined to become the follicular (granulosa) cells of the Graafian follicles. It is possible that some of them may develop into oogonia - this point should be studied further. The cords of Pfiliger are connected with the peritoneum by slender necks.


26-Day Emhryo. Testis. - The rete tissue has extended the entire length of the testis. Scattered primitive sex cells are found here and there in the part lying within the testis, but are not present in the mesonephric portion.


The interstitial cells are found to occasionally divide by mitosis.


There is an extensive karyolitic degeneration of sex cells of the seminiferous tubules. Not only is the nucleus affected in the manner already described, but the cytoplasm undergoes modification a,s well, in that it assumes the property of staining more deeply in these degenerating cells than it does in the normal ones.


Ovary. - The medullary cords are now clearly separated from one another by rather wide intervals filled with stroma tissue. The cords of Pfiiiger have increased in extent and have, to a large extent, fused with one another until their original limits are marked only by dense plates and strands of connective tissue. As in the pig, we shall hereafter refer to this zone of densely-packed cords of Pflüger as the cortex, in contradistinction to the inner core of looser tissue made up of stroma and medullary cords.


Rabbit at Birth. Testis. - The rete tubules become more distinctly limited from one another and have begun the process of lumen formation simultaneously in all parts of the rete tissue. In this process the cells pull apart from the central axis of the cord which is a line of weakness due to the manner in which these cords are formed. The lumen of any given rete tubule is not continuous at first, being formed disconnectedly along the course of the cord. Each tubule is provided with a connective tissue sheath. The typical epithelial cells of these tubules form a syncytium in which the deeply-staining, columnar nuclei are arranged side by side, usually in a single layer, although one, at times, finds a superposed layer. A few primitive sex cells occur in the portion of the rete tissue lying within the testis. A complete series of transitional forms are found to connect the primitive sex cells with the germinative cells in this as in the 17, 21, 23 and 26day stages.


In both male and female the mesonephros proper has almost wholly disappeared, leaving a connective tissue network in whose meshes lie masses of fat. The caput epididymis (Text Fig. 4), made up of contorted tubules (rete efferentia) lined with epithelial cells, still remains. These are the persistent uriniferous tubules of the anterior part of the mesonephos, the great bulk of the tubules posterior to these having degenerated, together with a few of those which were formed in the most anterior end of the mesonephros. A few shrunken glomeruli still persist in connection with the rete efferentia.


Rabbit 3 and 8 Days after Birth. Testis. - These stages are interesting chiefly because they show a marked diminution in the number of interstitial cells which, however, are still found to be in process of division by mitosis.


Rabbit 10 Days after Birth. Ovary. - The rete tubules are in all cases devoid of a lumen. They are frequently joined to hollow tubules of larger diameter which extend into the general rete mass. These are the rete efferentia. Their identity is shown by the large size of the lumen, and by the fact that the nuclei are larger than those of the rete tubules. These rete efferentia have been brought to lie within the hilum by the extension of the ovary to partially enclose the shrunken mesonephros. They probably grow into the ovary of their own account as well. The differences between medullary cords and rete tubules disappeared as far back as the 23-day stage of embryonic life.


The cortical layer is distinctly bounded from the medullary substance by a dense layer of connective tissue which follows a zig-zag course to accommodate itself to the large rounded projections comprising the cords of Pflüger. These cords are still attached to the peritoneum by their narrow basal portions.



â– "Fir.. 4. .Sagittal section of the testis of the rabbit, 3 days after birth, al., albuginea ; m., mesonephric remains; ?•., rete ; r. e., rete efferentia ; s. tu., seminiferous tubule ; W. d.. Wolffian duct, X28.


The medullary and rete tubules are usually two cells wide and without any trace of a lumen. Their nuclei are oval and ra'ther uniform in size. Here and there one finds primitive sex cells singly or in groups of two to four. They are frequently in the same stage of synapsis as are the more advanced nuclei of the cortex.


In the 26-day female embryo the Wolffian duct has almost disappeared by degeneration, together with the great bulk of the urinifer'ous tubules. In the female, 10 days after birth, the mesonephric structures are found to have completely degenerated save for a few vestiges of uriniferous tubules lying within the rete tissue and the mesentery, posterior to the hilum. Aside from these vestiges, the mesonephros consists of loose connective tissue enclosing great masses of fat - the remains of former Malpighian corpuscles and mesonephric tubules.


13 Days after Birth. Ovary. - Follicles are forming in the cortex at this stage. They are, of course, very s^ple, each consisting of a large oocyte surrounded by a single layer of granulosa cells. Exactly similar follicles are also found in the medullary cords.


Very many nuclei in all parts of the cortex and medullai-y cords are suffering karyolytic degeneration.


17 Days after Birth. Ovary. - The process of follicle formaition has continued, resulting in the breaking up of the inner ends of the cords of Plliiger. No sex cells are found in the medullary cords from this stage on. The ova of certain of the innermost follicles have disappeared, leaving clumps of follicle cells such as are found in corresponding stages in the pig. Frequently one finds two or even more oocytes in the same follicle.


24- Days after Birth. Testis. - This stage shows the testis to have pretty largely assumed the characters prominent in adult life. The caput epididymis is made up of the much-contorted tubules of the rete efferentia, the latter being traceable down to their points of connection with the rete testis. This, as already stated in the" description of previous stages, is made up of a mass of anastomosing rete tubules from which proceed the tubuli recti that connect Avith the seminiferous tubules. In the posterior two-thirds of the testis, the rete is wholly surrounded by the seminiferous tubules that have closed in around it. Here and there primitive sex cells (spermatogonia) can still be found in the rete testis.


Up to the stage of 8 clays after birth, there were found numerous transition forms connecting the germinative cells with the primitive sex cells. This is by no meins true of the 2-i-day stage (Plate YI, Fig. 24), where there is a very sharp distinction between the two types of cells which are singularly uniform among themselves. This is true not only in the characters already enumerated, but also in the staining reaction as Avell. The germinative nuclei take the iron hematoxylin stain with avidity while the primitive sex cells (spermatogonia) are not affected b}^ it at all.


25 Days after Birth. Ovary. - The cords of Pfiilger are now almost entirely broken up to form large numbers of small follicles surrounded by connective tissue that has permeated the entire mass of each cord of Pfiilger.


31 Days after Birth. Ovary. - It was noted in an earlier stage (17day ovary) that certain ova of the innermost follicles degenerated leaving clumps of follicular cells. These clumps are quite evident in the 31-day stage, lying along the border between the medulla and cortex.


Jf5 Days after Birth. Ovary. - Many of the follicles have increased in size until they have acquired as many as three layers of granulosa cells, among which appears an incipient follicular cavity. Already the stroma forms a capsular investment (theca) about each follicle, and this investment has begun to show a ditferentiation into a theca interna and a theca externa. The nuclei of the cells of the theca interna are roimded and the cell body has become fuller in contrast to the attenuated fibrous character of the cells of the theca externa, whose nuclei have remained elongated and in every regard like those of the general stroma tissue of which they are an integral part. Transition forms l^etween both varieties of theca cells can be readily found. It might be Avell to call attention to the fact that there is a thin layer of attenuated stroma cells between the theca interna and the membrana propria of the granulosa layer. It will be termed the follicular capsule.


At this period many cells of the theca interna have developed into interstitial cells similar to those described in the testis of the pig embryo. Each has a rounded nucleus, clear cell outlines, centrosphere. centrosome and numerous fatty granules deposited in its cytoplasm. The formation of these interstitial cells is genetically connected with a process of follicle degeneration which continues from the time of the earliest formation of follicles on through adult life.


60 Days after Birth. Ovary. - The medulla becomes still more reduced in this sta2:e bv the invasion and growth of follicles in its border. The ground substance is a rather compact connective tissue in which are imbedded the slender transversely-placed medullary and rete tubules, which are quite inconspicuous and devoid of a lumen. Large open lymph spaces are formed in the stroma. Typical interstitial cells are not at all uncommon.


75 Days after Birth. Ovary. - Certain of the innermost follicles have increased greatly in size, having in some cases almost reached maturity. They encroach upon the limits of the medullary region to such an extent as to make the latter band-like in cross-section. In Plate VII, Fig. 26, is represented a portion of the ovary showing the more important tissues composing it. The granulosa cells have welldefined boundaries, being polygonal in shape as the result of mutual pressure. The nuclei are rounded and are found to divide by mitosis. The follicle is bounded externally by a clearly-defined membrana propria, external to which one finds a very thin layer of attenuated connective tissue cells. This investment (follicular capsule) is similar to that which surrounds the seminiferous tubules.


Outside of this occurs the theca interna, which is from one to four cells thick, the component cells being elongated in a direction parallel to the surface of the follicle. They are rich in cytoplasm. The large, rounded or oblong nuclei stain more lightly than do the nuclei of the granulosa ceils.


The slender branching and anastomosing medullary and rete cords are distinguishable from the surrounding stroma by the clear cytoplasm and small oblong, deeply-staining nuclei of their cells which have apparently remained unchanged in character from the earliest stages onward.


Certain young follicles of a stage just before the formation of the follicular cavity have begun to degenerate, the process first afllecting the oocyte which in some cases has disappeared wholly or in part. The mass of follicular cells becomes irregular in outline, but shows no signs of degeneration. It is quite likely that certain cords of cells with nuclei larger than the true nuclei of the medullary cords in the midst of which they lie, have originated from these degenerating follicles, the resemblance between their nuclei and those of the normal follicles being very striking.


85 Days after Birth. Ovary. - The ovary as a whole has changed but little in form and size, this stage being most remarkable on account of the great increase in the number of interstitial cells. This increase is due to a very extensive process of follicle degeneration which seems to be at its height, affecting follicles in all stages of developnient. The granulosa cells are the first to show signs of degeneration, the nucleus drawing up into a small globular homogeneous mass in the center of the cell. The cytoplasm changes in such a manner as to become more deeply stained than formerly and the whole cell becomes rounded. This is no doubt the process of chromatolysis described by Flemming, 85. It certainly results in the eventual liquefaction of the cells affected.


Large numbers of connective tissue elements from the capsule and theca interna penetrate into the follicular cavity. As the granulosa cells degenerate further, the follicular cavity becomes smaller and the capsule, theca interna and theca externa contract, thus encroaching upon the follicular cavity until the latter has become greatly reduced. The above mentioned elements that have migrated into the follicular cavity from the capsule and theca interna persist after the granulosa cells have all disappeared. The cells from the theca interna later undergo fatty degeneration and finally disappear, leaving the slender connective tissue cells that had migrated from the capsule; these persist and probably remain to form part of the general stroma tissue, lying between the columns of interstitial cells, whose method of formation will be described later.


A series of fine connective tissue fibres join the follicular capsule with the theca externa passing rather obliquely between the cells of the theca interna. When the capsule closes in upon the follicular cavity these threads are drawn taut and arrange the cells of the theca interna in radial rows. The whole mass may become laterally compressed by the growth of neighboring follicles. In very advanced stages of atresia, when the follicular capsule has become reduced to a crumpled remnant lying in the midst of the cells of the theca interna, the latter lose their cell walls and become irregular in shape. In this condition they undergo a process of rapid amitotic division (Plate VII, Fig. 27), the resulting nuclei being much smaller than before this process took place. These will develop into the prominent interstitial cells as seen in later stages.


6 months' Virgin. - In the ovary of this animal (Text Fig. 5) it is possible to trace out the further development of the interstitial cells. They cease to divide and undergo a process of growth in size both of the nucleus and of the cell body. At this stage^ - just after division has ceased - there occurs a deposition of a substance occurring in the form of small spherules, which stain deeply with haematoxylin. They were found only in this one specimen and are in no wise to be confounded with the very numerous fat granules which now begin to fill the cytoplasm of these interstitial cells.


These fat grannies are very characteristic of the interstitial cells. In this stage certain of these cells which have become isolated from the general mass are found to have enlarged greatly and to have become stuffed full of large fat spherules which are very readily dissolved by xylol. Each of these cells contains a large excentric nucleus with a centrosphere and centrosome (Plate VII, Fig. 28). Most of the interstitial cells are crowded together by mutual pressure and are hence prevented from attaining the full size of the one figured, which lay free in the connective tissue.


8 Months Old; 1st Pregnancy; 1^ Days Pregnant. - The corpora lutea appear to be in the height of their development. The lutein cells composing them are rounded and suffer very little mutual pressure, being separated by fair intervals in many cases. Between them is a loose mass of fibrous connective tissue. The interstitial cells on the other hand, lie in dense masses between the corpora lutea. They are arranged in parallel strands in the manner already noted. Certain interstitial cells that become separated from the general mass are found to be rounded and of almost the same size as the lutein cells of the corpora lutea.


One is struck by the great resemblance of the interstitial and lutein cells (Plate VII, Figs. 29 and 30), a resemblance that practically amounts to identity aside from the matter of size, which difference can, in large part, be attributed to the factor of external pressure. The description of the interstitial cells of the 6-months' virgin practically applies to the interstitial cells of this pregnant rabbit and to the lutein cells of the corpora lutea as well.


Ovaries of Older Pregnant Rabbits. - The ovaries of a number of animals in various stages of pregnancy were examined. In the older of these animals the lutein and interstitial cells are apparently indistinguishable in the deeper-lying regions where the cells are the oldest. In the most central zones they are found to be undergoing a process of hyaline degeneration, the cell limits becoming indistinct, the cytoplasm ragged, and the nucleus very faint. Finally the innermost regions are found to contain the shrivelled remains of these cells.


Fig. 5. Transverse section of ovary of a six-months old virgin rabbit. /'., follicle ; d. /., degenerate follicle ; y. c, germiriative epithelium; i. »!., masses of interstitial cells ; I. s., Ijmpli spaces ; m., mesonephros remains. X 24.


Corpora lutea and masses of interstitial cells are successively forming at the periphery and disintegrating in the interior of the ovary.


Atretic follicles were found in adult pregnant females of various ages.


V. Discussion of Results

1.Indifferent Stage. - TliB genital ridge first appears as an area of thickened peritoneum and underlying mesenchyme (stroma), extending the entire length of the mesonephros, and situated on the ventromedian face of that organ. The tissues composing it are in no wise different from those forming the remainder of the investment of the mesonephros. In section, the peritoneum is found to be separated from the stroma by a more or less distinct basement membrane formed by the interlacing of protoplasmic fibrils proceeding from the nuclei of peritoneal and stroma cells (Plate I, Figs. 2 and 3).


The cells composing the peritoneum and stroma tissues are almost wholly without evident boundaries. Only here and there does one find scattered cells with distinct cell boundaries, centrosphere, centrosome, large nucleus, and clear c^-toplasm - the so-called primitive sex cells. They occur in all parts of the genital ridge Init are most numerous in that region in which the sex gland will form.


The distinction between peritoneum and stroma is not based upon any essential difference in the character of their component cells at this early period, but is based upon their arrangement, the nuclei of the peritoneum being arranged with their long axes parallel to one another and perpendicular to the basement membrane, while those of the stroma tissue lie with their long axes usually parallel to the basement membrane of the peritoneum which is very faint at certain points where active division of the peritoneal cells is taking place. This is due to the fact that peritoneal nuclei are being crowded through the membrane by mutual pressure, caused by their rapid multiplication.


In the 10 cm. stage, a regional differentiation begins to appear in the genital ridge. This is marked by the formation of numerous crowded peritoneal invaginations (Plate I, Fig. 4) in the middle third; less numerous and deeper invaginations in the anterior third; and the almost total lack of them in the posterior third. The regions from front to rear, as thus marked off, are the rudiments of the rete, sex gland and mesenteric ridge, respectively.


These invaginations are caused by a progressive multiplication of the peritoneal nuclei. Although the first formation of these cords is a true process of invagination, further gi'owth is centrifugal, the peritoneum moving outward, and at the same time adding to the cords already laid down, by a continuation of the invagination process.


These cords are truly of a tubular nature, a lumen, though not present, being conditioned by the arrangement of the cells. Transverse sections of these incipient tubules show a bounding meml)rana propria which is continuous with the basement membrane of the peritoneum. Inside of this is a single layer of peritoneal cells with their bases attached to the membrana propria, while their apices meet at a common central point - the rudiment of the future lumen.


The invaginations are much fewer in the rete region than in the sex gland rudiment, and also differ from those of the latter region in the fact that they penetrate through the stroma to the walls of the Malpighian corpuscles (Plate II, Fig. 6), from which the rudimentary sex cords are separated by a layer of stroma. The limit between the rete and sex gland rudiments may be roughly placed at a point opposite the 12th glomerulus, in the rabbit, and opposite the 20th, in the pig ; however, the sex gland rudiment slightly overlaps the rete region; hence the impossibility of drawing a sharp limit between the two.


There has been a great diversity of opinion in regard to the origin of the sex cords and rete tubules. Practically all writers except Egli, Janosik, Coert and von Moller have derived the rete tubules from the Malpighian corpuscles. The above named, considered them as products of the peritoneum covering the mesonephros. There has been greater unanimity in regard to the derivation of the sex cords. We shall not enter into detail upon this subject, but shall simply point out the fact that Waldeyer, 70 and 02, Kolliker, 98, Balfour, 78, Rouget, 79, and others hold that the sex cords arise from the Malpighian corpuscles, and that they receive primitive sex cells which migrate to them from the peritoneum.


According to Mihalkovics, 85, the cells of the sex cords arise from the germinal epithelium, not through direct invagination but in an indirect manner, through infiltration of the stroma by peritoneal cells which later become segregated to form strands.


Schulin, 81, and Coert, 98, hold a somewhat different view, namely that the entire sex gland is formed from a homogeneous mass of cells blastema) derived from the peritoneum. This view differs from that of Mihalkovics, 85, in that the latter does not consider the stroma to be derived from the peritoneum, while Coert considers such a peritoneal origin of the stroma to be very probable, and extends this idea to explain the formation of the rete tubules and the sex cords as well.


The continued growth of the sex cords at their bases and the accompanying outward movement of the peritoneum results in a thickening of the genital ridge in the sex gland region. This becomes more and more pronounced until the sex gland appears hemispherical in transverse section, later appearing as a disc attached to the mesonephros by a relatively slender bridge - the mesentery. It gives the impression of having become constricted from the surface of the mesonephros. This appearance is, however, delusive, as the mesentery is in reality slightly broader than was the base of the rudimentary sex gland.


The cells composing the indifferent sex gland (Plates III and IV, Figs. 9 and 11) may be classed under three heads: (1) Primitive sex cells; (2) syncytial cells with small nuclei; (3) syncytial cells with nuclei of various sizes.


The primitive sex cells, already described, are found chiefly in the sex cords, although they occur sparingly in the connective tissue of the mesentery. They divide infrequently by mitosis throughout these early stages.


The nuclei of cells of class (2) stain deeply and are often attenuated. They form the albuginea and occur in the stroma, sex cords and, to a limited extent, in the peritoneum.


Cells of the third class occur together with those of the second class, which they resemble in that they are without definite boundaries. Their nuclei are larger and usually stain less deeply, showing all gradations between those of the primitive sex cells and the small, deeply-staining syncytial cells of class ( 2 ) . It is almost certain that both forms originate from these cells of intermediate character of which the peritoneum is almost exclusively formed.


In the basal portions of the sex cords at the time when the latter are being separated from the peritoneum, there is a direct transition of the nuclei of class (3) into connective tissue nuclei of class (2), which forms the mesentery and the albuginea (Plate III, Fig. 9). Certain small nuclei of the sex cords resemble tliese connective tissue nuclei in a most striking manner and probably arise by a similar process of differentiation. These last named belong to the germinative cells of the seminiferous tubules and to the follicular cells of the medullary cords.


The albuginea and mesentery of the sex glands are derived from the peritoneum of regions immediately dorsal and ventral to the sex gland (Plate II, Fig. 7). The cells of these mesentery rudiments proliferate rapidly throughout the early stages, causing a rapid growth of the mesentery. Here and there primitive sex cells are formed in these regions and are carried down into the mesentery with the connective tissue in the midst of which they lie.


As stated above^ the albuginea is formed by the transformation of the cells occupying the basal parts of the sex cords, into connective tissue elements, which are liberated by the rupture of the membrana propria encasing them (Plate III, Fig. 9). In this manner the sex cords become separated from the peritoneum and undergo further growth and differentiation independent of that layer.


The formation of the mesentery and albuginea and the separation of the sex cords from the peritoneum are far more clearly shown in the pig than in the rabbit, yet I have been able to verify these processes throughout in the latter animal. Coert, 98, has come to essentially the same conclusions, but is cautious in expressing himself in regard to these points, as he well may be, because of the difficulty of following these processes in the rabbit, upon which form he worked.


The separation of the sex cords from the peritoneum takes place at a slightly earlier period in the female than in the male. Coert, 98, has laid considerable stress upon this fact in the case of the rabbit. However it is not of primary importance in the pig. In the latter animal, it takes place in embryos of 1.6 to 1.7 cm. in length.


As previously stated, the rete tubules are serially homologous with the sex cords, differing from them at this stage chiefly in the fact that they are less numerous, being isolated from one another by considerable intervals, filled in with connective tissue. The portion of the genital region occupied by the rete tubules becomes elevated to such a aegree as to be quite evident in gross dissections. Coert considers the rete tubules of the rabbit to arise from a mass of unorganized rete blastema by a process of differentiation which slowly progresses inward from the periphery. According to liim, this differentiation process is not completed until after birth. I found these tubules to be distinct and clearly limited in the rabbit embryo of 16 days. This difference between our results may have been due to a difference of technique. Coert used Kleinenberg's picro-sulphuric solution as a fixing agent, while my material was fixed in Flemming's fluid followed by Heidenhain's iron hsematoxylin stain.


Primitive sex cells are present in the rete tubules from the first, but are uncommon, the great mass of cells being similar in character to those of the peritoneum from which they arose. This similarity applies not only to the absence of cell limits in the rete cells of the pig and rabbit, but in the former animal, to the size and staining reaction of the nuclei as w«ll. In the rabbit, these nuclei stain more deeply and are slightly smaller than are the nuclei of the peritoneum and of the germinatiA^e and follicular cells of the sex cords. As will be seen in a discussion of later stages these differences tend to disappear.


The rete tubules extend in a posterior direction, their bases remaining attached to the peritoneum for a long time after the sex cords have become completely separated from it. These rete tubules penetrate quite deep into the stroma, reaching the walls of the Malpighian corpuscles, to which they are often so closely approximated as to give the appearance of arising from them. The glomeruli underlying this rete region comprise those from the 6th to the 20th, inclusive, in the pig, and from the 6th to the 12th in the rabbit.


Since the rete remains indifferent in character long after the ovary and testis have become differentiated from one another, a common description will suffice to make clear its development in both male and female, up to a relatively late stage. Primitive sex cells begin to form anew from the syncytial cells of the rete tubules in the pig embryo of 3 cm. length. When the embryo has reached 4 cm. length, the rete tubules break away from the peritoneum along the posterior threequarters of the length of the rete region. I was unable to determine whether this process is similar to that by which the sex cords become separated from the peritoneum. In any case it takes place at a much later period as above shown.


At this stage and a little earlier, evaginations arise from the capsules of Bowman of the Malpighian corpuscles at points close to the mass of rete tubules (Plate V, Fig. 15). Their number varies from one to three. In fact many Malpighian corpuscles give off no evaginations at all, although they arise in close proximity to the rete tubules. Similar evaginations occur in the rabbit embryo of 16 and 17 days, where they were first observed by Coert, 98. I am inclined to ascribe to these a morphological significance, yet they are of no particular functional importance, because a union of the rete tubules with the capsules of Bowman is, in very many cases, established by the former coming in direct contact with the latter.


It is interesting to note that l^ranches from the rete tubules grow out to meet the tips of the evaginations from the Malpighian corpuscles. The cells from these two sources assume similar characters and are later indistinguishable from one another. Such later stages are very deceptive, having no doubt given rise to the incorrect view that the rete tulmles arise from the Malpighian corpuscles.


The rete tubules 1)ianch and anastomose in their course, behaving much like the sex cords in this regard. The tubules of the anterior end of the rete mass remain in connection with the peritoneum throughout later stages while posterior to this point they are separated from it by a considerable interval and are united to form a cylindrical mass, the posterior end of which projects into the anterior end of the sex gland.


In the rabbit, the conditions are essentially similar yet by no means so clearly shown as in the pig. Difficulties in the study of these processes in the rabbit are caused by the compactness of the tissues, the smallness of the component cells, and the indistinctness of the limits of the rete tubules.


2. Sexual Differentiation. - It now remains to follow the ovary and testis separately as they diverge in the process of further development. Both are homologous, in that they have originated from an indifferent rudiment in which a considerable complexity of structure has become evident before it is possible to distinguish sexual differentiation.


Unmistakable differences between ovary and testis can be discerned in the 2.5 cm. embryo of the pig, less-marked differences being evident in the embryo of 1.8 cm. lengih. A clear distinction between ovary and testis is observable in the rabbit embryo of 14| days' age.


Previous to the period of sex differentiation, the sex gland has taken definite form, having become constricted off from the mesonephros, to which it remains attached by the relatively narrow mesentery. A transverse section shows it to be composed of the following tissues: (1) the peritoneum, or germinal epithelium as it has been generally termed, especially in the case of the ovary; (2) the albuginea, a term usually applied to the subperitoneal connective tissue of the testis, but equally applicable to the same zone in the ovary; (3) the sex cords; (4) the interstitial stroma; (5) the distal ends of certain rete tubules that have grown from the rete region into the anterior end of the sex gland.


The prime features of sex differentiation are shown in the 2.5 cm. pig embryo. The fundamental points are the further development of the sex cords in the testis to form the seminiferous tubules and the development of the peritoneum in the ovary to form the cords of Pflüger. These two sets of cords having a similar origin, but one which is successive in point of time, are the structures in which the functional sex products form. On the other hand, the sex cords of the ovary cease in their growth and become the medullary cords - assuming the character of the cords of Pflüger. In the testis the peritoneum ceases to develop and becomes flattened - finally almost disappearing in later stages.


The albuginea layer is far thinner and more compact in the testis than in the ovary. This is due to the fact that in the former it is much more closely crowded against the peritoneum by the seminiferous tubules than by the medullary cords in the case of the ovary.


The peritoneum, cords of Pflüger and medullary cords of the ovary, together with the peritoneum and seminiferous tubules of the testis, contain numerous globules of fat resulting from a process of fatty degeneration in these structures. Loisel, oo and 02, found the spherules of fat to occur throughout the rudimentary sex gland of the 98-hour chick embryo, and in the 5-day embryo of the California quail, and in sparrow and guinea-pig embryos as well. According to him the primitive sex cells lose them when they become spermatogonia, while certain germinative and Sertollian cells contain fat globules up to the end of embryonic life, when they disappear, to later reappear at the time of puberty and at successive periods of sexual activity. There is thus a periodicity in their formation at least in the sparrow, upon which form the greater part of Loisel's work was done. He shows that the interstitial cells are filled with fat globules at the same time that the cells of the seminiferous tubules contain them, hence there is an interrelation between the two sets of cells. Interstitial cells are rare if not non-existent in the testes of adult birds, although they occur in great numbers in the testes of adult mammals.


He interprets the fat spherules described above as secretion products and not as the products of degeneration. It seems to me unsafe to hazard an opinion upon the physiological aspects of this process, yet it certainly does result in the destruction, both immediate and remote, of a large number of cells. It would hardly seem that in the present state of our knowledge, Loisel is justified in his assertion that the bright plumage assumed by birds during the breeding season is due to any trophic stimulus imparted by this fatty substance. Coincident with this process of fatty degeneration in these structures, certain cells of the stroma suffer extensive modification, their cytoplasm becoming granular, acquiring a centrosome, centrosphere and definite cell limits. The nuclei of these cells also enlarge and become spherical. These interstitial cells are very numerous in the testis, but quite rare in the ovary. In Iwth sex glands, they divide by mitosis.


Plato, 97, Coert, 98, and Limon, 02, are unanimous in agreeing that the interstitial cells arise from the stroma in both ovary and testis.


3. Further Development of the Sex Glands. A. Testis. - The peritoneum becomes less and less important in later stages, finally forming a broken and almost vestigial covering of the sex gland. The albuginea becomes thicker and more compact, but need be given little further attention as its development is a very simple process.


We have still to consider the development of the following elements :

  1. Seminiferous tubules.
  2. Rete tubules.
  3. Interstitial cells and stroma.



1. Seminiferous Tubules

The seminiferous tubules of the 2.5 cm, pig embryo are rather distinctly limited by a membrana propria, still exterior to which is a thin layer of small connective tissue cells forming a capsular investment. The cells of the tubules are of two general classes - germinative cells and primitive sex cells, between which classes are found all intermediate forms. The germinative cells are identical with those classed above under groups 2 and 3, in fact the conditions are not altered in this stage. All classes of cells are dividing by mitosis while many of the germinative cells appear to be undergoing amitotic division.


Conditions in every regard similar to those outlined above, hold good in the rabbit material, the corresponding period being about the 16th day of embryonic life.


Transition forms connecting the germinative cells with the sex cells occur in the pig embryos between 2.5 cm. and 13 cm. length (see Plate IV, Fig. 14), later stages showing no transition forms. The rabbit material being more extensive, shows the condition of the cells of the seminiferous tubules up to the period of sexual maturity. Intermediate forms of cells are found to connect these two types in all stages from the 16-day embryo up to the stage 8 days after birth inclusive. Testes of the 40-day rabbit show absolutely no connecting links, the two classes of cells being there found in their purity. The germinative cells occur in a single layer with their bases attached to the membrana propria, while the primitive sex cells - spermatogonia - lie in the more axial portions of the seminiferous tubules.


A striking feature of the seminiferous tubules is their tendency to branch and anastomose (Plate IV, Fig. 12), such tendencies manifesting themselves in the very earliest stages, when the sex cords are first laid down.


As has been previously shown, the rete tubules are pushed into the anterior end of the sex gland. In the pig, their tips project into an axial space left between the inner tips of the radially arranged seminiferous tubules, while in the rabbit, the mesentery is broader than in the pig ; hence there is left a space at the base of the testis, for the occupancy of the rete tubules.


2. Rete Tubules

In the testis of the pig, the rete tubules remain within the hilum until a period between the 8.5 cm. and 10 cm. stages, during which time they grow rapidly down the axial space, almost reaching the distal end. It is at this period that the rete tubules begin to acquire a lumen and also to send out branches^ - tubuli recti - to the inner ends of the seminiferous tubules. As manv as four of these tubuli recti were seen to arise from a single rete tubule, being apparently called forth wherever needed. A distinction between the tubuli recti and the seminiferous tubules can be readily drawn from several criteria. The chief difference lies in the greater diameter, lack of lumen, and far greater number of sex cells of the seminiferous tubules as contrasted with the fact that the rete tubules are narrower by half, possess a lumen, and contain very few primitive sex cells (Plate V, Fig. 18). The two structures resemble one another in the character of their component cells, the germinative cells of the seminiferous tubules being practically identical with the epithelial cells of the rete tubules, and the primitive sex cells of both structures showing an exact correspondence. This homology is also seen in the fact that both structures are limited by membrana propria and capsular connective tissue investments formed in the same manner in each.


This process of the extension of the rete tubules takes place in the rabbit 3 days after birth. Later, the seminiferous tubules grow about the eccentrically placed mass of rete tubules in such a manner as to enclose it. Eepeated anastomosis of the rete tubules results in the union of their lumina to form a large cavernous, irregular space imperfectly divided by the walls of the component tubules. The nuclei of the rete cells still have the general characters of those belonging to the germinative cells of the seminiferous tubules; but are far more elongated by lateral compression. Primitive sex cells are found in the rete tubules of the rabbit 24 days after birth, but are not present in those of the 140day rabbit; hence it is safe to conclude that the sex cells of the rete testis are not functional.


3. Interstitial Cells

In the pig, the interstitial cells are found to multiply by mitosis from the time of their first appearance up to the stage of the 7.5 cm. embryo, and in the rabbit testis as late as 8 days after birth. There may be new interstitial cells formed between the period of their first appearance and sexual maturity, but this seems highly improbable, no evidences of such having been seen. They begin to degenerate in the 15 cm. pig embryo, and in the rabbit 24 days after birth. This process of degeneration first manifests itself by a shrinkage of the cytoplasm. In the process of development of these interstititial cells, their cytoplasm becomes filled with fat globules that have a tendency to run together (Plate IV, Fig. 13). At the same time, the centrosphere becomes clearer and more sharply defined from the surrounding cytoplasm. Plato, 97, does not represent the centrospheres in his figures of the interstitial cells of the cat, rabbit, steer, horse and other forms studied bv him.


B. Ovary. - The tissues to be considered in this organ are as follows :

  1. Cords of Pflüger and peritoneum.
  2. Medullary cords.
  3. Rete tubules.
  4. Interstitial cells and stroma.


1. Cords of Pfiuger, and Peritoneum

The cords of Pflüger, were seen to arise in the 2.5 cm. pig embryo as columns of cells growing into the stroma from the peritoneum. During later stages, they lengthen by centrifugal growth, cell multiplication taking place largely at their points of attachment to the peritoneum. One can find all stages in the development of the oogonia (see Plate VI, Fig. 22) from the stage when they are indistinguishable from the other cells of the peritoneum from which they originate, to that in which more mature forms of oocytes are found in the deeper-lying portions of the cords of Pflüger. There is a gradual transition in these cells; the degree of maturity corresponding with the distance from the surface of the ovary. In the rabbit, certain small nuclei of cells without cell boundaries divide by amitosis (Plate VI, Fig. 22). These cells of the cords of Pflüger correspond to the germinative cells of the seminiferous tubules. In the ovary, they are destined to form the granulosa cells, although it might well remain an open question whether some of these amitotically dividing cells do not also transform into sex cells.


In certain of the later stages of the pig (13 and 15 cm. embryos), tubular cords of Pflüger make their appearance (Text Fig. 3). They extend from the peritoneum for some distance into the medullary substance. By the development of the cells forming these peritoneal tubules they become transformed into solid cords containing primitive sex cells and other elements similar to the peritoneal cells of which these invaginations were originally composed. The latter are destined, in part at least, to form the granulosa. In the manner above described, these hollow tubules become transformed into solid cords of Pflüger, in every way homologous with the cords laid down at an earlier period of development. In still later stages, the cords of Pflüger are found to have widened, branched, and anastomosed to such an extent as to form an almost unbroken cortical zone, through which plates of connective tissue extend in a radial direction, marking out the original limits of the cords. Their inner ends are broken up to form nests of cells which become surrounded by layers of the invading stroma. In this manner are formed the follicles with their connective tissue theca, the oocyte and granulosa cells being derived from the cords of Pflüger.


Kolliker, 98, Mihalkovics, 85, Eouget, 79, Biihler, 94, hold the view that the granulosa cells are derived from the medullary cords. Xone of the above named authors subjected this question to a critical study of numerous stages in any species of animal; but studied isolated and more or less mature stages. Wini-^'arter, oo, Balfour, 78, Coert, 98, Nagel, 99, and many others hold, on the contrary, that the granulosa cells arise from the cords of Pflliger.


2. Medullary Cords

The medullary cords which we found to be at a standstill in development at the time of their separation from the peritoneum develop into structures in all regards similar to the cords of Pfiiiger. Although homologous with the seminiferous tubules, they are distinctly female in character.


There is a constant degeneration of follicles in these medullary cords and in the deeper portions of the cortex as well. This results in the complete destruction of the sex cells in the former before the follicles have developed far enough to possess more than a single layer of granulosa cells. The few such young follicles found in the medullary cords and inner portions of the cords of Pfiiiger of the 13 cm. embryo, are found to have disappeared in the 15 cm. stage, leaving small clumps of more or less elongated granulosa cells enclosed in the membrana propria and connective tissue investment that was previously formed about them. These clumps remain throughout later stages, the persistent granulosa cells taking on more or less the appearance of connective tissue.


The fate of the medullary cords is quite similar in the rabbit. It will be more explicitly dealt with in connection with the rete tubules. Suffice it to say that the sex cells never pass beyond the stage of synapsis characteristic of young oocytes in a certain early stage of development, the few simple follicles that make their appearance lieing destined to degenerate as in the pig.

3. Rete Tubules

The subject of the rete tubules of the ovary is one of the most interesting of the whole account. i\.s previously stated, they contain primitive sex cells during the early stages in the development of both male and female. These are present in both the extra- and intraovarian portions. In that part of the rete lying within the sex gland, they increase in number during the 4 cm. and 5 cm. stages of the pig, becoming much more numerous than in that portion lying within the mesonephros. The proximity of any given portion of the rete tissue to the sex gland appears to condition the relative number of primitive sex cells found in it. The rete tubules of the ovary are at all times devoid of a lumen, and the intra-ovarian portions take on more and more the appearance of the medullary cords and cords of Pflliger. This similarity becomes very evident in the 13 cm. stage, at which period the intra-ovarian jDortions of the rete are almost wholly composed of primitive sex cells and of smaller cells in every regard identical in kind with the follicular cells. Strands of stroma tissue serve to separate the rete tubules from one another.


Later -stages show the intra-ovarian portion. of the rete tissue to become constricted off from the extra-ovarian (mesonephric portion). The primitive sex cells in the former continue to develop until in the 18 cm. embryo there are found t3'pical follicles (Plate VI, Fig. 20), each with its oocyte and a single layer of granulosa cells. These oogonia and oocytes are short-lived, however, being already in process of degeneration, resulting in their total destruction before the 20 cm. stage, where all traces of the sex cells have disappeared from the rete tissue, both in the ovary and in the mesonephros. The only trace of rete tissue found in the ovary at this time consists of clumps of connective tissue and elongated modified granulosa cells. These resemble the vestiges remaining after the degeneration of the medullary cords and of the inner follicles of the cords of Pflüger.


In the rabbit, the process is not so striking, the sex cells having disappeared from the rete tissue in the 17-day embryo, long before there has been any trace of follicle formation. The rete tissue is bunched at the anterior end of the ovary in contact with the medullary cords. Coert, 98, states that the rete extends by no means so far distally in the ovary as it does in the testis. I have also observed this fact in the rabbit, while in the pig it is very marked as already shown. The close resemblance between the cells of the rete and medullary cords makes it difficult and, in later stages, impossible to distinguish between them. The only criterion is the presence in the latter of scattered sex cells, these being absent from the rete tubules after the 17th day of embryonic life. This distinction, however, is quite unreliable.


The subsequent history of the mass of tissue formed by the union of the rete and medullary cords is an uneventful one. After the primitive sex cells of the medullary cords degenerate in the young rabbit, 17 days after birth, the rete and medullary cords are seen as slender strands, lying in the dense stroma between the lymph spaces of the medullary portion of the ovary (Plate VII, Fig. 26). Their nuclei often become columnar through the pressure exerted upon them. These rete-medullary cord rudiments have now reached a period of quiescence in Avhich they are remarkably persistent, remaining until after the animal has passed the stage of puberty. In both pig and rabbit, they persist as vestigial structures, playing absolutely no further role in the development of the sex gland.

4. Interstitial Cells and Stroma

The first generation of interstitial cells in the pig ovary appears in the 2.5 cm. emhryo. They divide by mitosis, but are on the whole short-lived, disappearing in the stage of 4 cm. The stroma consists of fibrous connective tissue filling in all the space between the remaining structures, and forming a very important element of the ovary. No interstitial cells are found in the rabbit ovary until the stage of 45 days after birth, when a few cells are to be found, which can unmistakably be assigned to this class. Their presence is associated with the degeneration of certain follicles in which a theca interna has developed from the stroma investment. Such a theca interna is not formed until the follicle has acquired about three layers of granulosa cells and the rudiment of a follicular cavity. Their development can be best understood in the ovary of the 85-day rabbit. Fully-formed follicles at this stage are seen to be surrounded by a connective tissue investment which consists of an inner layer of modified cells - theca interna - and an outer layer of ordinary connective tissue cells. All transition forms between these two kinds exist (Plate VII, Fig. 26), showing that the cells of the theca interna have originated from the general stroma by a process of transformation in which the cell body becomes cylindrical instead of fibrillar, the cytoplasm becomes clearer, the nucleus larger and spherical, and a centrosome makes its appearance. In realit}^, the theca interna is separated from the follicle by a thin layer (follicular capsule) of attenuated connective tissue cells which send fibres in a diagonal direction through the theca interna to the theca externa. This arrangement has been previously described l)y a number of authors, Paladino, 87, Clarke, 98, and Rabl, 98.


Very many of these follicles are degenerating at this stage. As soon as the granulosa cells have begun to degenerate by chromatolysis, those of the theca interna begin to enlarge slightly. A few cells from the innermost fibrous layer (follicular capsule) and from the theca interna break through the basement membrane and enter the cavity of the follicle. The theca interna derivatives undergo fatty degeneration, eventually disappearing together with the granulosa cells. The only elements that ultimately persist in the mass of degenerating cells enclosed by the follicular capsule are the thin connective tissue cells that have become separated from the capsule. These remain unaltered after all the cells that have migrated from the theca interna have disappeared by fatty degeneration and the granulosa cells by chromatolysis.


During the degeneration of the cells enclosed by. the follicular capsule, the cells forming the latter join to form a thick densely-staining membrane which contracts and thiq]vens as degeneration proceeds. It persists after all the granulosa and inner theca cells and even the ovum have entirely disappeared. It shows a more or less fibrous structure in these later stages, finall}' disappearing without leaving a trace. The above described closing-in of the follicular capsule stretches the connective tissue strands joining the capsule with the theca externa. In this manner, the cells of the intermediate theca interna are arranged in radiating columns. These develop into the interstitial cells to be described later. This view has already been advanced by Schottlaender, 91, Clarke, 98, Plato, 97, Limon, 02, and a number of others.


At a stage immediately succeeding the disappearance of granulosa cells and ovum, the nuclei of the still undeveloped interstitial cells become amoeboid and then undergo a rapid process of amitotic division (Plate VII, Fig. 27). This is not described in any of the literature, although Dr. Frank E. Lillie and Dr. C. M. Child of this laboratory both inform me that they have noted the same phenomenon.


Van der Stricht, 01, finds that in the formation of the corpora lutea of the bat (Vespertilio) the cells of the theca interna having taken on the form of lutein cells, divide by mitosis for a short period, after which division ceases entirely. Sobotta, 96, also has found scattered mitotic figures in the theca interna of the rabbit after discharge of the ovum, although he does not ascribe to this layer the formation of the lutein cells. Eabl, 98, finds them to divide before the beginning of atresia, not after that process has set in.


After this process of amitotic division is completed the interstitial cells rapidly increase in size and finally develop into the mature form in which the cytoplasmic body is voluminous, clearly bounded, and stuffed full of fat globules; the nucleus is enlarged and roimded; and a welldefined centrosphere and centrosome have appeared. These interstitial cells are similar to the lutein cells of the corpora lutea in all regards save size. Even this criterion is not a safe one by which to distinguish the two sets of elements. Certain interstitial cells of the ovary of a 6-months' old virgin, having become separated from the mass and lying free in the loose stroma, are found to have enlarged to the dimensions of the smaller lutein cells of the corpora lutea.


Although the work of Sobotta, 96, Honore, 00, and others has led them to assert that the lutein cells of the corpora lutea originate solely from the granulosa cells of the discharged follicle, there are a large number of workers who hold the view that they originate solely from the cells of the theca interna. Among such authors may be mentioned Clarke, 98, Van Beneden, 80, and Kolliker, 98. Van der Stricht, 01, and Schulin, 81, consider them to arise from both sources. This question does not properly come within the limits of this work, yet I cannot refrain from pointing out the very close resemblance between the interstitial and lutein cells (Plate VII, Figs. 29 and 30). It seems quite improbable that two groups of cells, almost identical as these are, could have arisen from such diverse elements as the connective tissue cells of the theca interna, on the one hand, and the granulosa cells on the other.


Successive generations of lutein and interstitial cells push the earlierformed groups of cells toward the center of the ovary, where they undergo hyalin degeneration. De Sinety, 77, finds in the human subject that the number of atretic follicles is greater during pregnancy than at other times. I cannot substantiate this; but am inclined to consider pregnancy to make no difference in their number in the rabbit. This view I can support by a number of ovaries, taken from immature rabbits, mature virgins, immature virgins, pregnant animals and one taken from a rabbit that had borne young but had been isolated for three months.


4. Primitive Sex Cells. - The primitive sex cells occur from the earliest stages studied (pig, 6 mm. length; rabbit, 13-day embryo) on through all later stages of development. Similar cells have been found in the earliest stages of the Elasmobranchs by Beard, 00, 02, 03, Eabl, 98, Woods, 02, and in the Teleost by Eigenmann, 91. I have myself found the large yolk-filled primitive sex cells of these authors in turtle embryos (Trionyx) and may say that I am now at work upon this subject. It is too early to give results, but it may be stated with certainty that these cells occur in the embryo of 3 mm. length and are seen to be apparently migrating from the entoderm through the splanchnopleuric mesoderm to the point where the latter joins the somatopleuric mesoderm. It is here that the sex glands are to form. This observation corresponds with that of Beard, 03.


The nuclei of such cells are larger than those of the entoderm and mesoderm among which they lie, but resemble them at this stage in the fact that they show very little chromatin material as contrasted with the very pronounced chromatin network which they show in later stages (1.5 cm. embryo). In this stage the large yolk spherules are found to be breaking up into small granules which remain in one or two clumps in the cytoplasm. These cells at this stage show a very marked resemblance to the primitive sex cells of the pig and rabbit. In fact I have no hesitation in identifying them as their homologues. In the pig and rabbit they are found not only in the genital ridge, but outside of it as well, in the earlier stages. So far as my own work goes, I have found them in the mesentery of the alimentary canal. Eigenmann, 91, found them even in the brain region of the young Teleost (Micrometrus). There seems to be no doubt of their sexual character, because they are present in such great numbers in the sex glands, and have all the characteristics of those cells (spermatogonia and oogonia) from which the sex products eventually form.


It has been shown in the pig and rabbit, however, that these sex cells, appearing in the indifferent stages, do not contribute to the formation of functional sex products in the ovary. The same is probably true of the testis, it being at least certain that the great bulk of the spermatogonia are formed from the germinative cells - cells derived originally from the peritoneum and maintaining, first, their indifferent character at least so far as our technique is able to show. Schonfeld, oi, and Loisel, oo, have shown, the former in the case of mammals, the latter in birds, that spermatogonia and Sertoli i an cells arise during adult life from these germinative cells (indifferent cells of Schonfeld). What interpretation shall we then put upon the so-called primitive sex cells (primitive ova, XJreier, Urkeimzellen, etc.) ? I consider them to be spermatogonia in the testis and oogonia in the ovary. They have almost reached that degree of specialization at which we might call them oocytes and spermatocytes. The fact that they are still found in process of mitotic divisions excludes them from these latter classes of cells. They should more properly be termed spermatogonia and oogonia of the second order, in accordance with the use of these terms by Loisel in the case of birds and by many authors who have written upon the subject of the sex cells of invertebrates.


These primitive sex cells found in the early stages of embryonic life have, then, undergone a process of precocious development; but for some reason, this process is not carried beyond a certain point. The stimuli or favorable conditions that brought about the formation of these secondary spermatogonia in regions outside of the sex gland, may be later present only in the sex gland itself, or indeed, only in certain parts of it. The influence exerted by the sex gland in bringing about the development of the sex cells is beautifully illustrated by the process above described by which egg follicles and spermatogonia develop in large numbers in the rete tubules lying within the sex gland, while they deN-elop sparingly in those parts lying within the mesonephros. In such cases as we have seen, the degree of development and number of sex cells forming in any given region of the rete tissue is dependent upon the proximity of that region to the sex gland. It must be clearly understood that I do not deny the possibility of an early specialization and segregation of the sex cells as claimed by Nussbaum, 80, Eabl, 96, Beard, oo, 02, 03, Eigenmanu, 91, Woods, 02, and others; but I do assert that the proof of such an assertion has not yet been furnished. The development of the sex cells must be followed from the earliest embryonic stages to the period of sexual maturit}^, before one can prove that the cells under consideration are the only ones that give rise to the sex products, or that they give rise to them at all.


5. MoRPHOLOC4iCAL Eelationships. - The morphological significance of the sex gland structures may be expressed in terms of the biogenetic law somewhat as follows : The genital ridge represents a primitive condition in which the sex gland extended along the entire length of the mesonephros. In this sex gland, the gonads appeared in the form of tubules or vesicles opening into the body-cavity in the case of both ovary and testis. It was impossible, in this piece of work, to determine whether there is any segmental arrangement of the sex cords in the forms studied. Such, however, would most probably be the condition in the primitive t^-pe.


The evaginations from the Malpighian corpuscles no doubt represent the " segmental strange," " genital kanale," etc., of those authors who have worked upon the development of the sex glands in the Anamnia. In the Amphibia, for instance, they have a truly segmental arrangement, according to Hoffman, 87, Spengel, 76, Semon, 92, while Hoffman, 89, Braun, 77, Mihalkovics, 85, Semon, 87, Weldon, 85, show a like segmental arrangement to occur in the Eeptilia. Much might be said in this connection, as the literature upon the subject is quite rich in suggestive facts.


According to Shreiner, there are 2 to 3 glomeruli in each somite of the pig. My own work has shown that a connection takes place between the rete tissue and those glomeruli with which it comes in contact, there may be as many as three evaginations called forth from a single glomerulus or there may be none at all. For these reasons we cannot assert a strict homology between these evaginations and the " segmental strange " of the Anamnia, yet the former probably represent a modification of the same process, as that by which the formation of the latter are formed.


Returning to the subject of the genital ridge as a whole, it would seem fair to conclude that the sex cords which in the ancestral forms lay posterior to the present limits of the sex gland, have disappeared. The rete tubtiles would represent sex cords anterior to the sex gland that had retained more or less of their ancestral condition, but have become modified to meet the requirements of their function as efferent ducts for the male sex products. The union of the rete tubules with the sex cords is more intimate in the male than in the female, owing to the fact that the connection is not, and probably never was, of functional importance in the latter.


One is struck with the fact that there is a complete, or almost com})lete, separation of the medullary cords of the ovary from the peritoneum at almost the same time that the seminiferous tubules of the testis break away from it. This separation takes place essentially in the same manner in both sexes. Coert, 98, considers the medullary cords to represent a system of ducts which served in phylogenetically earlier periods, to carry the female sex products to the Wolffian duct. Waldeyer, 70, considers them to represent vestigial seminiferous tubules arising from the mesonephros. Paladino, 87, and Harz, 83, confound them in part with the long rows of interstitial cells of the adult animal. One might assume that there was one stage in the jjhylogenetic history of the sex glands in which both medullary cords and seminiferous tubules furnished sex products that were conducted to the rete efferentia through the rete tubules. If one hold this view, he must grant that the formation of the cords of Pfl tiger or second generation of ovarian cords represents a return to the primitive condition in which the female sex products are again discharged into the body-cavity from the surface of the sex gland. This view would hardly seem to be a reasonable one, hence I, at least, would prefer to consider the cords of Pflüger to be mere interrupted continuations of the medullary cords. Winiwarter, 00, holds a view very similar to the last, his well-known diagram practically expressing my own conception of the process. There is no exact correspondence in number between the cords of Pflüger and medullary cords, the former being much more numerous that the latter.


The medullary cords never assume the characteristics of seminiferous tubules.


The assumption that the separation of the medullary cords from the peritoneum is not and never was phylogenetically of functional importance, leads to many interesting questions pertaining to the influence of heredity in the transmission of sexual characters. Is it possible that developmental processes of functional importance to the testis alone must be transmitted to the female or vice versa, simply because the germ cells which have united to produce the embryo transmit tendencies to the formation of both male and female, both of which assert their power, though one be ever so feeble? In this connection it is of great interest to note that Laulaine, 86, has found that the peritoneum of the 7 to 8 day chick testis thickens after the separation as though it were about to develop along the line of the ovary. This is merely temporary, as it again thins out and loecomes relatively unimportant in later stages. In any case, there is a remarkably close correspondence between the general processes of development of the testis and ovary.


I wish to express my deep obligations to my professor. Dr. Frank K. Lillie, for constant guidance and valuable assistance throughout the course of this work.


VI. Summary and Conclusions

1. The sex glands and rete originate from the genital ridge composed of the thickened mesonephric peritoneum and underlying tissue, proliferated from it.


2. The testis is composed of (A) the seminiferous tubules, (B) stroma.


A. The seminiferous tubules are formed as solid invaginations of the peritoneum, which later became separated from it, and undergo subsequent growth by the activity of the component cells. These are of two kinds, (1) primitive sex cells, spermatogonia of the second order; and (2) germinative cells. Intermediate forms connect these two kinds and may be interpreted as the primitive cells from which both varieties origirate. They occur' up to a certain stage in development, and may possibly recur periodically in adult stages.


B. The stroma consists of (1) loose connective tissue, (2) the albuginea - formed from the cells comprising the proximal portions of the seminiferous tubules together with possible additions of other connective tissue from the stroma, (3) interstitial cells formed from the stroma. These are formed contemporaneously with the appearance of fatty degeneration in both peritoneum and seminiferous tubules.


3. The ovary is made up of homologous groups of structures.

A. The medullary cords and cords of Pflüger are both derived by invagination of the peritoneum, the former being in all regards homologous with the seminiferous tubules. The cords of Pflüger are invaginations of the peritoneum, formed after the medullary cords have become separated from it. Both medullary cords and cords of Pflüger contain oogonia and follicle cells. Follicles formed in the medullary cords are never functional and cease to form in later stages. They degenerate together with other young follicles of the inner ends of the cords of Pflüger. Both medullary cords and cords of Pflüger contain (1) primitive sex cells (oogonia) ; (2) follicular cells - probably homologous with the germinative cells of the seminiferous tubules; while intermediate forms of cells are found in the peripheral part of the ovary.


B. The stroma consists of (1) loose connective tissue, from which are derived the theca interna and theca externa of tlie follicles; (2) a zone homologous with the albuginea, but of a loose consistency, which renders it indistinguishable from the remainder of the stroma; (3) interstitial cells homologous with those in the testis. In the pig ovary, these interstitial cells appear very sparingly, at the same time that they appear in the testis, but very soon disappear. They develop in later stages in both pig and rabbit ovary. Details of this phenomenon were studied only in the rabbit, in which animal the cells in question were first found 45 days after birth. They are formed from the cells of the theca interna in response to conditions created by the degeneration of the follicles about which they lie. Many striking points of similarity link these interstitial cells with the lutein cells of the corpora lutea. Both finally sufi'er hyalin degeneration in the interior of the ovary.


4. Rete tubules are formed in connection with both testis and ovary, A. The tubules forming the rete ovarii and rete testis originate in the region of the genital ridge anterior to their respective sex glands. They are homologous with the sex cords which they also reseml^le in the fact that they contain primitive sex cells. In the testis they form a core surrounded more or less completely by the seminiferous tubules, and extend almost the entire length of the -organ. They project but a short distance from the hilum into the ovary. In both sexes they are connected with certain glomeruli of the anterior part of the mesonephros, by means of more or less vestigial evaginations from the capsules of Bowman.


B. Each cord of the rete testis acquires a lumen and sends numerous side branches (tubuli recti) to the seminiferous tubules, from which they differ only in diameter and in the relative number of primitive sex cells contained in them. This similarity is in later stages confined to tliose portions, of the rete tubules that lie within the testis. These finally undergo modification in form and lose their sex cells by degeneration.


C. Homologous relations exist between the -rete ovarii and the medullary cords. The rete ovarii never acquire a lumen, remaining solid, like the medullary cords which they greatly resemble. So great is this resemblance in the case of the rabbit, that the two structures cannot be distinguished from one another in post-embryonic stages. The slender strands of these indistinguishable elements persist in a quiescent state in the ovary of the adult. In the pig, numerous oogonia appear in the intra-ovarian rete tubules, many of them even developing into young follicles, all of Avhich degenerate before birth, leaving masses of follicular cells similar in all regards to the remains of the medullary cords.

Conclusions

  1. The sex cords - medullary cords and seminiferous tubules - are homologous structures formed as tubular invaginations of the peritoneum.
  2. The cords of Pflüger are formed from the same source and in the same manner as the sex cords, but at a slightly later period of time.
  3. The rete cords are formed at the same time as the sex cords and in the same manner.
  4. The connective tissue of the sex-gland - stroma and albuginea - is derived from the peritoneum.
  5. The interstitial cells of both ovary and testis are formed from connective tissue in reference to a process of degeneration occurring in the sex gland.
  6. The primitive sex cells seen in the earliest stages are precociously developed oogonia or spermatogonia of the second order, similar cells developing during later stages, from apparently undifferentiated peritoneal cells.
  7. The sex glands exert a specific influence, causing follicles to form in the intra-ovarian rete of the pig, and bringing about the development of spermatogonia in the intra-testicular rete of both pig and rabbit. Such sex elements are not functional, because of the fact that they suffer early degeneration.

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Explanation of Plates

All figures except No. 1, were outlined with the aid of a camera lucida.

al., albuginea. m. r., mesenteric ridge.

c, blood corpuscle. nil., nucleus.

c. B., capsule of Bowman. oc, oocyte.

cen., centrosphere. p., peritoneum.

c. P., cord of Pflüger. p. s., primitive sex-cell.

c. p., epithelial plate. r., rete.

ev., evagination of capsule of r c, rete cord.

Bowman. r. t., rete tubule.

f. c, follicular capsule. s. c, sex cord, gr., glomerulus. st., stroma.


ger., germinative cell. s. tu., seminiferous tubule.


gr., granulosa cell. s. g., sex gland.


i. c, interstitial cell. t., testis.


m., mesonephros. t. r., tubulus rectus.


m. f., mesenteric fundament. th. i., theca interna.


m. p., membrana propria. th. e., theca externa.


Plate I

Fig. 1. Reconstruction of the anterior end of the left mesonephros. Pig embryo, length 12.5 cm.


Fig. 2. Fundament of the rete ridge. Pig embryo, length 0.7 cm. Transverse section, x 893.


Fig. 3a. Fundament of sex gland. Pig embryo, length 0.7 cm. Transverse section, x §93.


Fig. 3b. Same as 3a, showing primitive sex cell in portion of peritoneum. X893.


Fig. 4. Fundament of sex gland. Pig embryo, length 1.0 cm. Transverse section, x 893.

Plate II

Fig. 5. Fundament of sex gland. Pig embryo, length 1.25 cm. Transverse section, x ^08.


Fig. 6. Fundament of rete. Pig embryo, length 1.4 cm. Transverse section. X893.


Fig. 7. Fundament of sex gland and mesentery. Pig embryo, length 1.4 cm. Transverse section, x 893.


Plate III

Fig. 8. Fundament of sex gland (peripheral portion). Pig embryo, length 1.6 cm. Transverse section, x 608.


Fig. 9. Fundament of sex gland (peripheral portion). Pig embryo, length 1.7 cm. Transverse section, x 893.


Fig. 10. Medullary cord of ovary. Pig embryo, length 5 cm. x 893.

Plate IV

Fig. 11. Rete cord in rete ridge. Pig embryo, length 1.8 cm. x 893.


Fig. 12. Seminiferous tubules and stroma of testis. Pig embryo, length 1.8 cm. X S93.


Fig. 13. Seminiferous tubule and stroma of testis (transverse section). Pig embryo, length 3 cm. x 893.


Fig. 14. Seminiferous tubule of testis (longitudinal section). Pig embryo, length 7.5 cm. x 893.

Plate V

Fig. 15. Capsule of Bowman and rete cords. Pig embryo, length 4 cm. X 893.


Fig. 16. Capsule of Bowman and rete cords. Pig embryo, length 7.5 cm.


Fig. 17. Intra-ovarian portions of rete cords. Pig embryo, length 8.5 cm. X893.


Fig. 18. Tubulus rectus and seminiferous tubule. Pig embryo, length 13 cm. X 893.

Plate VI

Fig. 19. Seminiferous tubule. Pig embryo, length 15 cm. x 893.


Fig. 20. Follicle in intra-ovarian rete. Pig embryo, length 15 cm. X 893.


Fig. 21. Rete tubule of testis. Pig embryo, length 13 cm. x 893.


Fig. 22. Portion of cortex of ovary. Rabbit embryo, 23 days. X 893.


Fig. 23. Seminiferous tubule (longitudinal section). Rabbit embryo, 21 days. X 893.


Fig. 24. Seminiferous tubule (transverse section). Rabbit, 24 days after birth. X 893.

Plate VII

Fig. 25. Cord of Pflüger, with lumen. Pig embryo, length 15 cm. x 893.


Fig. 26. Tissues of a rabbit ovary. 78 days after birth, x 893.


Fig. 27. Interstitial cells in process of amitotic division. Rabbit ovary, 93 days after birth, x 893.


Fig. 28. Interstitial cell of ovary. Virgin rabbit, 6 months old. x 893.


Fig. 29. Interstitial cell of ovary. Rabbit in 14i/^ day of first pregnancy. X893.


Fig. 30. Lutein cell of corpus luteum, same animal as for Fig. 29. X 893.

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