Book - The development of the chick (1919) 13
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Part II The Forth Day to Hatching, Organogeny, Development of the Organs
Chapter XIII The Urinogenital System
The history of the pronephros and the origin of the mesonephros have been already described (Chap. VI). We have now to consider (1) the later history of the mesonephros, (2) the development of the metanephros or permanent kidney, (3) the development of the reproductive organs and their ducts, and (4) the development of the suprarenals. All these organs form an embryological unit, by virtue of their mode of origin and their interrelations. Thus we find that the intermediate cell-mass is significant for the development of all: its growth causes the formation of the Wolffian body, on the median face of which the gonads arise. The secreting tubules and renal corpuscles of the permanent kidney are also derivatives of the intermediate cell-mass. The Wolffian duct is derived from the same source, and by change of function becomes the vas deferens, after functioning for a while as the excretory duct of the mesonephros. Certain parts of the mesonephros also enter into the construction of the testis. And the Miillerian duct, which forms the oviduct of the female, is derived from the epithelium covering the Wolffian body.
I. The Later History of the Mesonephros
In Chapter VI we traced the origin of the nephrogenous tissue, and the differentiation of the first mesonephric tubules within it. We saw that in each of the segments concerned a number of balls of cells arises by condensation within the nephrogenous tissue, and that these become converted into vesicles. We saw also that each vesicle sends out a tubular sprout from its lateral side to the Wolffian duct, with which it unites; and that its median face becomes converted into a renal corpuscle. These processes take place sucessively in antero-posterior order within the somites concerned, so that a series of stages in the development of the tubules may be studied in the same embryo. Moreover, all the tubules of a given somite do not develop simultaneously: primary tubules are formed in each somite from the most ventral portion of the nephrogenous tissue; then secondar}tubules later from an intermediate portion, and tertiary tubules later yet from the dorsal portion.
Fig. 217 represents a transverse section through the middle of the Wolffian body at the stage of ninety-six hours, showing a primary, secondary, and tertiary tubule. The primary tubule is typically differentiated; the secondary has formed the secreting tubule and the rudiment of the renal corpuscle, but the tubule does not yet open into the Wolffian duct, though it is connected with it; the tertiary tubule is still in the vesicular stage. Some undifferentiated nephrogenous tissue remains above the rudiment of the tertiary tubule, which makes it possible that quarternarv tubules mav be formed later.
Fig. 217. — Transverse section through the middle of the Wolffian body of a chick embryo of 96 hours.
Ao., Aorta. Coel., Coelome. Col. T., Collecting tubule. Glom., Glomerulus, germ. Ep., Germinal epithelium. M's't., Mesentery, n. t., Nephrogenous tissue. T. 1,2, 3, Primary, secondary, and tertiary mesonephric tubules. V. c. p., Posterior cardinal vein. W. D., Wolffian duct.
Referring still to the same figure, it will be noted that the Wolffian duct itself has formed a considerable evagination dorsomedially (collecting tubule), with which both secondary and tertiary tubules are associated as well as the undifferentiated nephrogenous tissue. Similar evaginations are formed along the entire length of the functional portion of the mesonephros.
Figs. 218 and 219 illustrate the form of these evaginations in duck embr3^os of 40 and 50 somites respectively, as they appear in reconstructions of the posterior portion of the mesonephros.
It will be seen that they gradually form sacs opening into the Wolffian duct. Subsequently, by elongating, these sacs form collecting tubules that gather up the secretions of the mesonephric tubules proper and conduct them to the Wolffian duct. These conducting tubules are stated to branch more or less; it is also said that they are more highly developed in the duck than in the chick. Felix proposes to call them mesonephric ureters.
In the case of the secondary and tertiary tubules, three parts may be distinguished : parts one and two (derived from the nephrogenous tissue) I;. o\C ^rc the renal corpuscle and secreting tubule respectively; the third part is the collecting tubule derived by evagination from the Wolffian duct. In the case of the primary tubules, a conducting part appears to be formed secondarily, though in what way is not clear.
The formation of new tubules ceases on the fifth day, all the nephrogenous tissue being then used up. Up to the eighth day at least the tubules grow rapidly in length and become more differentiated. The result is a relatively enormous protrusion into the bodv-cavity on each side of the dorsal mesentery. Degeneration of the tubules sets in about the tenth or eleventh days, and the tissue is gradually absorbed; this process extends over the whole of the latter period of incubation, and is completed at hatching. Parts, however, remain in the male in connection with the testis; non-functional remnants may also be detected in the female (p. 401). It is difficult to state the exact period of beginning and cessation of function of the mesonephric tubules. Judging from the histological appearances, however, it is probable that secretion begins in the tubules on the fifth day and increases in amount up to the eleventh day at least, when signs of degeneration become numerous. Presumably the functional activity diminishes from this stage on, being replaced by the secretion of the permanent kidney.
Fig. 219. — Profile reconstruction of part of the mesonephros and diverticulum of the ureter of a duck embryo of 50 somites. (After Schreiner.)
CI., Cloaca. Int., Intestine. Mn. T., Meso nephric tubules, n. T., Nephrogenous tissue. Ur Ureter W. D.. Wolffian duct. XXXII, XXXIII, XXXIV, Somites of the same number.
Fig. 218 — Profile reconstruction of part of the Wolffian duct and primordia of mesonephric tubules (represented by circles) of a duck embryo of 45 somites. (After Schreiner.) YXTV XXV etc., position of the correspondmg somites. Lines 114 A,
114 B 114 C ^represent the positions of the sections shown in these figures.
Fig. 220. — Transverse section through the mesonephros and neighboring parts of a 6-day chick, in the region of the spleen.
Ao., Aorta, bl. V., Blood vessels (sinusoids). Caps., Capsule of renal corpuscle. Coel., Coelome. col. T., Collecting tubule. D., Dorsal. Giz., Gizzard. Glom., Glomerulus. Gon., Gonad. L., Left. Spl., Spleen. Sr. C, Cortical substance of the suprarenal, s. t., Secreting tubule. T. R., Tubal rid^e. V., Ventral. V. c. p., Posterior cardinal vein V. s'c. 1., Left subcardinal vein. W. D., Wolffian duct.
Figs. 220 and 221 represent sections through the mesonephros on the sixth and eighth days respectively (see also Fig. 222, eleven days). The renal corpuscles show the typical capsule and glomerulus, and relation to the secreting tubules. The latter are considerably convoluted on the sixth day, much more so on the eighth day. The conducting tubules can usually be distinguished by their smaller caliber and thinner walls. The Wolffian duct is situated near the dorso-lateral edge of the mesonephros, and the opening of a collecting tubule into it is shown in Figure 220. The renal corpuscles are situated next the median face of the Wolffian body. The space between the tubules is occupied almost entirely by a wide vascular network of sinusoidal character; that is, the endothelial walls of the vessels are moulded directly on the basement membrane of the tubules without any intervening connective tissue. The circulation is described in the chapter on the vascular system.
Fig. 221. — Transverse section through the metanephros, mesonephros, gonads and neighboring parts of an 8-day chick, bl. v., Blood vessels (sinusoids). B. W., Body-wall. col. T. M't'n., Collecting tubules of the metanephros. M. D., Miillerian duct. M's't., Mesentery, n. t. i. z., Inner zone of nephrogenous tissue (metanephric). n. t. o. z., Outer zone of the nephrogenous tissue. Symp. Gn., Sympathetic o^anghon of the twenty-first spinal ganglion. V. C, Centrum of vertebra. Other abbreviations as before.
II. The Development of the Metaxephros or Permanent Kidney
The metanephros or permanent kidney supplants the mesonephros in the course of development. It is derived from two distinct embryonic primordial (1) the nephrogenous tissue of the two or three posterior somites of the trunk (31 or 32 to 33), which furnish the material out of which the renal corjxiscles and secreting tubules develop; and (2) a diverticulum of the posterior portion of the Wolffian duct (Fig. 219), which develops by branching into the collecting tubules and definitive ureter. The development of the kidney takes place in a mass of mesenchyme, known as the outer zone of the metanephrogenous tissue, that furnishes the capsule and connective tissue elements of the definitive kidney, in which also the vascular supply is developed (Figs. 221 and 222). The cortical tubules of the kidney are thus derived mainly from the nephrogenous tissue, and the medullary tubules and ureter from the metanephric diverticulum.
Thus the definitive kidney is analogous in mode of development to the mesonephros, and is best interpreted as its serial homologue. This point of view may be regarded as definitely established by the work of Schreiner, to which the reader is referred for a full account of the history of the subject.
The metanephric diverticulum, or primordium of the ureter and collecting tubules, arises about the end of the fourth da}^ as a rather broad diverticulum of the Wolffian duct at the convexity of its terminal bend to the cloaca (Fig. 219). It grows out dorsally, forming a little sac, which, however, soon begins to grow forward median to the posterior cardinal vein and dorsal to the mesonephros (Fig. 224); by the end of the fifth day its anterior end has reached the level of the csecal appendages of the intestine, and on the eighth day its anterior end has reached its definitive position at the level of the vena cava inferior, near to the anterior end of the mesonephros (twenty-first definitive somite or twenty-fifth of the entire series; cf. Fig. 150).
It should be noted that the metanephric diverticulum is similar in its mode of origin to the so-called mesonephric ureters. It may in fact be regarded as the posterior member of this series, but it is separated from those that form the collecting tubules of the mesonephros by at least two somites in which no diverticula of the mesonephros are formed (Fig. 219). During its growth forward a series of small diverticula arise from its wall and extend dorsally (Fig. 223); these branch secondarily in a generally dichotmoiis manner, and it is from them that the collecting tubules of the kidney arise; the posterior unbranched portion of the metanephric diverticulum represents the definitive ureter.
Fig. 222. -Transverse section through the metanephros, mesonephros gonads and neighboring structures of an 11-day male chick, a. A. S., Abdominal air-.sac. Ao., Aorta B W Rndv wall r^^I n duct. Mst., .Mesentery. M't'n., Metanephros. Sp., Spine of neural areh
W DwoMandScrotr Vna ca"™ Merioiman duct. Other abbreviations as before.
Fig. 223. — Profile reconstruction of the Wolffian duct and primordium of the metanephros of a chick embryo of 6 days and 8 hours. (After Schreiner.)
XXV to XXXIH, twenty-fifth to thirty-third somites. Al. N., Neck of allantois. CI., Cloaca. Int., Intestine. M's'n., Mesonephros. n. T., Nephroojenous tissue of the metanephros included within the dotted lines. W. D., Wolffian duct. Ur., Ureter.
The following data concerning these branches should be noted:
- the first ones are formed from the posterior portion of the metanephric diverticulum, and the process progresses in an anterior direction. This is the reverse direction of the usual order of embryonic differentiation, but the reason for the order is the same, viz., that differentiation begins in the first formed parts.
- A posterior, smaller group of collecting tubules is separated at first by an unbranched portion of the ureter from an anterior larger group (Fig. 223). The unbranched region corresponds to the position of the umbilical arteries which cross here. (3) During the fifth and sixth days the terminal portion of the Wolffian duct common to both mesonephros and metanephros is gradually drawn into the cloaca, and thus the ureter obtains an opening into the cloaca independent of the Wolffian duct and posterior to it (Fig. 223).
The Nephrogenous Tissue of the Metanephros. The nephrogenous tissue of the thirty-first, thirty-second, and thirty-third somites is at first continuous with the mesonephros (Figs. 218 and 219), but on the fourth and fifth da3^s that portion situated immediately behind the mesonephros degenerates, thus leading to a complete separation of the most posterior portion situated in the neighborhood of the metanephric diverticulum. This constitutes the metanephrogenous tissue proper (inner zone). It is important to understand thoroughly its relations to the metanephric diverticulum. This is indicated in Fig. 219, which represents a graphic reconstruction of these parts in a duck embryo of 50 somites. It will be seen that the metanephrogenous tissue covers nearly the entire metanephric diverticulum; a transverse section (Fig. 224) shows that it lies on its median side. The outer dotted line (Fig. 219) gives the contour of a dense portion of mesenchyme related to the diverticulum and nephrogenous tissue proper. In section this forms a rather ill-defined area shading into the nephrogenous tissue on the one hand and into the surrounding mesenchyme on the other.
Fig. 224 shows the relations of the three constituent elements of the kidney at the end of the fifth day, as seen in a transverse section. The metanephric diverticulum lies on the median side of the cardinal vein, and is in contact, on its median face, with the proper nephrogenous tissue (inner zone); the latter shades into the outer zone, the cells of which are arranged concentrically with reference to the other parts. The relations subsequently established may be summarized in a few Avords; the inner zone of tissue grows and branches pari passu with the growth and branching of the metanephric diverticulum, so that the termination of every collecting tubule is accompanied by a portion of the inner zone, which is, however, always distinct from it. This conclusion is established by the fact that from the start the two elements, collecting tubules and inner zone, are distinct and may be traced continuously through every stage. The outer zone differentiates in advance of the two more essential constituents at all stages, and thus forms a rather thick investment for them.
Fig. 224. — Transverse section through the ureter and metanephrogenous tissue of a 5-day chick.
A. umb., Umbilical artery. Coel., Coelome.
M's't., Mesentery, n. t. i. z., Inner zone of the nephrogenous tissue, n. t. o. z., Outer zone of the nephrogenous tissue. Ur., Ureter. V. c.p., Posterior cardinal vein. W. D., Wolffian duct.
Fig. 225. — Sections of the embryonic metanephros of the chick to show developing tubules. (After Schreiner.)
A. Nephric vesicle or primordium of secreting tubule (ur. t ) and collecting tubule (col. T.); 9 days and 4 hours.
B. Elongation of nephric vesicle; same embryo.
C. Indication of renal corpuscle at the distal end of the forming tubule.
D. The secreting tubule appears S-shaped.
E. Secreting tubule well formed; 9 davs and 21 hours.
F. Secreting tubule opening into collecting tubule; 11 days.
The formation of the secreting tubules from the inner zone of the metanephrogenous tissue takes place in essentially the same manner as the formation of the mesonephric tubules. The first stages may be found in seven and eight-day chicks in the portion of the kidney behind the umbilical arteries. The inner zone tissue begins to arrange itself in the form of minute balls of cells in immediate contact with the secreting tubules; a small lumen then arises within the ball, transforming it into a thickwalled epithelial vesicle with radially arranged cells. The vesicle then elongates away from the collecting tubule and gradually takes on an S-shape. The distal end of the S becomes converted into a renal corpuscle as illustrated in Figure 225 and the proximal end fuses with the wall of the collecting tubule; an opening is then formed between the two.
On the eleventh day of incubation, secreting tubules are thus formed throughout the entire length of the kidney; but the histological structure does not yet give the effect of an actively secreting gland, although degeneration of the mesonephros has already begun. The full development of the nephric tubules in the chick has not been studied.
At all stages in its development the kidney substance is separated from the mesonephros by a distinct layer of undifferentiated mesenchyme, which is, however, at certain times extremely thin. But there is no evidence that at any time elements of the mesonephros, e.g., undifferentiated nephrogenous tissue, extend up into the metanephric primordium which so closely overlies it (cf. Figs. 221 and 222).
The kidney is entirely retroperitoneal in its formation, and its primary capsule is established by differentiation of the periphery of the outer zone. This may be seen in process at eleven days (Fig. 222) : the primary capsule is definitely established on its median and lateral sides; but is defective dorsally and at the angle next the aorta. With the subsequent degeneration of the mesonephros, and projection of the kidney into the coelome, its ventral surface acquires a secondary peritoneal capsule.
III. The Organs of Reproduction
The gonads are laid down on the median surface, and the ducts on the lateral surface of the Wolffian body, which thus becomes converted into a urinogenital ridge. The composition of the urinogenital ridge is at first the same in all embryos, whether destined to become male or female. It has three divisions: (1) the anterior or sexual division, containing the gonad, involves about the anterior half of the Wolffian body; (2) a non-sexual region of the Wolffian body occurs behind the gonad, and (3) behind the Wolffian body itself the urinogenital ridge contains only the Wolffian and Mullerian ducts. A transverse section through the anterior division shows the following relations (Fig. 221): on the mecUan surface the gonad, on the lateral surface near the dorsal angle of the body-cavity the Wolffian and Mullerian ducts, the latter external and dorsal to the former: between the gonad and ducts lie the tubules of the Wolffian body destined to degenerate for the most part.
There is an incUfferent stage of the reproductive system during which the sex of the embryo cannot be determined, either bv the structure of the gonad or the degree or mode of development of the ducts. In those embryos that become males the gonad develops into a testis, the Wolffian duct becomes the vas deferens, the tubules of the anterior part of the Wolffian body become the epididymis, those of the non-sexual part degenerate, leaving a rudiment known as the paradidymis, and the Mullerian duct becomes rudimentary or disappears. In embryos that become females, the gonad develops into an ovary; the Wolffian duct disappears or becomes rudimentary, the Mullerian duct develops into the oviduct on the left side and disappears on the right side, and the tubules of the Wolffian body degenerate, excepting that functionless homologues of the epididymis and paradidymis persist, known as the epoophoron and paroophoron respectively.
It is not correct to state, as is sometimes done, that the embryo is primitively hermaphrodite, for, though the ducts characteristic of both sexes develop equally in all embryos, the primitive gonad is, typically, only indifferent. Nevertheless, if the gonad be physiologically as well as morphologically indifferent in its primitive condition, the possibility of an hermaphrodite development is given. The primitive embryonic conditions appear to furnish a basis for any degree of development of the organs of both sexes.
Development of Ovary and Testis
The reproductive cells of ovary and testis alike arise from a strip of peritoneal epithelium, known as the germinal epithelium, which is differentiated on the fourth day by its greater thickness from the adjacent peritoneum (Fig. 217). The germinal epithelium lies between the base of the mesentery and the mesonephros at first, but as the latter grows and projects into the body-cavity the germinal epithelium is drawn on to its median surface. It is difficult to determine its antero-posterior extent in early stages; it begins near the point of origin of the omphalomesenteric arteries, and its posterior termination is indefinite, but it certainly extends over seven or eight somites.
Two kinds of cells are found in the germinal epithelium, viz., the ordinary peritoneal cells and the primordial germ-cells. The latter are typically round, and several times as large as the peritoneal cells (Figs. 226 and 227); the cytoplasm is clear but contains persistent yolk granules and a large attraction sphere, and the nucleus contains one or two nucleoli; they are sharply distinguishable from the peritoneal cells, and they may be traced through a continuous series of later developmental stages into the ova and spermatozoa. The origin of these primordial germ-cells is therefore a matter of considerable interest.
Two views have been held: (1) that they are derived from the peritoneal cells, and (2) that they have an independent history antecedent to the differentiation of a germinal epithelium, representing in fact undifferentiated embryonic cells that reach the germinal epithelium by migration from their original source. The former view was due to Waldeyer, and was supported by observations of cells intermediate in structure between the primordial germ-cells and cells of the peritoneum (e.g. by Semon). These observations have, however, been shown to be erroneous. The second view has been demonstrated for a considerable number of vertebrates; and quite recently Swift has shown that the primordial germ-cells of the chick arise from the germ-wall at the anterior margin of the pellucid area in a late stage of the primitive streak; that they later enter the blood stream and are carried into the embryo; some, which reach various inappropriate positions, degenerate; but others leaving the blood near the base of the mesentery reach the germinal epithelium by migration. The independent and early origin of germ-cells has an obvious bearing on the theory of the continuity of the germ-plasm of Weismann.
Two other epithelial constituents enter into the composition of the indifferent gonad, viz.: the rete tissue or cords of the urinogenital union, and the sexual cords. These lie between the germinal epithelium and the glomeruli of the Wolffian body. Between these elements is a sparse mesenchyme continuous with the surrounding mesenchyme, constituting the stroma of the gonad.
Fig. 226. — Cross-section through the genital primordium of Limosa segocephala. (After Hoffmann, from Fehx and Biihler.)
The stage is similar to that of a chick embryo of 4.5 days. Germ., Germinal epithelium. Mst., Mesentery. S. C., Rete cords. v., Posterior cardinal vein. W. D., Wolffian duct.
Some primordial germ-cells occur in the stroma, though most are in the germinal epithelium.
The rete cords appear within the gonad on the fifth day; they are solid cords of epithelial cells that fill up the interior of the gonad and cause it to protrude from the surface of the Wolffian body (Fig. 226); the cords extend from the germinal epithelium towards the hilum of the gonad (represented at this time by the broad surface opposed to the Wolffian body), and into the Wolffian body where they enter into close connection with the renal corpuscles. In the Wolffian body and intermediate zone they are very irregular in their course and connected by numerous anastomoses, corresponding to the rete region of the future testis. Strands of these cells pass dorsally, and, according to some authors, form the cortical cords of the suprarenal capsules (Fig. 226).
The following views of the origin of the rete cords in birds have been held: (1) That they arise as outgrowths of the capsules of renal corpuscles (Hoffmann, Semon) and the neck of the Wolffian tubules also (Semon); (2) that they are ingrowths of the germinal epithelium (Janosik); (3) that they differentiate from the stroma (Prenant, Firket). The subject is a somewhat difficult and complicated one, but the view that the rete cords arise as outgrowths of the capsules of renal corpuscles brings the birds into line, in this respect, with the reptiles and amphibia. Hoffmann's observation that the rete cords lie at first on the lateral side of the blood-vessels intervening between the germinal epithelium and the Wolffian body, and that the cells of the cords are directly continuous with those of the capsules, should be conclusive.
The sexual cords arise as proliferations of the germinal epithelium which appear as buds projecting into the stroma (Fig. 227). They are definitely limited in time of origin between the middle of the fifth and sixth days of incubation (Swift). They carry with them numerous primordial germ-cells from the germinal epithelium. About the end of the sixth day all free themselves from the germinal epithelium, and a layer of stroma begins to separate them sharply from the latter. They are destined to form the seminiferous tubules in the male, and the so-called medullary cords in the female.
The period of morphological indifference of the gonad is relatively long and the actual sexual differentiation appears slowly. It manifests itself (1) in differences in the behavior of the germinal epithelium; (2) of the sexual cords; (3) larger size of the left ovary and ultimate disappearance of the right one; (4) behavior of the stroma, particularly the albuginea. The sex of the embryo can first be definitely determined about the 156th hour, by the relative sizes of the two gonads, by the behavior of the germinal epithelium and by the presence of a larger number of primordial germ-cells in the germinal epithelium of the female. (Swift.)
Fig. 227. — Portion of a transverse section through an ovary of a 6^ day chick embryo (after Swift), germ, ep., germinal epitheHum. m. c, sexual cord. pr. o., primordial germ-cells.
As already stated, the sexual cords form the seminiferous tubules of the testis; they are made up of two kinds of cells, viz.: the primordial germ-cells and the ordinary peritoneal cells derived from the germinal epithelium. After the seventh day they constitute most of the bulk of the testis, and the rete cords are pressed towards the hilum by the sexual cords which radiate in that direction. The sexual cords now begin to branch and anastomose, and soon form a reticulmn with mesenchyme in the meshes. About the thirteenth day the primordial germ-cells, which have been inactive, begin to divide, and a rapid increase in numbers ensues.
Fig. 228. — Portion of a transverse section through the right testis of a 20 day chick embryo. The section shows a seminiferous cord in which a lumen is beginning to develop. Note the position and polarization of the spermatogonia (after Swift). Int. c, interstitial cells. L., beginning of lumen. M. C, Mitochondrial granules within a spermatogonium, p. c, supporting cells, derivatives of peritoneal cells of the sexual cords, s. c, seminiferous cord, sp., spermatogonia, str., stroma.
The sexual cords are solid up to about the twentieth day of incubation; a lumen then begins to appear and they become transformed into tubules (Fig. 228). The primordial germ-cells form the spermatogonia, and the peritoneal cells form the supporting cells of the seminiferous tubules (Swift).
After the sixth day the germinal epithelium of the testis rapidly retrogresses and becomes reduced to a thin peritoneal endothelium. The stroma of the primitive testis remains scanty up to the eleventh day. It then increases rapidly between the sexual cords and also forms a layer between germinal epithelium and seminiferous tubules, which becomes the albuginea. Interstitial cells appear in the stroma of the testis about the thirteenth day and increase so rapidly as to form an immense amount by the twentieth day (Swift).
As the testis increases in size it projects more from the surface of the Wolffian body, and folds arise above and below it as well as in front and behind, that progressively narrow the surface of apposition, which in this way becomes gradually reduced to form the hilum of the testis, through which the rete cords pass to the neighboring renal corpuscles (cf. Figs. 221 and 222).
As the testis is attached to the anterior portion of the Wolffian body, the latter may be divided in two portions, an anterior sexual and a posterior non-sexual portion. In the latter part of the period of incubation the non-sexual portion undergoes absorption while the anterior portion becomes converted into the epididymis.
The irregularly anastomosing rete cords in the region of the hilum are united to the neighboring renal corpuscles by the original strands and these form the vasa efferentia. In order to complete the urinogenital union it is necessary that the rete cords unite with the seminiferous tubules. The exact manner in which this takes place has not been worked out for the chick; but there is no doubt that this union does take place so that the seminiferous tubules connect by way of the rete with the mesonephric tubules and thus with the Wolffian duct.
As regards the formation of the epididymis: the renal corpuscles of the Wolffian tubules concerned diminish in size, the glomerulus disappears and the cells of the capsule become cylindrical. These changes progress from the lateral side of the Wolffian body towards the testis; that is to say, the more lateral corpuscles are first affected. A rudiment of the non-sexual part of the Wolffian body persists in the . mesorchium of the male, between testis and kidney. It is known as the paradidymis.
The development of the ovary in the chick has been studied in recent years by Firket and by Swift.
The right ovary never undergoes much development after the indifferent stage; it is destined to retrogress, and finally it disappears.
In the indifferent gonad the sexual cords are formed in the same way whether the organ is to become ovary or testis; but, whereas in the case of the testis these cords are destined to form the functional seminiferous tubules, in the case of the ovary they form only the cords of the medulla. The cortex of the ovary which includes the functional follicles develops from a second proliferation of the germinal epithelium. The sexual cords cease to grow, and become converted into tubes with a wide lumen, and low epithehum; shortly after hatching they entirely disappear.
Fig. 229. — Cross-section of the ovary of a young embryo of Numenius arcuatus. (After Hoffmann.) bl. v., Blood-vessel, germ. Ep., Germinal epithelium, r., rete ovarii. s. c, Sexual cord.
The characteristic feature of the development of the ovary is a second period of intensive growth of the germinal epithelium accompanied by a rapid increase of the primordial germ-cells contained in it. This goes on very rapidly during the eighth to the eleventh days of incubation. The inner surface of the germinal epithelium, or ovigerous layer of the ovary, begins to form low irregular projections into the stroma, or the latter begins to penetrate the ovigerous layer at irregular distances so as to produce elevations. This condition is well illustrated in Fig. 229.
In the course of development the ovigerous layer continually increases in thickness, and the projections into the stroma form veritable cords of ovigerous tissue, which correspond to the cords of Pfltiger in the mammalian ovary. The cords carry the primitive ova with them. The surface of the ovary also begins to become lobulated by the extension of the stroma trabeculae. Successive stages in the growth and differentiation of the primitive ova occur from the surface towards the inner ends of the ovigerous strands. Fig. 230 represents a section through the ovary of a fledgling of Numenius arcuatus three or four days old. The germinal epithelium covers the surface and is continuous with the ovigerous strands projecting far into the stroma. The strands are broken up in the stroma into nests of cells; next the germinal epithelium are found characteristic primitive ova, but in deeper situations the primitive ova are larger and each is accompanied by a group of epithelial cells, which are distinctly differentiated as granulosa cells of young follicles in the deepest. Thus the young follicles arise by separation of nests of cells from the ovigerous strands within the stroma; each nest includes a young ovocyte and a group of epithelial cells which arrange themselves in a single layer of cuboidal cells around the ovocyte. On each side of the free border of the ovary the embryonic state persists, and it is not known whether this condition is maintained permanently, as in some reptiles, or not.
Fig. 230. — Cross-section of the ovary of a fledgling of Numenius arcuatus 3-4 days old. The germinal epithelium is below. (After Hoffmann.) s. c, Sexual cords.
The atrophy of the Wolffian body is much more complete in the female than in the male; no part of it remains in a functional condition, but the part corresponding to the epididymis of the male remains as a rudiment, known as the epoophoron. It has almost the same structure in young females as in young males, but the rete cords uniting it with the ovary do not become tubular. A rudiment of the non-sexual part of the Wolffian body is also found in the hen between ovary and Iddney in the lateral part of the mesovarium; it has been named the paroophoron.
Development of the Genital Ducts
The Wolffian Duct
The origin and connections of the Wolffian ducts have been already sufficiently described. In the male they are connected with the seminiferous tubules by way of the epididymis, vasa efferentia, and rete, and function as vasa deferentia exclusively, after degeneration of the mesonephros. Subsequently they become somewhat convoluted, acquire muscular walls and a slight terminal dilatation. The details of these changes are not described in the literature. In the female the Wolffian duct degenerates; at what time is not stated in the literature, but presumably along with the Wolffian body.
The Mullerian Duct
The Mullerian duct, or oviduct, is laid down symmetrically on both sides in both male and female embryos; subsequently both right and left Mullerian ducts degenerate in the male; in the female the right duct degenerates, the left only remaining as the functional oviduct. We have now to consider, therefore, (1) the origin of the ducts during the indifferent stage, and (2) their subsequent history in the male and in the female.
The origin of the Mullerian duct is preceded by the formation of a strip of thickened peritoneum on the lateral and superior face of the Wolffian body extending all the way to the cloaca (cf. Fig. 220). This strip, which may be called the tubal ridge, appears first at the anterior end of the Wolffian body on the fourth da}", and rapidly differentiates backwards; it lies immediately external to the Wolffian duct. The anterior part of the Miillerian duct arises as a groove-like invagination of the tubal ridge at the cephalic end of the Wolffian body immediately behind the external glomeruli of the pronephros. The hps of this groove then approach and fuse on the fifth day, so as to form a tube which soon separates from the ridge. This process, however, takes place in such a way as to leave the anterior end of the tube open and this constitutes the coelomic aperture of the oviduct, or ostium tuh(£ abdominale. Moreover, the closure of the groove does not take place uniformly, and one or two openings into the Miillerian duct usually occur near the ostium on the fifth clay. Typically, however, these soon close up, though persistence of one of them may lead, as a rather rare abnormality, to the occurrence of two ostia in the adult. There is no ground for the view (see Balfour and Sedgwick) that the two or three openings into the anterior end of the Miillerian duct correspond to nephrostomes of the pronephros; they are situated too far posteriorly and laterally to bear such an interpretation.
The anterior part of the Mullerian duct is thus formed by folding from the epithelium of the tubal ridge; it constitutes a short epithelial tube situated between the Wolffian duct and the tubal ridge, ending blindly behind. The part thus formed is relatively short; the major portion is formed by elongation of the anterior part, which slowly grows backwards between the Wolffian duct and the tubal ridge, reaching the cloaca on the seventh day. The growing point is solid and appears to act like a wedge separating the Wolffian duct and the tubal ridge, being thus closely pressed against both, but apparently without receiving cells from either. Balfour's view, that it grows by splitting off from the Wolffian duct or at the expense of cells contributed by the latter, has not been supported by subsequent investigators. A short distance in front of the growing point the Mullerian duct receives a kuiien, and mesenchyme presses in from above and below, and forms a tunic of concentrically arranged cells around it (Fig. 221).
The Mullerian duct thus begins to project above the surface of the Wolffian body, and, as it does so, the thickened epithelium of the tubal ridge becomes flat and similar to the adjacent peritoneum; whether it is used up in the formation of the mesenchymatous tunic of the epithelial Mullerian duct is not known. Up to this time the development is similar in both sexes and on both sides of the body.
In the male development of these ducts ceases on the eighth day; retrogression begins immediately and is completed, or at any rate far advanced, on the eleventh day. In this process the epithelial wall disappears first, and its place is taken by cells of mesenchymatous appearance, though it is not known that transformation of one kind into the other takes place. Retrogression begins posteriorly and proceeds in the direction of the head; the ostium is the last to disappear. The mesenchymatous tunic shares in the process, so that the ridge is no longer found (see Fig. 222). In the male the IMullerian ducts never open into the cloaca.
In the female the development of the right Mullerian duct ceases after the eighth day, and it soon begins to degenerate. Its lumen disappears and it becomes relatively shorter, so that its anterior end appears to slip back along the Wolffian body. On the fifteenth day slight traces remain along its former course and a small cavity in the region of the cloaca. It never obtains an opening into the cloaca (Gasser).
With the degeneration of the anterior end of the Wolffian body the ostium tubse abdominale comes to be attached by a Ugament to the body-wall (Fig. 231); farther back the ligamentous attachment is to the Wolffian body.
The fimbriae begin to develop on the eighth day on both sides in both sexes. It is only in the left oviduct of the female, however, that development proceeds farther, and differentiation into ostium, glandular part, and shell gland takes place. This appears distinctly about the twelfth day. The lower end expands to form the primordium of the shell gland at this time, but does not open into the cloaca. Indeed, the opening is not established until after the hen is six months old (Gasser.)
Fig. 231. — Photograph of a cross-section of an embryo of 8 clays through the ostia tubae abdominaha.
a. A. S., Xeck of abdominal air-sac. O. T. a., Ostium tubae abdominale. M's't.ac, Accessory mesentery, pi. C. r., 1., Right and left pleural cavities. Rec. pn. ent. r., Right pneumato-enteric recess. V. c. a. 1., Left anterior vena cava. R., rib. Other abbreviations as before.
IV. The Suprarenal Capsules
The suprarenals of the hen are situated medial to the anterior lobe of the kidney, in the neighborhood of the gonad and vena cava inferior. They have a length of about 8-10 mm. The substance consists of two kinds of cords of cells, known respectively as cortical and medullary cords, irregularly intermingled: the so-called cortical cords make up the bulk of the substance, and the medullary cords occur in the meshes of the cortical cords.
The terminology does not, therefore, describe well the topographical arrangement of the components; it was derived from the condition found in many mammals, the cortical cords of the birds corresponding to the cortical substance, and the medullary cords to the medullary substance of mammals. The medullary cords are often called phseochrome or chromaffin tissue on account of the specific reaction of the constituent cells to chromic acid, and their supposed genetic relation to tissue of similar composition and reaction found in the carotid glands and other organs associated with the sympathetic system.
The embryonic history has been the subject of numerous investigations, and has proved a particularly difficult topic, if we are to judge from the variety of views propounded. Thus for instance it has been maintained at various times: (1) that cortical and medullary cords have a common origin from the mesenchyme; (2) that they have a common origin from the peritoneal epithehum; (3) that the origin of the cortical and medullary cords is absolutely distinct, the former being derived from the sexual cords by way of the capsules of the renal corpuscles and the latter from the sympathetic ganglia; (4) that their origin is distinct, but that the cortical cords are derived from ingrowths of the peritoneum, and the medullary cords from sympathetic ganglia. The first view may be said now to be definitely abandoned, and no one has definitely advocated a common epithehal origin since Janosik (1883). Thus it may be regarded as well estabUshed that the two components have diverse origins, and it seems to the writer that the fourth view above is the best supported. (See Poll and Soulie.) The comparative embryological investigations strongly support this view.
Origin of the Cortical Cords.
According to Soulie, the cortical cords arise as proliferations of a special suprarenal zone of the peritoneum adjacent to the anterior and dorsal part of the germinal epithehum. This zone is distinguishable early on the fourth day, and begins about half a millimeter behind the glomeruH of the pronephros, extending about a millimeter in a caudal direction. Proliferations of the peritoneal epithelium are formed in this zone, and soon become detached as groups of epithelial cells lying in the mesenchyme between the anterior end of the Wolffian body and the aorta. Such proliferation continues up to about the one hundredth hour or a httle later, and a second stage in the development of the cortical cords then begins: The cords grow rapidly and fill the space on the mediodorsal aspect of the AVolffian body, and then come secondarily into relation with the renal corpuscles of the latter and the sexual cords.
According to Semon and Hoffmann the relation thus established is a primary one, that is to say, that the cortical cords arise from the same outgrowths of the capsules of the renal corpuscles that furnish the sexual cords. Rabl agrees essentially with Soulie, and it seems probable that Semon and Hoffmann have overlooked the first stages in the origin of the cortical cords of the suprarenal corpuscles.
During the fifth, sixth, and seventh days there is a very rapid increase of the cortical cords accompanied by a definite circumscription of the organ from the surrounding mesenchyme; however, no capsule is formed yet. The topography of the organ on the eighth day is shown in Figs. 150 and 182. Whereas during the fourth, fifth, and sixth days the arrangement of the cortical cells is in masses rather than in cords, on the eighth day the cords are well developed, in form cylindrical with radiating cells, but no central lumen. The organ has become vascular, and the vessels have the form of sinusoids, i.e., they are moulded on the surface of the cords with no intervening mesenchyme.
Origin of the Medullary Cords.
The medullary cords take their origin unquestionably from cells of the sympathetic nervous system. During the growth of the latter towards the mesentery, groups of sympathetic cells are early established on or near the dorso-median surface of the cortical cords (Fig. 226). The ingrowth of the sympathetic medullary cords does not, however, begin until about the eighth day. At this time there is a large sympathetic ganglionic mass on the dorso-median surface of the anterior end of the suprarenal, and strands of cells characterized sharply by their large vesicular nuclei and granular contents can be traced from the ganglion into the superficial part of the suprarenal. These cells are precisely like the specific cells of the ganglion, perhaps a little smaller, and without axones. On the eleventh day these strands have penetrated through a full third of the thickness of the suprarenal, and are still sharply characterized, on the one hand by their resemblance to the sympathetic ganglion cells, and on the other by their clear differentiation from the cells of the cortical cords. These occupy the relations characteristic of the differentiated medullary cords, and there can be httle doubt that they develop into them.