Bennet Mills Allen 113 



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 man- 

 ner described in the pig embryo. The latter are distinctly separated 

 from one another by their clear-cut membrana propria. No primi- 

 tive 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 an- 

 terior 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 seminifer- 

 ous 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 de- 

 generate 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, cen- 

 trosphere and centrosome, and a spherical nucleus somewhat smaller 

 than that of the primitive sex cells. 



OvAEY. — The chief advance over the preceding stage is found in 

 the extension of the cords of Pfliiger 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. 



