92 Embryonic Development of Ovary and Testis of Mammals 



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 termin- 

 ates 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 homo- 

 logues in the ovarv are termed the medullarv cords. These have all 



