132 Embi\voiiic Development of Ovai\y and Testis of Mammals 



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 be- 

 come 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 degenera- 

 tion, 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 remain- 

 ing after the degeneration of the medullary cords and of the inner fol- 

 licles of the cords of Pfliiger. 



In the rabbit, the process is not so striking, the sex cells having dis- 

 appeared 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 be- 

 tween 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-medul- 

 lary 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 develop- 

 ment of the sex gland. 



