140 Embryonic Development of Ovary and Testis of Mammals 



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 inter- 

 stitial 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. Eete 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 meso- 

 nephros, by means of more or less vestigial evaginations from the cap- 

 sules 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 medul- 

 lary 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 follicu- 

 lar cells similar in all regards to the remains of the medullary cords. 



