REPRODUCTION lOI 



upward between the two peritoneal layers of the dorsal mesentery and 



Fig. 74. — Section of genital ridge of a chick of five days incubation, e, peritoneal 

 epithelium of ridge; c, genital cords; o, primordial germ cells. (From Kingsley, after 

 Semon.) 



then laterally outward to enter the genital ridge. As development pro- 

 gresses, the genital ridges bulge more and more 

 into the coelom and eventually the substance of 

 the gonad becomes entirely detached from the 

 body-wall except as the peritoneal investment 

 of the gonad remains in continuity with the 

 parietal peritoneum forming a two-layered 

 supporting membrane, the mesovarium for the 

 ovary and the mesorchimn for the testis. 



The gonads find outlet by way of ducts 

 which arise in relation to the kidneys. The 

 seminiferous tubules of the testis acquire connex- 

 ion with the neighboring mesonephric tubules 

 and thereby gain exit by way of the Wolffian 

 duct which therefore, in adult vertebrates 

 which retain the mesonephros, serves as a 

 urinogenital duct. In amniotes the adult male 

 retains, in the epididymis, that part of the 

 embryonic mesonephros which provided connex- 

 ion between the testis and the Wolffian duct. 

 With metanephros and ureter serving the 

 urinary function, the Wolffian duct is left as a Waideyer.) 

 vas deferens or sperm duct only. 



The oviducts of Anamnia and Amniota are perhaps not exactly homo- 

 logous. In elasmobranchs and probably some amphibians the pronephric 



Fig. 75. — Transverse sec- 

 tion through the urinogeni- 

 tal region of a four-day 

 chick embryo. g, mesoder- 

 mal epithelium (peritoneum) 

 of genital ridge; m, infolding 

 of peritoneum to form Miil- 

 lerian duct; ms, mesentery; s, 

 mesenchyme cells which give 

 rise to the stroma (non-geni- 

 tal tissue) of gonad; t, meso- 

 nephric tubules; W , Wolffian 

 duct. (From Kingsley, after 



