REPRODUCTION 



73 



the peritoneum and mesenteries to the coelomic organs. It is clear that 

 no organ can be said to lie in the coelom except as the peritoneum investing 

 that organ is regarded as a part of the organ. In strict sense, median 

 organs lie between the peritoneal sheets of the right and left halves of the 

 body. 



The peritoneum plays a part in the development of the gonads although 

 it is not necessarily the source of the germ cells. The prospective gonads 

 first appear as longitudinal thickenings or genital ridges in the dorsal 

 peritoneum, one on each side and between the dorsal mesentery and the 

 mesonephros (Figs. 60, 61 and 66). The earlier belief that the germ cells 







Fig. 66. — 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.) • 



are derived from the peritoneal layer has been shaken by evidence that 

 the primordial germ cells first appear in the mid-dorsal enteric endoderm 

 whence they migrate into the genital ridge. The deeper substance of the 

 definitive gonad is derived either from the thickened peritoneum of the 

 genital ridge or, especially in the male, from the mesoderm of the closely 

 adjacent mesonephros. 



The gonads find outlet by way of ducts which arise in relation to the 

 kidneys. The seminiferous tubules of the testis acquire connection with 

 the neighboring mesonephric tubules and thereby gain exit by way of 

 the Wolffian duct which therefore, in Anamnia, serves as a urinogenital 

 duct. In amniotes the adult male retains, in the epididymis, that part 

 of the embryonic mesonephros which provided connection between the 

 testis and the Wolffian duct. With metanephros and ureter serving the 

 urinary function,' the Wolffian duct is left as a vas deferens or sperm duct 

 only. 



