282 



Basic Structure of Vertebrates 



Fig. 233. Section of genital ridge of a chick of five days' incubation, (c) Genital 

 cords; (e) peritoneal epithelium of ridge; (o) primordial germ-cells. (After Semon. 

 Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, The 

 Blakiston Company.) 



extend across coelomic space freely and without support, but it is 

 always encased in peritoneum left surrounding it after the adjacent 

 region of mesentery degenerated. The embryonic mesenteries provide a 

 scaffolding, necessary during the period of construction, but to a large 

 extent removed after the work is finished. Figure 232 shows the ideal 

 relations of 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 a 

 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 prospec- 

 tive 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. 230, 231, 233). The earlier belief 

 that the germ-cells are derived from the peritoneal layer has been 

 shaken by evidence that the primordial germ-cells first appear in the 

 middorsal enteric endoderm whence they migrate, via the mesentery, 

 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 gonad eventually becomes detached from the body- 

 wall but retains its peritoneal covering, which remains in continuity 

 with the parietal peritoneum forming a supporting membrane, the 

 mesorchium for the testis and the mesovarium for the ovary 

 (Fig. 232). 



