lOO 



COMPARATIVE ANATOMY 



particular function such as digestion or secretion — lies between the 

 peritoneal sheets of the right and left halves of the body. The peri- 

 toneum is not so much the property of the organ which it invests as it is 

 specifically the immediate wall or lining of the coelom. 



The peritoneum plays a part in the development of the gonads 

 although it is not necessarily the source of the germ cells. At the earliest 



MVJ 



Fig. 73. — Diagrammatic transverse section of the body of a vertebrate showing 

 relations of organs to the peritoneum and coelom. .4, dorsal aorta; C, coelom; EN, 

 endodermal epithelium of digestive tube; G, gonad; /, integument; K, kidney; L, liver; 

 M, muscle layer of digestive tube; MD, dorsal muscle of body-wall; MV, ventral muscle 

 of body- wall; NC, position of embryonic notochord; NT, neural tube (spinal cord); 

 PP, parietal peritoneum; PV, visceral peritoneum; R, rib; VC, vertebral column. 



stage when they are easily recognizable, the prospective gonads appear as 

 longitudinal thickenings or genital ridges in the dorsal peritoneum, one 

 on each side and between the dorsal mesentery and the mesonephros 

 (Figs. 67, 68 and 74). The earlier belief that the germ cells are derived 

 from the peritoneal layer has been shaken by evidence that they are of 

 endodermal origin. 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 meso- 

 nephros (Fig. 75). It has been claimed, however, that the earliest recog- 

 nizable primordial germ cells, in embryos of various vertebrates, lie in the 

 mid-dorsal enteric endoderm. From this location they are said to migrate 



