THE MESOMERE (INTERMEDIATE CELL MASS) 



227 



ridge dorsal to the midgut at about the time of hatching (6 mm. 

 stage). These cells presumably arise from the yolk sac splanchno- 

 pleure, or from the gut endoderm. Some evidence in favor of this 

 theory is the fact that the primordial germ cells are heavily laden with 

 yolk. As the two lateral mesodermal plates converge above the gut, 

 this group of cells migrates to a position directly dorsal to the dorsal 

 mesentery, between the posterior cardinal veins at the 8 to 9 mm. 

 stage. Since this ridge of primitive gonadal material is rather long, 

 it is now called the sex cell cord. This shortly divides longitudinally 

 into two, and each part moves ventro-laterally to a position between 

 the developing Wolffian body and the dorsal mesentery projecting 

 into the body cavity as the genital ridges. Each mass will enlarge and 

 project ventrally into the coelomic cavity, carrying with it a covering 

 of peritoneal epithelium. This membrane becomes double and sus- 

 pends the gonad to the body wall as the gonad grows into the coelom. 



NEUROCOEL 



NOTOCHORD 



MESONEPHRO: 



WOLFFIAN DUCT 

 GONAD 



MESONEPHROS 



COELOM 



STOMACH 



HEPATIC PORTAL VEIN 



EXTERNAL 

 GILLS 



PERITONEUM 



LIVER 



INTESTif 



Schematized diagram through the level of the gonad primordium of the 1 1 mm. 



frog tadpole. 



