156 DEVELOPMENT OF THE FROG'S EGG [Cn. XIV 



plate is first formed. It is well marked at the time when the 

 medullary folds have rolled in, but have not yet fused. A 

 thickening of the somatic layer of the lateral mesoderm near the 

 second mesoblastic somite marks the beginning of the prone- 

 phros (Fig. 48, A). At a later stage, the mesodermic thick- 

 ening becomes larger, and the anterior end arches over toward 

 the coelomic cavity, to form the first nephrostome. The ventro- 

 posterior part of the nephrostomal thickening is continued 

 backward as a thickening of the somatic wall as far as the 

 seventh somite, to form the segmental duct. A canalization 

 now takes place in the nephrostomal portion and in the seg- 

 mental duct. Three short tubes or canals appear in the 

 pronephric mass running outward from the ccelom (Fig. 41). 

 Constrictions appear between the first and second, and between 

 the second and third canalized tracts (Fig. 48, B), and short 



A B C 



FIG. 48. Three stages in the formation of the pronephros. (A and C after Field.) 



hollow stalks are formed leading ventrally into the longitu- 

 dinal canal of the segmental duct. 



A proliferation of cells from the somatic layer of the meso- 

 blastic somites, dorsal to the, pronephros, gives rise to a cover- 

 ing of mesoderm for the pronephros, the pronephric capsule. 

 A little later a protrusion of the splanchnic wall opposite to 

 the funnels of the pronephros forms the glomus (Fig. 47, B). 

 The glomus becomes filled with blood, and seems to have a 

 direct connection with the dorsal aorta. The bulging portion 

 of the glomus protrudes into the coelom, and its cavity is sepa- 

 rated from the coelomic cavity by only a single layer of cells. 



At the time when the embryo is hatched, the duct of the 

 pronephros, the segmental duct, has fused with the wall of 

 the cloaca, and the lumen of the duct opens into the digestive 



