The Development of the Frog 23 



with white cells. The mass of white cells 

 which fills the blastopore is known as the 

 yolk-plug {^\g. 8, YP). 



By a rapid division of the black cells around 



«5^ •■ "^ 



felt f 



YP — 



i • ' ' r' 





Fig. 8. — Longitudinal vertical section through 

 a frog embryo at a later stage in the formation 

 OF THE MESENTERON. (After Marshall.) 



/^, invaginate entoblast. /I/, mesoblast. M N^ mesenteron. 

 iV, notochord. SC^ segmentation cavity. VP, yolk-plug^ filling up 

 the blastopore. 



the rim of the blastopore, especially at the dor- 

 sal lip, the exact nature of which process is in 

 some dispute, the diameter of the blastopore is 

 gradually reduced and the yolk-plug is with- 

 drawn into the egg (Figs. 8 and 9). 



This overgrowth of the black cells continues 



