The Development of the Frog 23 



with white cells. The mass of white cells 

 which fills the blastopore is known as the 

 yolk-plug (Y\g. 8, YP\ 



By a rapid division of the black cells around 



YP m 



FIG. 8. LONGITUDINAL VERTICAL SECTION THROUGH 

 A FROG EMBRYO AT A LATER STAGE IN THE FORMATION 

 OF THE MESENTKRON. (After Marshall,) 



H, invaginate entoblast. /)/, mesoblast. M N^ mesenteron. 

 N^ notochord. SC^ segmentation cavity. YP, 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 



