26 



Vertebrate Embryology 



packed cells, its floor is a mass of undiffer- 

 entiated yolk-cells. The cells forming the 

 roof of the mesenteron are the beginning of 

 the lower germ-layer, or entoblast (Fig. 9). 

 The mesenteron, opening to the exterior by 



Fig. g. — Longitudinal vertical sec tion through 



A FROG EMBRYO, SHOWING THE COMPLETION 01' THE 



MESENTERON. (After Marshall,) 



,5, blastopore, ^^, epidermic layer of ectoblast. ^A^, nervous 

 layer of ectoblast. /^, invaginate entoblast. VJ/, mesoblast. MN^ 

 mesenteron. .A'', notochord. 



the narrow blastopore (at the posterior end 

 of the embryo), rapidly enlarges by forward 

 growth and by the depression of the yolk-cells 

 forming its floor, until it becomes a large 

 cavity whose growth has caused the oblitera- 

 tion of the .segmentation cavity (Figs. 8, 9, and 



