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. 9. LONGITUDINAL VERTICAL SECTION THROUGH 

 A FROG EMBRYO, SHOWING THE COMPLETION OF THE 

 MESENTERON. (After Marshall.) 



B, blastopore. E E, epidermic layer of ectoblast. EN, nervous 

 layer of ectoblast. H, invaginate entoblast. M^ mesoblast. M N^ 

 mesenteron. N^ 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 



