2 2 Vertebrate Embryology 



arrangement. At this time a crescentic groove 

 appears on one side of the egg at the bor- 

 der between the white and dark cells. This 

 groove, whose convex side is upward, is the 

 dorsal lip of the blastopore, and is the begin- 



FiG. 7. — Longitudinal vertical section of a frog 



EMBRYO, SHOWING COMMENCING INVAGINATION. X28. 



(After Marshall). 



B, blastopore. EE^ outer or epidermic layer of ectoblast. El^^ 

 inner or nervous layer of ectoblast. 5 C, segmentation cavity. }', 

 yolk-cells. 



ning of the process of mvagination (Figs. 6, 

 7, and 8). 



The horns of the crescent extend towards 

 each other until they meet to form a circle 

 (Fig. I, //), the blastopore, bounded on the 

 outside by pigmented cells, and filled inside 



