A DESCRIPTIVE SKETCH 



17 



mouth of the blastopore forming the so-called yolk-plug. At the 

 same time, the original cavity of the blastula, the blastocoel, has 



b 

 Fig. 3 

 Diagrams to show the directions of movement and displacement of the parts of 

 the blastula in the process of gastrulation in Amphibia, a. Seen from the vege- 

 tative pole, b, From the left side. The thick lines show the tracks followed on the 

 surface: the thin lines the tracks beneath the surface after invagination at the 

 blastopore rim. (From Vogt, Arch. Entzomech. cxx, 1929.) 



been more or less obliterated by the formation of the new cavity, 

 the archenteron or gut (fig. 4). 



Since it is on the dorsal side of the embryo that the overgrowth 



