196 TORREY 



TEXT-FIGURE 4. 



A, the invaginating plate. B, actual section of a gastrulation stage. C, an 

 early gastrulation stage ; the X group, ventral plate, is bounded by heavy lines ; 

 fil, blastopore; <?, cells migrating from the prgetrochal region; c^r.;;/., oesophago- 

 blast median. D, a later stage showing the shifting of the blastopore toward the 

 prototroch on the ventral side and the further migration of cells, /, from the prse- 

 trochal region. 



