GASTRULATION IN VARIOUS CHORDATA 



405 



FUTURE 

 ANTERIOR 



POLAR 

 BOD 



Fig. 190. Similar to fig. 189, showing the presumptive organ-forming areas of the 

 blastula and their position during gastrulation. The position of the respective organ- 

 forming areas in a transverse section through the midregion of gastrula shown in G is 

 depicted in fig. 195A. (Based upon data obtained from Conklin, '32.) 



c. Resume of Cell Movements and Processes Involved in Gastrulation 



of Amphioxus 

 1) Emboly: 



(a) Invagination. The entodermal plate of cells gradually invaginates and 

 folds inward into the blastocoel. 



(b) Proliferation of cells. This is true particularly of notochordal, meso- 

 dermal, and neural cells near the blastoporal area during initial stages 



