80 SUPER-ORGANIC EVOLUTION 



inwardly until it penetrates the blastula's cavity of 

 segmentation, and puts itself in contact with the 

 other portion that remains fixed, so that instead of 

 one cellular stratum there are two. When both 

 portions are in contact, the embryonic animal form 

 resulting from it is called gastrula (see fig. 1). 



ev 



FIG. 19. 

 ca, animal cell ; cv, vegetable cell ; cs, cavity of segmentation. 



" This form has a great importance in embryonic 

 history, as Haeckel has shown in his celebrated 

 theory of the gastrula. One finds it, in fact, in all 

 the great ramifications of the animal kingdom. So 

 it constitutes a period common to all animals. It is 

 from this form that the divergence of all the animal 

 kingdom commences" (0. Hertwig). 



Many inferior animals exist in the form of 



