^^^^ ZOOGENESIS ©ll 



other. All, however, as well as the coelenterates, are 

 derived through a gastrula stage or its equivalent, 

 which is the last stage common to them all. 



Nothing exactly comparable to a gastrula exists as 

 an adult animal. It is true that all the coelenterates 

 are essentially adult forms reducible to a gastrula, 

 but all of them are developed very far beyond the 

 gastrula. The coelenterates are probably to be con- 

 sidered as having progressed quite as far beyond the 

 gastrula as the bilaterally symmetrical animals, al- 

 though they retain the original symmetry of the 

 gastrula sometimes quite unmodified, but usually 

 slightly modified (fig. B, p. 140) . 



In the coelenterates the body symmetry always 

 remains essentially that of the gastrula no matter 

 how far development in other lines may go. But in 

 all other types which are derived through a gastrula 

 the body symmetry changes over from the radial 

 symmetry of the gastrula to a different, usually a 

 bilateral, symmetry during the course of the develop- 

 ment from the gastrula to the adult. 



In the bilaterally symmetrical animals during the 

 course of the development from the gastrula to the 

 adult profound modifications in the internal structure 

 occur and these follow a different and divergent path 

 in each of the seventeen major groups of animals 

 concerned. 



In none of these seventeen major groups of animals 

 is there any indication of a cessation in the course of 

 the development between the gastrula and the adult, 

 except for the appearance in most cases of a special 



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