324 HEREDITY AND ENVIRONMENT 



be totipotent, that is, it is capable of giving 

 rise to an entire animal. 



On the other hand in certain animal phyla 

 such as the ctenophores, mollusks, annelids 

 and ascidians isolated cleavage cells give rise 



Fig. 71. Double embryos of frog developed from eggs in- 

 verted when in the 2-cell stage. A, twins with heads turned 

 in opposite directions. B, twins united back to back. C, 

 twins united by their ventral sides. D, double headed tad- 

 pole. (From Wilson after O. Schultze.) 



