582 JAMES F. GEMMILL. 



Doubling (partial or complete) of the gastrula invagination is the 

 great step on which the differentiation of twin bipinnarise depends. 



This differentiation shows very markedly the working of " regu- 

 lation " processes in the course of which, when union of structures 

 occurs, the union is always between structures of homologous origin. 

 Thus preoral and postoral bands, enterocoeles, and particular regions 

 of the alimentary canal, unite each with its own counterpart. 



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