DOMINANCE AND PHYSIOLOGICAL ISOLATION 331 



structure and determining regeneration need not necessarily be specific 

 but may merely serve to maintain metabolism in the part concerned at a 

 level which maintains or makes possible the development of a certain 

 structure. For example, muscle may differentiate in the embryo, but later 

 it atrophies in the absence of innervation because in the earlier stages the 

 intrinsic metabolism of the cells is sufficiently intense to determine muscle 

 structure but later is not sufficient to maintain it in the absence of innerva- 

 tion. In the simpler animals dominance is apparently chiefly or wholly 

 nervous or neuroid, so far as axiate pattern is concerned. In the embry- 

 onic development of so-called "mosaic eggs" and of vertebrates the re- 

 gional cytoplasmic differentiation of the egg at the beginning of develop- 

 ment or chemical relations resulting from this differentiation may play a 

 part in bringing about the dominance of the nervous system. 



