144 'J'HE GERM-PLASM 



long as the processes which he describes admit of more than 

 one interpretation, we cannot reject the hypothesis of the pre- 

 destination of the cells by means of the distribution of certain 

 determinants and groups of determinants to them, for this view 

 is supported by so large a number of facts, and even by the 

 earlier experiments of Roux himself. It would certainly, how- 

 ever, have to be rejected if we could prove that the cells of the 

 germinal layers were really capable of being determined in their 

 nature by the region which they accidentally reach, or by their 

 accidental surroundings. 



Further research along the line opened up by Roux will, I am 

 convinced, show us the facts in another light, and will enable us 

 to reconcile them to the rest of our conceptions as to the causes 

 of ontogeny. But I do not consider it worth while at present 

 to enumerate all the possible causes which must be taken into 

 account in an attempt to explain 'post-generation.' 



