304 Explanation of Inheritance 



whereas embryonal differentiations, on the other hand, 

 take place without or almost without such differentiating 

 stimuli, there is reason to believe that these results are 

 produced in another way which, while undoubtedly con- 

 trolled by natural laws, is nevertheless for the time 

 incomprehensible by us. Consequently the essence of 

 embryonic differentiation and its immediate physico- 

 chemical causes are for the moment quite inaccessible to 



us." 22 



Centroepigenesis, on the contrary, teaches us, as we 

 have seen, that embryonic and functional differentiation 

 are essentially the same. And in support of this view we 

 may recall among others the following principal orders 

 of facts: 



"All the organs which fulfill their specific function 

 already in the embryo have there a life dependent upon 

 stimulus (Reizleben) in proportion to this function." 221 

 This indicates that an organism in process of develop- 

 ment may be at one and the same moment in the 

 embryonic period in respect to some of its parts and in 

 the functional period in respect to other parts, without 

 ceasing for that reason to behave in all its manifestations 

 as a whole of a thoroughly individual nature. 



A number of characters begin to develop embryon- 

 ically which later require the help of the functional 

 stimulus to complete their development : "In embryonic 

 development some parts are ontogenetically formed and 

 developed to a certain grade of functional capacity, which 

 have been formed phylogenetically entirely by functional 

 adaptation. Functional stimuli seem to be necessary only 

 for that remainder of development which belongs from its 



220 Roux : Ibid. P. 166. 



221 Roux: Der Kampf der Teile im Organismus. P. 182. 



