Mediate Formative Influence in Post-generation 21 



formative action exercised mediately and at a distance by 

 one part of the organism upon another during the entire 

 development of this latter is furnished by the famous ex- 

 periments of Roux in the post-generation of his half 

 embryos. 



The words in which he describes the process of this 

 post-generation which he has observed in half embryos 

 obtained from frogs' eggs after he had killed one of the 

 first two blastomeres with a hot needle, deserve to be 

 reproduced here in full. 



But we must remember at the outset that while the 

 uninjured blastomere develops into only a half embryo, 

 the injured blastomere lying beside its neighbor produces 

 often a late fragmentation of its protoplasmic mass con- 

 sisting only of undifferentiated cells. And this is effected 

 in one of the following two ways, either through only 

 partial killing of the nucleus concerned some individual 

 fragments of it continuing to live and multiply, or 

 through an emigration of naked nuclei from the part of 

 the egg remaining intact into the protoplasm of the in- 

 jured blastomere. Now we quote Roux. 



"Postgeneration of germ layers of half organisms 

 proceeds always from the already differentiated germ 

 layers of the normally developed half of the egg. It 

 extends thence first to the yolk mass subsequently cel- 

 lulized, especially where such a germinal layer is in con- 

 tact with such a mass by a broken surface and conse- 

 quently by the lateral surfaces of its cells." 



'The formation commencing at this point proceeds 

 steadily and continuously through the yolk mass of the 

 undeveloped half of the egg. About the free margin of 

 the advancing germinal differentiation there are to be 

 found gradual transition stages between the undiffer- 



