138 THE GERM-PLASM 



in doing so : he asks, in fact, whether the uninjured segmenta- 

 tion-sphere of the frog would not behave exactly in the same 

 manner as that of the sea-urchin if it could be actually isolated, 

 instead of remaining in close connection with the other injured 

 sphere. Thus even the apparently incontrovertible result of this 

 experiment may be doubted. 



It seems to me that careful conclusions, drawn from the 

 general facts of heredity, are far more reliable in this case than 

 ; j are the results of experiments, which, though extremely valuable 

 and worthy of careful consideration, are never perfectly definite 

 and unquestionable. If what was said in support of the theory 

 of determinants in the first chapter of this book be borne in 

 mind, the conviction that ontogeny can only be explained by 

 evolution, and not by epigenesis, seems to force itself upon us. 

 It would be impossible for any small portion of the skin of a 

 human being to undergo a hereditary and independent change 

 from the germ onwards, unless a small vital element correspond- 

 ing to this particular part of the skin existed in the germ-sub- 

 stance, a variation in this element causing a corresponding 

 variation in the part concerned. Were this not the case, '■ birth- 

 marks ' would not exist. If, however, determinants are contained 

 in the germ-plasm, these can only take part in controlling the 

 formation of the body if, in the course of embryogeny, they 

 reach those particular cells which they have to control, — that 

 is to say, if the differentiation of a cell depends primarily on 

 itself^ and not on any external factor. 



If therefore ontogeny is not, as Roux aptly expresses it, a 

 < new formation ' of multiplicity, or an epigenesis, but is merely 

 the unfoldi7ig of multiplicity, i.e. an evolution, — or, as it might 

 also be called, the appeaj-ance of a previously invisible multi- 

 plicity., — the principle of self-determination is certainly only 

 established with regard to the egg as a whole: the self-deter- 

 mination of each cell, and its control of ontogeny, do not neces- 

 sarily follow from this conclusion. We can only thereby arrive 

 at the very simple assumptions, that the primary constituents of 

 the germ-plasm are distributed by means of the processes which 

 can actually be observed in the nuclear divisions, so that they 

 come to be situated in those regions which correspond to the 

 various parts of the body, and that those primary constituents 

 are present in each cell which correspond to the parts arising 

 from it. 



