DIFFERENTIATION OF NEUROBLASTS 87 



have activated its development, there are strong logical grounds 

 against such an hypothesis. If tissues are self-differentiating, 

 it is impossible to account for their interdependence in the adult 

 without postulating a physiological discontinuity in the life of 

 the individual cells. In this connection the relation between nerves 

 and muscles will serve as an illustration. It is well known that the 

 life of a nerve can not be maintained after the extirpation of its end 

 organ ; and if it can not be, then it must be due to its inability to 

 carry on its normal metabolic processes. If it can do this indepen- 

 dently during development, then there must come a point at which 

 embrj^onic physiological processes are checked and adult processes 

 begin. To avoid this difficulty a functional and a pre-functional 

 period have been postulated, but since the functions which the 

 cell performs for the body as a whole are merely incidental to 

 the processes necessary for its own existence, it is evident that if it 

 performs different offices at different times, it must be due solely 

 to the fact that its metabohsm differs. The cell once differen- 

 tiated goes on building up protoplasm of the same architectural 

 pattern during the rest of the life of the organism, therefore, the 

 point at which it begins to assume the offices which it performs 

 for the adult must begin at the beginning of differentiation and 

 not at the end. Any cell which, in the adult organism, had no 

 physiological inter-relation with some other cell would be a foreign 

 body, not an organic part ; if the forces acting in its differentiation 

 are not identical with those acting in its physiological relations, 

 then the structure established at the time of differentiation could 

 not be maintained. 



In view of the above considerations, I would suggest as a work- 

 ing hypothesis, that the development of different kinds of tissue 

 may be regarded as the result of a chain of chemical reactions and 

 inter-reactions between ooplasmic or organ-forming substances, 

 and that any given tissue will be formed only when a given pri- 

 mordium is acted on by a definite force external to itself; and 

 while pressure, contact, and forces external to the organism, may 

 play a part, one of the sources of stimulation will always be found 

 to be the metabolic products of other tissues. 



How may the truth of such an hypothesis be tested? 



The Journal of Experimental Zoology, Vol. 10, No. I 



