DIFFERENTIATION OF NEUROBLASTS 93 



from the medullary canal in a comparatively late stage of de- 

 velopment lends a certain amount of support to this suggestion. 



A greater objection is found in the fact that with a commercial 

 extract of muscle, or indeed in any extract, many substances 

 might be introduced which are not specifically the products of 

 muscular metabolism. No attempt was made to discover what 

 substances in the beef extract were the source of the stimulus, 

 and, therefore, as far as these experiments alone are concerned, 

 it might equally as well be some inorganic salt which may be found 

 in it. But taken in connection with the fact that when the pri- 

 mordium of a muscle is destroyed, development of the nerves 

 does not occur, it is certainly added testimony for the belief 

 that the products of muscular metabolism are the specific source. 



But if there may still be some doubt regarding the source of the 

 stimulus, the present experiments have, I believe, confirmed the 

 conclusion previously reached that neuroblasts are not self-dif- 

 ferentiating. For the possiblity of the presence of a specific in- 

 hibitory substance in all the varieties of indifferent media used 

 can hardly be maintained. 



