DIFFERENTIATION OF NEUROBLASTS 89 



be harmonized is that it is the metabolic products of muscles 

 which act as a stimulus. My previous work furnished no experi- 

 mental evidence regarding the means by which the muscles in- 

 fluence the nerves, while Harrison's experiments are open to the 

 objection that in the lymph the products of muscular activity 

 might, and almost certainly would, be present. 



In order to test the truth of my hypothesis by this method, 

 both positive and negative results must be obtained. That is, a 

 medium must be found in which neuroblasts remain alive and 

 active, but do not develop nerve fibres, while if some substance 

 or substances produced by the activity of muscles is added, fibres 

 do develop. 



I first attempted experiments with portions of the medullary 

 canal of the chick, but these results were always negative, with 

 two possible exceptions. Pieces of the canal or isolated neuro- 

 blasts may be kept alive for days in a variety of culture media. 

 White of egg, gelatin solutions containing peptone or beef ex- 

 tract, solutions obtained by allowing muscular tissue from chicks 

 or adult fowls to stand for a few hours in normal salt solution, 

 lymph from a number of different kinds of animals, if kept with- 

 out infection, will all serve as nutrient material for a period of 

 from three to at least fourteen days. That the cells are alive can 

 not be doubted, for after a few careful observations, one can not 

 fail to recognize evidences of degeneration if it is present. J can 

 not affirm with certainty that division ever occurs under these 

 conditions, f 01 while I have occasionally seen contiguous cells which 

 from all appearances, were just completing the process of divis- 

 ion, I have never seen it in sufficiently early stages to be convinced. 



But while in hundreds of cases the neuroblasts have remained in 

 a vigorous condition for days after fibres would normall}^ develop, 

 on only two occasions, as stated above, has there been any indi- 

 cation of this. As no growth was seen after what may possibly 

 have been nerve fibers were first observed, and as no method 

 of staining was attemptd with these, and numerous repetitions 

 of the same conditions failed to produce the same results, I am 

 still very skeptical regarding them. It should perhaps, be added, 

 that in both of these cases extracts from muscles were present. 



