92 MARIAN L. SHOREY 



with the result as given above. Repetitions of this experiment 

 could not be continued because it soon became impossible to 

 obtain Necturus with undifferentiated neuroblasts. 



Regarding the fibers that develop, it might be added, that in- 

 stead of one long process, with, perhaps a number of much shorter 

 protoplasmic outgrowths, characteristic of motor cells of the 

 spinal cord, there are often a number of processes of equal length. 

 A possible explanation of this is that in the embyro the movements 

 of the lymph are such that a greater stimulus reaches the cell at 

 the point where the longest fiber forms, while in the culture 

 medium the substances which act as a stimulus are equally 

 diffused. Occasionally swellings were noted at the ends of the 

 fibers, as Harrison has described for the frog, but these were not 

 characteristic. 



At the same time a culture medium made according to the same 

 formula as the one used above, with the beef extract omitted, was 

 prepared, and drop cultures with neuroblasts from the same em- 

 bryos were made. But although many of these were prepared 

 and examined, more than of those containing beef extract, and 

 the cells were still in a vigorous condition at the end of two 

 weeks, no development of fibers, and no change in the shape of 

 the cells could be observed. 



Do these facts justify a belief that the neuroblasts are stim- 

 ulated to differentiate through the presence of the metabolic 

 pioducts of muscular end organs? Of themselves they do not 

 furnish crucial evidence, since in many instances differentiation 

 does not occur even in the presence of these products. This may, 

 however, be accounted for by considering the condition of the 

 cells at the time when they are placed in the culture medium. It 

 is probable that the the change in metabolism, from the com- 

 pletely undifferentiated to the adult cells, is a gradual one in both 

 nerves and muscles, and it is quite possible that the two keep 

 pace, and that a neuroblast must reach a certain stage of develop- 

 ment before it can be stimulated by the metabolic products of 

 adult muscles. The fact that in all cases in which I have been 

 successful in producing nerve fibers the neuroblasts were taken 



