ORIGIN OF NEURON PATTERN 199 



first polarization are not immediately reversible, these 

 cells may develop two growing regions at opposite poles 

 and become strictly biaxial. 



The second possibility is that the polarization giving 

 rise to the dorsomedially directed outgrowth is the first 

 effective polarization and that the peripheral outgrowth 

 is determined only after the central process attains con- 

 tinuity with the cord. If this is the case, the migration 

 of the cells before the outgrowths appear must be 

 regarded as due to mechanical or other factors and not 

 to polarization. 



The third possibility is that both processes may 

 represent reactions to polarization of the second type, 

 the central process with respect to the most negative 

 region of the cord, the peripheral with respect to the 

 less strongly negative surfaces of peripheral parts, e.g., 

 perhaps primarily the myomeres. 



Which of these possibilities or whether any of them 

 represent the actual course of events, it remains for the 

 future to determine. They are not entirely mutually 

 exclusive and each of them may be in part realized at 

 some stage. In any case, however, it is evident that 

 as soon as the centrally directed process attains con- 

 tinuity with the cord, the direction of growth of the 

 peripheral process is determined in the same way and 

 in the same general direction as that of the axons grow- 

 ing out from the ventrolateral regions of the cord 



(Fig. 58). 



As regards the differentiation of both outgrowths as 

 axons, various facts indicate that axon and dendrites 

 do not represent reactions of different kind but rather 

 of different degree, the more rapidly growing, more 



