198 THE ORIGIN OF THE NERVOUS SYSTEM 



are axons, as they appear to be, is answered differently 

 by different authorities. . Some, taking into account the 

 fact that the centrally directed outgrowth arises first and 

 conducts away from the cell body, while the peripheral 

 outgrowth appears later and conducts toward the cell 

 body, regard the central outgrowth as axon, the periph- 

 eral as dendrite. When the peripheral process is 

 separated from the cell body, chromatolysis occurs in 

 the latter, as in other neurons after separation of the 

 axon from the cell body, and this fact has led some to 

 regard the peripheral process as the axon. Still others 

 conclude from the structural characteristics that both 

 processes are axons. 



As regards the developmental factors concerned in 

 this process of differentiation, three possibilities suggest 

 themselves. The first is that the cells become electrically 

 polarized while they are still continuous with the cord 

 as the flaps mentioned above. Such polarization is of 

 the first type (Fig. 58), and the region of high positivity 

 determined by it must be ventrolateral. If this is the 

 case, we should expect these cells to give rise first to 

 axons growing ventrolateral^, but the ventrolateral 

 migration of the cells may be a reaction to this polariza- 

 tion and may delay the axon outgrowth. As these cells 

 lose their continuity with the cord, its electronegative 

 external surface must tend to produce a second polari- 

 zation in them , with the region of high positivity toward 

 the negative surface, i.e., the dorsolateral regions of the 

 cord. This is a polarization of the second type (Fig. 59) , 

 and it may also be a factor in retarding the outgrowth of 

 the peripheral process because it tends to obliterate and 

 reverse the primary polarization. If the effects of the 



