200 THE ORIGIN OF THE NERVOUS SYSTEM 



active, and probably more highly polarized region 

 becoming an axon, the less active and less highly 

 polarized, or perhaps not primarily polarized, a dendrite. 

 Both outgrowths being highly polarized and active in 

 this case, both develop as axons and the functional 

 polarity of these cells, i.e., the direction in which they 

 conduct, is determined, not by the fact that one out- 

 growth represents a dendrite, the other an axon, but by 

 their relations to other parts. The reason for the 

 occurrence in the cell body of chromatolysis following 

 the section of the peripheral and not that of the central 

 outgrowth, is not at present clear. It may be that the 

 peripheral outgrowth makes greater demands on the 

 cell body in the matter of nutrition and maintenance 

 than the central, and therefore the effect on the cell 

 following section of it is greater than that following 

 section of the central outgrowth. The later change of 

 the spinal ganglion neurons from the "bipolar" to the 

 "monopolar" type by the approach and union of the 

 bases of the two outgrowths is perhaps the result of 

 "neurobio taxis," which is considered below. 



KAPPERS' " NEUROBIOTAXIS " OF THE CELL BODY 



Kappers and others have shown that in many 

 neurons and neuron groups the cell body undergoes an 

 apparent migration in the direction of growth of the 

 chief dendrites, i.e., toward the region with which the 

 neuron or group is functionally correlated. So far as 

 this migration is an ontogenetic, not a phylogenetic, 

 process, it is doubtless determined by the factors which 

 direct the growth of the dendrites and probably results 

 from some degree of polarization of the cell body. On 



