ORIGIN OF NEURON PATTERN 193 



distance, or until it enters some other electrical field 

 capable of altering its direction. 



The development of a terminal -arborization on an 

 axon may result in some cases from its approach toward 

 an externally negative region of large size. Under 

 such conditions the growing tip will tend to spread 

 out and give rise to various outgrowths, all growing 

 more or less toward the negative surface. Again, 

 arborization may perhaps result from the cessation of 

 the action of the previously directing field, so that 

 growth becomes indefinite in direction or responds to 

 various local conditions, perhaps determined by adjoin- 

 ing neurons. 



If these suggestions are correct, the axon behaves 

 during its growth period in many respects like other 

 physiological axes, and its origin, like that of other axes, 

 is determined by a quantitative differential in the action 

 of a factor external to it. The appearance or non- 

 appearance of medullation on the axon is of course a 

 matter of differentiation and therefore in part deter- 

 mined by the hereditary constitution of the protoplasm, 

 though within the same individual differences in rate 

 of metabolism in the developing axon are perhaps con- 

 cerned in determining whether medullation shall appear 

 or not. 



DENDRITE DEVELOPMENT 



The problem of the factors concerned in the origin 

 and development of the dendrites appears much less 

 simple than the problem of the axon, but the facts 

 afford a basis for certain suggestions. In general the 

 dendrites appear much later than the axon, they are 

 evidently less highly specialized structures, their growth 



