194 THE ORIGIN OF THE NERVOUS SYSTEM 



is usually less rapid, in some neurons they arise from 

 all parts of the cell surface and some of them may show 

 definitely directed growth, while others belonging to 

 the same neuron grow in various directions, and in 

 many cases only certain dendrites persist, others under- 

 going resorption in the course of development. More- 

 over, the dendrites undoubtedly are of great importance 

 as nutritive organs of the neuron, as well as organs of 

 conduction. 



As regards the factors concerned in dendrite develop- 

 ment, it appears necessary to recognize two distinct 

 aspects or phases of the developmental process, the 

 origin and localization of the dendrites on the cell body 

 and the later orientation and definitely directed growth 

 of some or all of them. In connection with dendrite 

 origin two possible factors suggest themselves, viz., 

 physiological isolation and the nutritive relations of the 

 cell with its environment. Physiological isolation may 

 lead to budding on the cell body as well as on the axon. 

 The fact that the dendrites usually arise only when the 

 axon tip has attained a considerable distance from the 

 cell body, or has ceased to grow, favors this possiblity. 

 From this viewpoint the origin of the dendrites is in 

 some degree similar to the origin of the basal stolons in 

 hydroids and perhaps of the root system in plants. But 

 it is also possible that nutritive conditions in the cell 

 body play a part in determining these outgrowths as 

 they apparently do in root formation. 



As regards the orientation and definitely directed 

 growth of dendrites, we must apparently look to the 

 electrical factor. By the time the dendrites arise, the 

 developing neurons have become more or less separated 



