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 
