170 THE ORIGIN OF THE NERVOUS SYSTEM 



except possibly in earlier stages of its development. 

 Not only its growth but its maintenance depends upon 

 its connection with the cell body, as the experiments on 

 degeneration and regeneration have demonstrated. 

 There can be little doubt, however, that the dendrites 

 are important nutritive organs as well as conductors of 

 impulses. In all probability they are primarily nutritive 

 and continue this function after they become conductors. 

 The axon arises at a definite region of the cell and 

 usually grows in a definite direction for at least some 

 distance, but the dendrites very commonly, though not 

 in all neurons, arise on various regions of the cell surface, 

 often on all sides of it and grow in all possible directions, 

 at least during earlier stages. In some neurons, how- 

 ever, such as the pyramidal cell of the cortex (Fig. 46), 

 a primary dendrite develops on the opposite side of the 

 cell from the axon and grows at first or continuously in 

 a definite direction (Fig. 54) . In such cases other later 

 dendrites may show other and various directions of 

 growth. 



So far as we are able to conceive the conditions in 

 the developing nervous system, there are apparently 

 only two sorts of conditions in the environment of the 

 developing neurons which can possibly determine the 

 localization or the direction of growth of either axon or 

 dendrites. The one of these is chemical, the other 

 electrical. 



The attempt to interpret the localization and direc- 

 tions of outgrowth of axon and dendrites as a response 

 of some sort, whether chemotactic in the strict sense or 

 not, to chemical substances in the environment of the 

 cells meets with various difficulties. In the first place. 



