THE PROBLEM OF NEURON PATTERN 171 



the presence of the chemical substances to which reaction 

 is supposed to occur and the proper concentration gradi- 

 ents must be assumed. Moreover, in a neuron like the 

 pyramidal cell (Figs. 46, 54) the reaction of axon and 

 primary dendrite, if to the same substance, must be 

 opposite in character, or they must be reacting to 

 different substances. The absence of definite orienta- 

 tion of the dendrites of some neurons in earlier stages, 

 and its appearance later, and the sudden changes in 

 direction of growth of both axon and dendrites make it 

 necessary to assume that the reaction of the outgrowths 

 changes, or that the concentration gradients of the sub- 

 stances change their direction, or that new substances 

 appear with concentration gradients in different direc- 

 tions from those of the original substances. Again the 

 establishment of functional connections between differ- 

 ent neurons and between nerves and muscles and other 

 peripheral organs necessitates various other assumptions. 

 While directed response to chemical factors may play 

 some part in neuron development, any attempt to 

 interpret the process entirely in terms of such response 

 is a pure speculation with many difficulties in the way. 

 It may perhaps appear at first glance that the situa- 

 tion is not much better as regards the interpretation in 

 terms of electrical response, and so far as definite 

 experimental data on the neuron itself are concerned, 

 this is the case. Certain facts are at hand, however, 

 which seem to me to be highly significant in this 

 connection. In the first place, the existence of the 

 axial physiological gradients and of gradients in electrical 

 potential as one of their characteristics has been demon- 

 strated beyond question, and we find a very definite 



