172 THE ORIGIN OF THE NERVOUS SYSTEM 



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relation between the directions of growth of the axons 

 which arise in the earlier stages of development and 

 these gradients. Second, from what we know concern- 

 ing other developmental processes, there is good reason 

 to believe that the establishment of regions of rapid 

 growth in the nervous system, as well as in other organs, 

 involves a change in electrical potential in those regions. 

 Admitting this, it is evident that in the course of develop- 

 ment there must be changes, both in the intensity of 

 potential and in the direction of the potential gradients. 

 Third, it is a familiar fact that conduction of the nervous 

 impulse is accompanied by marked change in electrical 

 potential, and as soon as any neuron, neuron group, or 

 axon, or axon bundle becomes functionally active in this 

 way, changes in the potential gradients in its vicinity 

 must appear. In short, the existence of gradients in 

 electric potential with the resulting electric currents 

 and the changes in direction and intensity of these 

 gradients during development and in relation to function 

 must be a factor in determining the physiological con- 

 dition of the cells affected, and so constitute a real 

 basis of fact for attacking, at least tentatively, the 

 problem of neuron development. On the other hand, 

 it is not demonstrated, though many facts make it 

 highly probable, that the neuroblast or the neuron is 

 capable of response in any way to such electrical gradi- 

 ents, or, more strictly speaking, to the electrical cur- 

 rents which arise in connection with them. Moreover, 

 the difficulties which arise in the matter of the opposite 

 orientation of axon and dendrites in certain neurons and 

 the absence of orientation in some or all dendrites in 

 others, remain. But in spite of the lack of expcii- 



