176 THE ORIGIN OF THE NERVOUS SYSTEM 



Kappers' hypothesis may be illustrated by the 

 accompanying diagram (Fig. 57). In this diagram a 

 represents the source of stimulation which Kappers 

 postulates and which is electronegative externally; d, ax is 

 a neuroblast surrounded by a fluid medium containing 

 electrolytes. The current produced by a has its nega- 

 tive pole on the surface of a and its positive pole in 

 some more or less distant region, indicated by the 

 sign+. The return current must pass in the opposite 

 direction through body tissues. The neuroblast exposed 



ax 



FIG. 57. Diagram illustrating Kappers' conception of the action 

 of the bioelectric current on the neuroblast. For explanation see text. 



to the current arising from a develops an anodic exten- 

 sion ax, which, because of its greater permeability to 

 anions, becomes electronegative and gives rise to the 

 axon growing toward the positive pole because of the 

 attraction between it and this pole. Later the opposite 

 side of the cell gives rise to the dendritic outgrowth d, 

 which contains the supposedly acid and electropositive 

 Nissl granules and is therefore attracted by the cathode 

 and grows toward it. 



While this ingenious hypothesis of Kappers' is of 

 great interest as an attempt to interpret the polarity and 

 external morphology of the neuron in terms of a reaction 

 to an electric current, it seems to me to present certain 



