ORIGIN OF NEURON PATTERN 181 



region about the cathode, the region in which the nerve 

 becomes internally positive and externally negative, is 

 increased (catelectrotonus) , and if the current is strong 

 enough the cathode gives rise to a nerve impulse when 

 the circuit is closed. A region of the nerve about the 

 anode, however, where the nerve becomes externally 

 more positive and internally more negative, shows 

 decreased irritability while the current is passing, but 

 the anode may give rise to a nerve impulse when the 

 circuit is broken, that is, the disappearance of the 

 depression constitutes an excitation. Electrotonus is 

 apparently merely a special case of the physiological 

 effect of electrical polarization in highly specialized 

 tissues. 



Electrical conditions as factors in development and 

 differentiation have received comparatively little atten- 

 tion of late years, though they are probably of consider- 

 able importance, particularly in the case of highly 

 susceptible cells, such as the neuroblasts, which in early 

 developmental stages are the most susceptible cells of 

 the organism. Concerning the presence of differences 

 in electrical potential and of electric currents there can 

 be no doubt, for, as I have pointed out (p. 44), they are 

 characteristic features of the physiological gradients 

 from the earliest stages on, and their existence in con- 

 nection with the specialized activities of later stages 

 has been abundantly demonstrated. It is of course not 

 necessary to assume that electrical factors are the only 

 factors concerned in determining the morphological 

 polarity, directions of outgrowths, and connections of 

 the neurons. Various other factors may play a part, 

 but it appears impossible to find any other than the 



