ORIGIN OF NEURON PATTERN 191 



interesting case in the development of the Amphibia 

 is considered in another chapter (pp. 251-57). 



The aggregation of growing axons into nerve trunks 

 is apparently also readily accounted for in terms of 

 electrical polarization and growth. If the axon is 

 internally positive, either on account of growth activity 

 or later because of the passage of nervous impulses, it is 

 negative externally and must therefore increase the 

 positivity in that region of a later outgrowth which is 

 toward its surface (second type of polarization) . Conse- 

 quently the later axons will tend to grow along the sur- 

 face of those which have arisen earlier, except where 

 other regions of organs give rise to a sufficiently strong 

 electrical field to alter these relations. 



In the origin of the branches known as collaterals 

 from an axon local electrical fields more or less at right 

 angles to the axon direction may play a part in localizing 

 regions of high positivity and growth at various points 

 along the axon and so determining the occurrence of 

 budding. The axiate pattern of the axon suggests, 

 however, that internal physiological factors may also 

 play a part in this branching. The growing axon 

 unquestionably represents, for the time being, a physio- 

 logical axis with the most active region at its tip, and 

 some degree of dominance and subordination probably 

 exists between the tip and other regions within a certain 

 distance from it. I have repeatedly called attention to 

 the occurrence of physiological isolation in relation to 

 such gradients and to its significance for the processes 

 of budding and branching in multiaxial plants and 

 animals (see chap. v). The axon with its branches 

 bears a close resemblance to the growth forms of various 



