ORIGIN OF NEURON PATTERN 185 



these two types of electrical polarization in the cells of 

 the developing nervous system and their role in the 

 differentiation, direction of growth, and connections 

 of the neurons. 



AXON DEVELOPMENT 



In the localization and development of at least the 

 earlier axons in the vertebrate central nervous system 

 the conditions represented in Figure 58 seem to be more 

 or less exactly realized. The embryonic neural tube 

 constitutes a compact tissue and is the region of highest 

 internal positivity in the embryo. Moreover, the region 

 of highest internal positivity in this tissue is probably 

 primarily that adjoining the original external surface 

 (see p. 26), which becomes the surface of the central 

 canal after closure of the tube (see Figs. 17, 62, 63). 

 Secondarily, the edges of the neural folds which become 

 the dorsal regions of the neural tube are, to judge from 

 their susceptibility, the most active regions of the tube 

 preceding the closure and therefore must represent 

 regions of high positivity internally, as compared to 

 more ventral regions. We may expect then to find 

 that in the transverse section of the neural tube the 

 electropositive regions of the neuroblasts as determined 

 by electrical polarization, will appear primarily on the 

 sides of the cells away from the central canal and from 

 the dorsal region, or in the resultant of these two gradi- 

 ents. Assuming that the region of highest positivity in 

 each cell becomes the region of most rapid growth, 

 extensions or outgrowths of the cells will appear on the 

 sides of the cells away from the central canal, or the 

 dorsal region, or in the resultant of these two directions 

 and will grow laterally, or ventrally, or ventrolateral ly 



