ORIGIN OF NEURON PATTERN 187 



in the neural tube. Figures 60 and 61, cross-sections of 

 the cord of the chick and of the fish in early stages of 

 neuron development, show very clearly the localization 

 and directions of growth of the early axons and indicate 

 their very definite relation to the central canal in the cells 

 nearest it and to the dorsiventral direction in others. 

 In short, the neuroblasts are apparently reacting to the 

 physiological gradients, and the only factor in these 

 gradients which we can at present conceive as deter- 

 mining polarity and direction of outgrowth is the elec- 

 trical factor, acting through the polarization of the cells. 



According to this interpretation, the region of origin 

 of the axon in the neuroblast represents the high end of 

 a physiological gradient, electrically determined in each 

 cell, by the electric currents of its environment. The 

 direction of axon growth may likewise be regarded as 

 electrically determined but as a process of electrical 

 and consequent physiological polarization rather than 

 through a simple electrical attraction. Harrison (1908, 

 1910) has shown that the growing tip of the axon is 

 amoeboid and sends out pseudopodia, not merely in the 

 direction of growth, but in various other directions 

 (Fig. 56), and various axon tips in Figure 60 show 

 similar conditions. The direction of pseudopod exten- 

 sion in the growing tip is evidently not determined by 

 the electric gradient, but in general the pseudopodia 

 which extend in the direction of the gradient will tend to 

 persist, extend, and grow, because they, and particularly 

 their tips, represent the regions of highest positivity. 



An apparent migration of the cell body in the direc- 

 tion of axon outgrowth, i.e., toward the cathode of the 

 current, is a feature of these early stages, at least in the 



