256 THE ORIGIN OF THE NERVOUS SYSTEM 



Amphibia, this is the period during which the reflex 

 mechanism above described develops. In these experi- 

 mentally demonstrated physiological axial relations we 

 find, I believe, a simple basis for interpretation, not 

 only of the remarkable reflex mechanism of early stages 

 which Coghill has described, but of the progressive 

 changes and the definitive condition. If the suggestions 

 advanced in chapter xi concerning the significance of 

 electrical polarization in determining neuron patterns 

 are correct, there is no difficulty in understanding why 

 axons growing down a general axial gradient are long 

 and those growing in the opposite direction short. As 

 I have suggested (p. 190), the growth direction of those 

 growing up the general gradient is determined by local 

 gradients arising from other neurons in the vicinity, or 

 from other local conditions, and opposite in direction to 

 the general gradient. The axon tip will remain posi- 

 tively polarized and will therefore grow only as far as 

 the electrical factors of this gradient over-compensate 

 those of the general gradient. 



In the early stages of salamander development the 

 upward sensory paths in the cord are growing down 

 the general axial gradient as it exists at that time in 

 that region, and they grow approximately to the level 

 at which this gradient is compensated by the cephalic 

 gradient. The downward motor path, on the other 

 hand, is growing against the general gradient, and the 

 length of each axon probably represents approximately 

 the length at that tune of the local gradient which 

 determines it. In later stages, as this secondary 

 developmental gradient becomes less marked, the sen- 

 sory paths developing from the more posterior levels of 



