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 time 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 
