THE REFLEX ARC 257 
the cord do not extend so far anteriorly but end at 
lower levels. Still later, as the growth activity and the 
internal electropositivity decrease at one level after 
another in the posterior direction and the gradient of 
the head region in the primary direction extends farther 
and farther posteriorly, this reflex mechanism of the 
earlier stages disappears and others take its place, 
following the gradient changes, until the definitive condi- 
tion is attained. This case has been considered at some 
length because it seems highly significant. The appear- 
ance in the salamander of the early reflex mechanism so 
different in character from the definitive mechanisms and 
its complete disappearance later remain completely inex- 
plicable facts, except in terms of the gradients as experi- 
mentally demonstrated. Viewed in relation to the 
gradients, however, these facts receive a simple physio- 
logical interpretation and fall into line with many others. 
Various other features of nervous development will 
probably be found to be related to the general or to 
local gradients or both, as they exist at the various stages. 
In general a certain similarity exists between the 
structural and the functional characteristics of a path 
in relation to the gradients. This similarity appears in 
the differences between definitive upward and down- 
ward paths in man and other mammals. We may 
expect that a path which develops and functions up a 
general gradient will be made up of relatively short 
neurons and will show a high "resistance," while a path 
in the opposite direction will be relatively simple and 
open. Even the more primitive transmission paths 
exhibit a more or less definite functional direction with 
respect to physiological gradients present, excitation 
