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 



