230 THE ORIGIN OF THE NERVOUS SYSTEM 



certainty. The synapse may be a membrane of this 

 sort as regards excitation, but we know nothing of the 

 mechanism of such irreversibility. Since this theory of 

 the synapse is still in large measure based on assumption 

 rather than on experimental data, it is perhaps allowable 

 to point out the possibility of a different interpretation 

 of some of the characteristics of the reflex arc, ordinarily 

 attributed to the synapse. 



The functional polarity of the neuron is evidently 

 quite distinct from its developmental polarity, as it 

 exists during the growth period. It is clear that a 

 relation of functional dominance and subordination 

 exists between neurons, and such a relation may depend 

 in part upon the order in which the neurons of a given 

 path attain the stage of functional conduction of 

 impulses and in part upon the frequency or the quanti- 

 tative characteristics of functional impulses in other 

 neurons in their vicinity. The facts indicate that the 

 neurons of the higher levels of a particular gradient 

 develop more rapidly than those of lower levels, and it 

 is also probable that the nervous impulse represents, at 

 least at first, either a steeper or a higher wave of excita- 

 tion in a' neuron of higher than in one of lower levels of 

 a gradient. If differences of this sort exist, they must 

 constitute important factors in determining not only 

 the morphological connections of different neurons but 

 the functional relations between them and particularly 

 such matters as "openness" or "resistence" of func- 

 tional paths. 



As regards the pecularities of inter-neuronic conduc- 

 tion which are usually attributed to the synapse, the 

 possibility that certain characteristics of the dendrite 



