220 THE ORIGIN OF THE NERVOUS SYSTEM 



but in various other ways. The behavior of the impulses 

 with respect to the two directions not only constitutes 

 additional evidence for the existence of the gradient, 

 but is, I believe, inexplicable on any other basis. In 

 this behavior we find in the first place dominance and 

 subordination in the normal transmission, in which all 

 levels are subordinate to the apical end, although each 

 level is capable of initiating an impulse. Second, by 

 decreasing experimentally apical dominance or by excit- 

 ing subordinate levels to a sufficient degree, physiological 

 isolation may be brought about, and, according to con- 

 ditions, the isolated region either retains the original 

 gradient or develops a new one, as in the case of reversed 

 transmission. In fact, the plate row constitutes an 

 example, almost diagrammatic in its clearness, of the 

 dynamic relations of parts as determined by a physio- 

 logical gradient. Whether the relations depend upon 

 the presence of nervous structure or upon neuroid trans- 

 mission is of minor importance from the physiological 

 viewpoint. In any case the plate row exhibits the most 

 various degrees of integration of the organismic type, 

 and may, I believe, be regarded almost as a model of a 

 physiological gradient. 



CERTAIN AXIATE ORGANS AS PHYSIOLOGICAL 

 GRADIENTS 



It has already been shown that certain axiate organs 

 behave in many respects much like the ctenophore plate 

 row and probably further physiological analysis will show 

 similar conditions in many other axiate organs. The 

 vertebrate heart is a case in point. In its earlier 

 embryological stages the heart is a tubular structure 

 and Hyman has found that in both the fish and the 



