io8 THE ORIGIN OF THE NERVOUS SYSTEM 



is rather that nervous function develops in some way 

 from the functions of excitation and transmission which 

 are characteristic of all protoplasm. If this view is 

 correct, some sort of physiological continuity must exist 

 between excitation and transmission in general, and 

 development and function of the nervous system. 



In forms possessing it the nervous system is not only 

 the chief organ of excitation and transmission but also 

 the chief organ of physiological integration, and the 

 physiological factors concerned in its origin in ontogeny 

 must be factors connected with organismic integration 

 in general. Chemical, transportative, or hormonic re- 

 lations throw no light on the problem, and the only 

 logical procedure is to seek these factors in the general, 

 non-nervous excitation-transmission relations. Admit- 

 ting that the physiological gradients originate in 

 excitation-transmission relations and that they repre- 

 sent fundamental features of organismic pattern and 

 integration, the developmental relations between these 

 gradients and nervous structure and function become 

 highly significant. 



THE LOCALIZATION OF NERVOUS STRUCTURE 



In the first place the localization of nervous differ- 

 entiation shows a very definite relation to surface- 

 interior pattern. The primary receptors develop at the 

 surface, and the nervous system as a whole is primarily 

 of ectodermal origin and even in the higher animals 

 arises superficially. Even in the neutral tube of verte- 

 brates we find indications of the influence of the surface- 

 interior factor in the localization and direction of 

 out-growth of the earlier axons. These axons arise on the 



