NERVOUS SYSTEM AND OTHER PARTS 1 29 



the dominant regions of the physiological gradients which 

 are present before the nervous system appears. I have 

 endeavored to show that these gradients are in the final 

 analysis responses to the differential action of external 

 factors upon living protoplasm, and if we admit this it 

 follows that the localization and the general pattern of 

 the centralized and cephalic nervous system is deter- 

 mined physiologically by a response of protoplasm to 

 environmental action upon it. Moreover, the central 

 nervous system is the chief organ of excitation and trans- 

 mission and the chief organ of physiological integration, 

 because it represents the working out in ontogeny and 

 phylogeny of the primary organismic integrating factors, 

 the excitation- transmission relations. 



The special characteristics of nervous structure as 

 distinguished from those of other organs are of course 

 determined by the hereditary constitution of the proto- 

 plasm concerned. As I have already pointed out, the 

 physiological gradients merely provide a framework or 

 plan as a basis for the realization of certain hereditary 

 potentialities, and with evolutionary changes in the 

 hereditary mechanism, this physiological plan, and 

 therefore the localization, structure, and relations of the 

 nervous system, undergo alteration. But there can be 

 no doubt that the earlier stages of the development and 

 differentiation of the nervous system are in large meas- 

 ure independent of other organs, and I have endeavored 

 to show that this independence is due to the fact that 

 the nervous system, more than any other organ, is 

 the expression of the primary physiological factors of 

 organismic pattern and integration. 



