242 THE ORIGIN OF THE NERVOUS SYSTEM 



Moreover, the excitation arc may arise not only within 

 a single cell or within a small group of adjacent cells, 

 but also in relation to the axiation and integration of 

 the body as a whole. In other words, not merely intra- 

 cellular gradients and local multicellular gradients, but 

 the general axial gradient or gradients of the whole 

 body may constitute the basis of excitation arcs and 

 so of nervous function and structure. My point is 

 then that intracellular arcs, local multicellular arcs, 

 and what we may call general axial arcs may all exist 

 in the same individual, and so far as nervous develop- 

 ment and differentiation is concerned, all be equally 

 primitive. It is not necessary to assume that nervous 

 structure was at first completely diffuse and equally 

 distributed in all parts and that centralization occurred 

 only in the course of evolution. If the physiological 

 gradients in the body constitute the basis of the localiza- 

 tion of nervous structure, we must expect to find more 

 or less nervous centralization in any axiate animal in 

 which nervous structure differentiates. A completely 

 uncentralized system is possible only when axiate 

 pattern as regards the body as a whole is absent, but, 

 as a matter of fact, all animals in which a nervous 

 system differentiates are more or less axiate. Whether 

 the unicellular arcs, the local, or the general axial arcs 

 are the more important in a particular case must depend 

 upon the degree of development of physiological axiation 

 and so of physiological integration in the body as a 

 whole. In the medusa, a motile form, the development 

 of axiation is higher and the general arcs as represented 

 by the marginal sense organs, the ganglionic aggregation, 

 and the nerve net are evidently more highly developed 



