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 
