no THE ORIGIN OF THE NERVOUS SYSTEM 



sides of the neuroblasts which face away from the central 

 canal, i.e., the surface originally external, and the first 

 outgrowth of the axons is usually away from the central 

 canal, though this direction may be altered by the axial 

 relations (pp. 185-87). In forms in which axiation is 

 but little developed the nervous system may be a more 

 or less diffuse nerve not developing in relation to the 

 body surface with little centralization, as in various 

 coelenterates. 



In animals with more highly developed axiate pattern 

 we find the chief receptors and the chief aggregations of 

 nervous tissue differentiating from the high ends of the 

 chief body gradients. This relation is most conspicu- 

 ous with respect to the major or polar gradient, the 

 apical end, or the head, with its special sense organs 

 and nervous ganglia, arising from the high end of the 

 gradient, but it also appears more or less clearly with 

 respect to the symmetry gradients (see p. 149); the 

 general direction of growth and differentiation of 

 nervous structure is down the gradients. Differentia- 

 tion of nervous structure begins at the anterior end and 

 progresses posteriorly (Figs. 13-17) and growth of 

 nerves is in general from central to peripheral regions, 

 particularly in the higher animals. The direction of 

 early axon outgrowth in the vertebrate neural tube, 

 while showing primarily a more or less definite relation 

 to the surface-interior pattern, as already noted, reacts 

 very early to the axiate pattern and the axons grow in 

 general postero-ventro-laterally, that is, down a resultant 

 of the chief body gradients (pp. 185-87). 



This definitely directed growth of nerves in relation 

 to the gradients is not limited to the higher animals. A 



