152 THE ORIGIN OF THE NERVOUS SYSTEM 



origin. It is evident that in the more primitive con- 

 ditions the localization of these cells is determined 

 primarily by the surface-interior, rather than the axiate 

 pattern. The whole ectoderm as that part of the 

 organisms most directly related to the external world 

 may be regarded as potentially nervous. It is obvious, 

 however, that in the course of evolution the axiate 

 pattern has become predominant over the general 

 surface-interior pattern. The whole hereditary mechan- 

 ism concerned in reproduction provides in more and 

 more definite ways as evolution proceeds for the estab- 

 lishment of such axiate pattern in the egg or for its 

 persistence through the reproductive process. At the 

 same time the surface-interior pattern, while still 

 evident in many ways in the higher forms, becomes less 

 and less important as a primary factor in differentiation. 

 Doubtless the phylogenetic development of protective 

 coverings of the egg and early embryonic stages is 

 concerned in these changes, the greater degree of pro- 

 tection of the body surface by egg coverings, etc., from 

 the direct action of external factors in the early develop- 

 mental stages of higher forms, determining that the 

 general surface-interior gradients shall be less definite 

 and not so far above the general level of intrinsic activity. 

 With this phylogenetic change, the evolution of axiation, 

 the relation of nervous differentiation to the regions of 

 most intense metabolism in the embryo appears in the 

 shifting of the region of differentiation of the receptor 

 cells from the general body surface toward the high 

 regions of the axiate pattern and they, like the motor 

 neurons, send fibers peripherally, i.e., down the chief 

 axial gradients. 



