CENTRALIZATION AND CEPHALIZATION 153 



From this point of view this process of shifting in 

 position is physiologically similar to certain features of 

 the process of cephalization, e.g., the phylogenetic 

 migration of segmental ganglia toward, and their aggre- 

 gation in, more anterior regions. The localization of 

 nervous structure in any particular organism is pri- 

 marily an expression of the localization of the regions 

 of higher rate of the fundamental dynamic activities in 

 that organism and the progressive growth and differen- 

 tiation of nervous structure is primarily from the regions 

 of more intense to those of less intense activity. The 

 statement that the whole ectoderm is potentially nervous, 

 a statement which has been repeatedly made, means in 

 terms of this conception that wherever the fundamental 

 physiological activity of the ectoderm cells is sufficiently 

 intense there nervous structure will arise, provided the 

 protoplasm is capable of recording this activity in per- 

 manent structural differentiation. The evolutionary 

 progress of nervous centralization and cephalization 

 signifies that, as the axiate pattern becomes more and 

 more predominant over the primitive surface-interior 

 pattern, it plays a larger part in determining the localiza- 

 tion of the physiological conditions which underlie the 

 development of nervous structure. And since these 

 conditions are evidently associated with the higher levels 

 of metabolic or oxidative activity in protoplasm the 

 development of nervous structure becomes more and 

 more definitely localized in the high regions of the axial 

 gradients. 



CONCLUSION 



The phylogenetic changes in localization and aggre- 

 gation of nervous structure are of course not independent 



