264 THE ORIGIN OF THE NERVOUS SYSTEM 



undergo modification with the other features of the 

 gradients. The cerebral cortex apparently arises in 

 organisms in which the primary gradients become so 

 modified in the course of development that a truly 

 central region is localized, which may be affected to 

 some extent by changes of potential in any part of the 

 organism. This being the case, the electrical situation 

 in the region from which the cortex develops must be 

 characteristically different in some way from that in 

 other regions. More specifically stated, this region may 

 be conceived as a region which represents not merely a 

 certain level in the general axial gradients but which is 

 affected, for example, by the electrical changes con- 

 nected with localized developmental activity in various 

 organ primordia, first of all the receptors of the head. 

 Later, as the changes which have been briefly discussed 

 in the preceding section occur, as various parts of the 

 nervous system successively become functional in a 

 specifically nervous way and as the upward paths in 

 the cord develop, progressive modification and complica- 

 tion must occur. 



The cortex represents a region which differs from 

 others, not in kind, but in the variety and complexity 

 of the structural and functional relations between its 

 parts, and these relations must be determined by 

 physiological factors of the same sort as those concerned 

 in other parts of the nervous system, but acting under 

 less simple conditions. If the conclusions and sugges- 

 tions of earlier chapters concerning the part played in 

 the development of the nervous system by electrical 

 factors through their effects on physiological condition 

 are correct, the physiological factors concerned in the 



