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 
