CHAPTER VII 



THE PHYSIOLOGICAL RELATION OF THE CENTRAL 



NERVOUS SYSTEM TO OTHER PARTS 



IN DEVELOPMENT 



PHYSIOLOGICAL INDEPENDENCE AND DEPENDENCE IN 

 DEVELOPMENT IN RELATION TO THE GRADIENTS 



Attention has already been called (p. 97) to the fact 

 that the establishment of a physiological gradient deter- 

 mines the existence of a relation of dominance and 

 subordination in which the high end of the gradient, 

 because of its greater irritability and activity, is physio- 

 logically dominant over all levels of the gradient within 

 a certain range of distance, which varies with the 

 activity of the dominant region and the conductivity of 

 the protoplasm. Similarly, any level of the gradient is 

 to some extent dominant over more basal or posterior 

 levels and subordinate to more apical or anterior levels. 

 This relation of dominance and subordination appears 

 most clearly in the simpler axiate organisms, both plants 

 and animals, and its significance in ontogeny and in the 

 regulatory development of isolated parts as well as in 

 function has been considered at length elsewhere (Child, 

 1910a, 191 ic y d, 1915c, chaps, iv, v). 



This relation may be stated not only in terms of 

 dominance and subordination, but in terms of inde- 

 pendence and dependence. So far as one region or level 

 of the axis is dominant over another, it is independent 

 of the other. In other words, in so far as a more irritable 



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