PHYSIOLOGICAL GRADIENTS 105 



in later stages, after the nervous system develops, the 

 parts may be reintegrated into a new whole, as is appar- 

 ently the case with annelid segments (Child, 191 7 d). 

 It is probable, however, that in the higher animals the 

 size of the adult is determined rather by the cessation 

 of growth with increasing differentiation than by the 

 range of dominance, for the conductivity of the highly 

 specialized nerve fiber is so high that the range of domi- 

 nance is certainly much greater than the length of any 

 animal body, and if medullated nerves actually do con- 

 duct without decrement, it is indefinite. But even if 

 physiological isolation of parts of the body should 

 occur in the more advanced developmental stages of the 

 higher animals, it would doubtless not result in agamic 

 reproduction of new individuals, because the capacity 

 of the cells for dedifferentiation is limited. Certain 

 aspects of the tumor problem, particularly with respect 

 to the embryomas and the malignant tumor forms with 

 indefinite growth, are of interest in this connection but 

 cannot be considered at this time. 



CONCLUSION 



This brief survey of some of the more important 

 features of physiological integration and isolation will 

 serve to show in a general way how a quantitative 

 physiological gradient involving differences in the rate 

 of fundamental reactions and in protoplasmic conditions 

 associated with them may, on the one hand, initiate 

 localization and differentiation of parts and organs and 

 on the other constitute the basis of a relation of domi- 

 nance and subordination within the individual. Such a 

 gradient, however, is merely the initiator and ordering 



