642] 



SEROLOGICAL REACTIONt 



83 



or other external factor or factors) causes one region to 

 have the greatest metabolic activity. As does any stim- 

 ulated part with reference to a resting part (muscle, 

 nerve, etc.), this point of heightened activity sets the 

 pace, as it were, for the other parts. Thus an excitation 

 gradient is established which Child terms an axial gra- 

 dient. Since the excitation initiates transmission — 

 changes and thereby determines what shall happen at 

 successive levels along the path of transmission, the 

 region of highest excitation dominating and controlling 

 the rest, the axis in question may also be called a met- 

 aholic gradient. In this way a physiological unity is 

 established and maintained by bringing the different 

 regions within range of the gradient into definite physio- 

 logical relations. 



Since the chief source of energy in protoplasm is oxida- 

 tion, and inasmuch as many different tests have shown 

 that the rate of oxidation gradually diminishes along the 

 gradient from the region of lii^liot acti\ itv. Child infers 

 that differences in oxygen sui)])!} i>l;i> a wry important 

 part in the local metabolic dillVicnco wliicli arise. In 

 any event, a differential axis of activity arises in the egg 

 as the result of differences in environmental conditions 

 (either before or after ovulation) and determines the 

 axiate pattern of the developing organism. In such a 

 complex system of chemical and physical activities, where 

 unquestionably many associated simultaneous reactions 

 and interactions are going on, different rates or condi- 

 tions of reaction in different regions must result in unlike 

 end-products. Thus, at different levels of a gradient 

 which was quantitative in origin, qualitative differences 

 arise. For once a gradient is established, any one of 

 several purely quantitative changes in the system, such 

 as increased oxidation which acts differentially on sub- 

 stances at a given point, changes in temperature, in water 



