GRADIENTS, FIELDS, AND DETERMINATION 273 



as to their significance in later stages. It is by no means necessary to 

 assume, nor is it probable, that the component reactions are the same in 

 different gradients even in the same individual. On the other hand, it is 

 probable that specific differences begin to appear almost at once in a 

 gradient primarily quantitative; but the important point, so far as early 

 developmental pattern and order are concerned, is that quantitative 

 gradient factors appear to be primary and specific regional differences to 

 develop gradually. The graded differentials in rate of development, oxi- 

 dation-reduction, electric potential, susceptibility to toxic action, etc., 

 characteristic of these gradients give, of course, only general and partial 

 information concerning them; but since developmental order and pattern 

 show very definite relations to the gradient pattern of which these graded 

 differentials are partial expressions, it appears, beyond question, that the 

 physiological gradients are operative and effective factors in development 

 of axiate order and pattern. The metabolisms of different gradients in 

 the same individual may differ widely, but the gradient pattern is appar- 

 ently as essential to axiate development as the kind of reaction in it. 



As will appear in following chapters, gradients in living protoplasms 

 can be determined experimentally by various environmental factors and 

 under natural conditions are often determined by local activation in rela- 

 tion to factors in the organismic environment of the part concerned. 

 Presence of differentiation is not necessary for their initiation. All that 

 is necessary is localization of an activated region in some way ; from this 

 the activation spreads, irradiates, and is transmitted with a decrement 

 in intensity. The activated region corresponds to a region of excitation, 

 and the resulting gradient to excitation transmitted with a decrement. 

 If the activated region persists long enough, it may determine a persistent 

 gradient, which in turn determines axiate development. Whatever the 

 factor or factors initiating activation or a gradient, specific constitution 

 of the protoplasm in which it appears and the physiological condition, as 

 determined by various factors, are, of course, the chief factors in deter- 

 mining the character of the gradient, steepness of decrement and effective 

 length, as well as the kind of physiological activity characterizing it and 

 the kind of development resulting from it. As in nervous excitation and 

 transmission, the activating factor initiates and the protoplasmic mecha- 

 nism determines character of effect. 



A gradient in a certain direction appears to constitute a physiological 

 basis for definite order and sequence in development in that direction, and 

 a gradient system seems to serve as a sort of a physiological co-ordinate 



