PHYSIOLOGICAL GRADIENTS 93 



reserves, the loading and discharge of gland cells, are 

 cases in point, and there can be no doubt that many 

 other differentiations arise in essentially the same way. 

 That is, a region in which oxidation is occurring slowly 

 becomes different from a region of rapid oxidation 

 because of this difference in relation between intake 

 and rate of oxidation, which determines that in the one 

 region certain substances shall accumulate as part of 

 the structure, while in the other such substances are 

 more or less completely oxidized as rapidly as they are 

 formed. 



The changes concerned in excitation, whether they 

 are primarily electrical, chemical, or physical, may 

 similarly become the starting-point of a series of quali- 

 tative changes, particularly if they persist for a con- 

 siderable time, and it is evident that the different levels 

 of an excitation gradient or a more or less permanent 

 physiological gradient afford numerous possibilities for 

 the origin of qualitative differences at the various levels. 

 Such changes may or may not be readily or completely 

 reversible. The changes which constitute differentia- 

 tion are those which are not readily or completely 

 reversible under the conditions of development. 



As soon as these qualitative differences make their 

 appearance the basis for definite and orderly chemical 

 or transportative correlation of organismic magnitude is 

 established, and from this time on such correlation plays 

 an increasingly important role in the further sequence of 

 events. The definiteness, variety, and specificity of 

 chemical correlation must increase as differentiation 

 progresses and mechanisms of communication develop, 

 both in the individual and in the course of evolution, 



