INDIVIDUATION AND REPRODUCTION 209 



region adjoining the wound determines the origin and direction of 

 a new gradient and so the axis of a new individual. In many 

 cases also the origin and direction of the new gradient may be 

 controlled and determined experimentally in other ways. Undoubt- 

 edly, after it is once estabhshed a gradient may often persist from 

 one individual to another through the process of reproduction, 

 but there are no adequate grounds for beUeving that such gradients 

 are fundamental properties of protoplasm, although, on the other 

 hand, it is probable that no cell or cell mass can exist for any 

 great length of time in any natural environment without acquir- 

 ing, at least temporarily, one or more gradients, because external 

 conditions at different points of its surface can never remain uni- 

 form. In general it may be said that the axial gradients of an 

 organism are either the parental gradients persisting in the organ- 

 ism, as in many cases of fission, or that they are produced de novo 

 by conditions which determine different rates of metabohsm in 

 different parts of the cell or cell mass at some stage of its existence. 



The essential feature in the estabHshment of a gradient in meta- 

 boHc rate in Hving protoplasm is the estabHshment of the region 

 of highest rate. If such a region is estabhshed in an undiffer- 

 entiated cell or cell mass, a more or less definite gradient in rate, 

 extending to a greater or less distance from this region, arises 

 because the changes in the primary region spread or are trans- 

 mitted, but with a decrement in intensity or energy, so that at a 

 greater or less distance they become inappreciable. In this way 

 the region of highest rate becomes the chief factor in determining 

 the rate of other regions, and since the rate thus determined is 

 higher in regions nearer to it and lower in those farther away, a 

 gradient in rate results. In its simplest form, then, the gradient 

 may arise merely from the spreading or transmission of metaboHc 

 changes from the region of highest rate. 



If metabohc gradients are characteristic features of the axes in 

 Hving organisms, the question at once arises whether the axis in 

 its simplest terms is anything more than such a gradient. In other 

 words, are not physiological and morphological polarity and 

 symmetry primarily the expression of gradients in rate of metab- 

 ohsm ? At present it can only be said in answer to this question 



