FIELDS AND GRADIENTS 305 



Such facts can only be interpreted in terms of a field theory. 

 Some general activity must be distributed in a graded way through 

 the limb so as to constitute a gradient-field. The fate of the re- 

 generated tissue is determined in relation to the level of the gradient 

 at which regeneration is made to occur, not to the specific tissues 

 present on the cut surface. Further, the determination of the re- 

 generated portion is a unitary process. The regenerated portion is 

 determined as a field, the morphogenetic agencies in which are in 

 equilibrium with those operative in the stump, so that the fractional 

 field of the regenerated portion and that of the stump together 

 make a whole (see also Chap, x, p. 362). Both the products of 

 undifferentiated cells and also certain types of already specialised 

 cells contribute to the regenerated material.^ 



Presumably the morphogenetic gradients in the stump extend as 

 it were by extrapolation into the new tissue, so that it comes to be 

 permeated by the missing portion of the total field: when this 

 occurs, the gradient activities of the whole field are in equilibrium. 

 As regards its gradients, the regenerated portion then constitutes 

 a fraction of a field: but since it alone contains undifferentiated 

 tissue, in its subsequent morphogenesis it behaves as an auto- 

 nomous field system with basal boundary set by the level of the cut. 



The same type of behaviour is seen in the regeneration of a tail 

 in Planarians; the new tissue from the start is determined in re- 

 lation to the existing gradient-stem of the old piece. It would thus 

 appear that the basalmost regions of a limb are dominant, and 

 correspond, as regards their activities in the gradient-system, to the 

 anterior (apical) region of the whole body in animals capable of total 

 regeneration. 



It should be noted that in such cases quite a small fraction of 

 the field (e.g. a short disc cut from a limb) will be able to exert this 

 morphogenetic effect on material proliferated from its cut surface, 

 even when grafted into another region of the body altogether (e.g. 

 a short section of fore-limb stump taken with an indifferent re- 

 generation-bud that has been proliferated from it, and grafted into 

 the hind-limb field; seep. 273). It is also important to find that when 

 a section of a limb is cut out and engrafted elsewhere in reversed 

 orientation, with original proximal cut surface away from the body, 



^ Hellmich, 1930. 

 HEE 20 



