372 GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



These growth-fields continue to operate so long as growth con- 

 tinues. The processes underlying them are clearly of a different 

 nature from those concerned with the gradient-systems of the early 

 embryo, and in higher animals it is uncertain whether they are even 

 the directly-produced descendants of those gradients. However, 

 the growth-gradient of a Planarian, as revealed by the relation 

 between head-size and body-size (p. 287), co-exists with the axial 

 gradient, and is the reciprocal of it;^ which suggests that the two 

 gradient-systems may be connected.^ 



§7 

 Although it seems clear that the gradient- and field-systems of 

 the egg and early embryo may persist into later life, this does 

 not necessarily imply that they persist wholly unchanged. For 

 instance, the facts of serial heteromorphosis can best be explained 

 on the view that the primary gradient has been flattened. Then 

 we have facts such as those concerning the regeneration of skin 

 in lizards,^ which show that the type of scale regenerated varies 

 with the external conditions (e.g. temperature). The fact that re- 

 generated tails produce scales unlike those originally present is 

 thus presumably due not to "atavism" but to the fact that con- 

 ditions in the regeneration-bud are different from those in the 

 original tail-rudiment, a fact which in turn may be correlated 

 with an alteration of the gradient-systems concerned. 



The chief points elicited in this chapter may be summed up as 

 follows. In the first place, strong evidence is provided for the 

 persistence throughout life of the primary axial gradient and of 

 focalised gradient-fields responsible for the morphogenesis of 

 particular organs, although the precise form and effects of these 

 gradients may alter with age. Secondly, attention is drawn to the 

 persistence throughout life of growth-gradients controlling the 

 relative growth of parts of the body. Here again, both total growth- 

 gradients and local growth-gradients appear to exist. It is possible, 

 though not certain, that these growth-gradients stand in some close 

 relation to the morphogenetic gradients previously described. 



^ Abeloos, 1928. 



^ For a more detailed discussion see Huxley, 1932, Chap. vi. 



^ Noble and Bradley, 1933. 



