302 FIELDS AND GRADIENTS 



with rudimentary ganglia, is produced. Higher concentrations in- 

 hibit head-formation ahogether, and only healing occurs (fig. 143). 



(b) DiflFerential acceleration occurs in response to exceptionally 

 favourable conditions. The effects are the exact reverse of those 

 obtained by diflrerential inhibition, though such extreme departures 

 from the normal are not seen, since there are no regions which fail 

 to form. As illustration we may take the fact that optimal high 

 temperature applied during regeneration of Planarian fragments 

 leads to the formation of heads which are not only relatively large, 

 but have widely separated eyes and an unusually large pre-ocular 

 region. 



(c) Differential acclimatisation occurs in certain low concentra- 

 tions of depressants. In these it appears that the most active regions, 

 although the most susceptible, have the greatest power of acclimati- 

 sation, and after a time show differential development. For in- 

 stance, intact normal Planarians placed in weak alcohol or ether 

 first show a differential reduction in size of head, the whole pre- 

 ocular region disappearing. Later, new growth sets in, and this is 

 abnormally high in the most median and most anterior regions, 

 leading to "snouted" forms^ (fig. 143). 



In slightly stronger concentrations, this differential action will 

 not take place during exposure to the solution, but occurs on re- 

 placement in water. In such cases the process is strictly speaking 

 one of differential recovery instead of differential acclimatisation, 

 but the results are in most respects similar. 



(x) In addition to these statements, applicable to regeneration of 

 the complete type, within total fields permeating the whole body, 

 there must be mentioned another very important rule derived from 

 a study of partial regeneration in a local field. This is that the various 

 tissues of the regenerated region need not be proliferated from 

 corresponding tissues in the old region, but are determined in re- 

 lation to a gradient-system which extends out from the old region 

 into the proliferated material. Total regeneration appears normally 

 to take place in two phases — first the formation of a new apical 

 region, and secondly the remodelling of the old tissues under the 

 influence of this apical region. However, in partial regeneration, 

 e.g. of an amputated limb or tail, the new tissues are not known to 

 ^ Child, 1921 A. 



