32 PIGMENTARY GROWTH AFTER ABLATION OF 



hours partly expanded epidermal melanophores; if the tempera- 

 ture becomes normal, they contract to the usual condition; if 

 the temperature is maintained and sunlight used in addition, 

 they expand to the maximum. Thus we see that, although 

 high temperature alone evokes the reaction, it is incomplete 

 and slowly expressed, double the time being necessary. The 

 same condition prevails if light alone is used. The reactions are 

 evoked slowly and incompletely. Thus a combination of both 

 light and heat is necessary to evoke the maximal response in the 

 shortest time. 



It has been mentioned that the xantholeucophores of the 

 normal are uniformly nearly punctate in a dark-adapted animal. 

 If the large normal animal be subjected to high temperature 

 and direct sunlight, these cells become broadly expanded (fig. 

 19), never exhibiting, however, as great an expansion as invari- 

 ably is presented by these cells in the albino. Thus we see that 

 in the large normal animal there is frequently a reversal in the 

 reaction between the 'interference' cells and the epidermal 

 melanophores one expands and the other contracts. Such a 

 correlated opposite reaction of the pigment-cell groups no longer 

 holds in the case of the albino, inasmuch as the xantholeuco- 

 phores are always maximally expanded there. However, since 

 under the usual environment the epidermal melanophores of 

 the albino are almost maximally contracted, these two groups 

 of cells are consequently usually in an opposite physiological 

 condition, as is the case in the large normal larvae. We have 

 already given evidence for our belief that the failure of albino 

 xantholeucophores to react to stimuli is due to a tonic hormonal 

 disturbance present in the tissue juices. 



The deep melanophores with a white background and light 

 expand; in darkness they contract; thus, in both types of ani- 

 mals their reaction is identical under all the environmental 

 conditions employed by the author. 



Specimens that have been thyroidectomized at an early stage, 

 as is well known, exhibit no pigment deficiencies. Under the 

 conditions of light and heat or of darkness, they react as do 

 their unoperated brothers, with this difference: the reaction, 



