ENDOCRINE CO-ORDINATION 125 



within a few minutes. After being kept for several days on 

 a white background the animals remain fuscous in bright 

 light. 



Redfield has recorded the results of careful experiments 

 carried out with a view to locating the receptors involved in 

 these modes of response. He finds that the local exclusion 

 of light from and application of heat to restricted areas of the 

 skin produce a local contraction of the melanophores in the 

 region to which the stimuli are applied, without affecting the 

 colour of the skin in other parts of the body. Furthermore, 

 local illumination produces a local expansion of the 

 melanophores without affecting the pigmentary effectors of 

 other regions, while local reduction of temperature maintains 

 locally a state of melanophore expansion already established, 

 though it apparently cannot initiate. These experiments 

 admit the possibility that melanophore response to heat and 

 to light, in the case of animals kept on a neutral background, 

 is propriogenic in character and results from the direct reactivity 

 of the pigmentary effector organs to incident stimuli. It is 

 not conceivable that these results could be brought about by 

 hormonic regulation through the circulatory system. Red- 

 field states that such local responses can be evoked after the 

 entire nerve- supply of the affected region has been severed. 

 If this is so, there would seem to be no alternative to accepting 

 his conclusion that light and temperature can act directly 

 upon the melanophores, without the intervention of either a 

 freely-circulating hormone or a nicely-adjusted system of 

 reflex arcs. 



Nevertheless, Redfield is driven to the conclusion that 

 there is, superimposed on this primary reactivity of the 

 melanophores of the horned toad to fight and heat, a co- 

 ordinating mechanism which will account for the generalised 

 condition of pallor following " excitement," and the peculiar 

 modification of the normal reaction to light in virtue of the 

 background upon which the animal is kept. For the latter 

 response the appropriate receptor is the eye ; since blinded 

 individuals no longer display the apparently adaptive response 

 to the brightness or darkness of the substratum. If horned 



