MELANOPHORES OF THE HORNED TOAD 291 



have become pale; c and e, having been blindfolded, remain as 

 dark as the animals which have been kept upon cinders. Figure 

 6 shows a control experiment, in which both animals have been 

 upon white sand for ten days; g was blindfolded in one eye, yet 

 it has become as pale as h, which was not blindfolded. 



This experiment leaves no doubt that the eyes are the receptors 

 involved in the adaptive reaction of the melanophores to the color 

 of the environment. 



Summary 



1. The melanophores or some closely associated tissues are 

 receptors of photic and thermal stimuli. 



2. There are no specific receptors for noxious stimuli. 



3. The eyes are receptors for stimuli which cause adaptive 

 reactions of the melanophores. 



V. COORDINATING MECHANISMS OF MELANOPHORE REACTIONS 



In the foregoing pages there have been considered the stimuli 

 which initiate reactions of the melanophores of the horned toad 

 and the receptors upon which these stimuli act. There remain to 

 be considered the mechanisms involved in transmitting the effect 

 of a stimulus to the pigment cells. By what means are the reac- 

 tions of the melanophores coordinated, so that a localized stimulus 

 can cause a change in the color of the entire skin? Obviously 

 this question need not be answered for those reactions in which 

 the melanophores or some closely associated tissues are stimu- 

 lated directly, as by light and heat. Noxious stimuli and stimuli 

 arising from the color of the environment, however, may act 

 on a restricted sensory area and yet cause a reaction of pigment 

 cells over a large surface of the body. How is this integration 

 accomplished? 



Two systems of the body are concerned in coordinating its 

 parts. These are the circulation and the nervous system. The 

 relation of these systems to the reaction of the melanophores 

 of the horned toad will now be considered. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 26, NO. 2 



