MELANOPHOEES OF THE HORNED TOAD 285 



By means of carefully controlled experiments, however, 

 it is possible to gain some idea of the significance of the direct 

 action of stimuli upon the melanophores without removing them 

 from the organism. If these pigment cells react in only those 

 places on which stimuli act, a direct response of the melanophore 

 may be suspected; if the pigment throughout the entire body is 

 affected by a local stimulus, it may be concluded that the reaction 

 is due to the intermediation of nerves, or hormones. The fol- 

 lowing experiments are instructive from this point of view: 



November 1, 1913. Three horned toads, the pigment of which 

 was contracted, were shielded locally from Kght by placing a small 

 piece of modeling clay over two or three lateral scales. After an hour 

 of exposure to sunlight the melanophore pigment of all the lateral scales 

 of all the animals had expanded, with the exception of the shielded 

 scales, the pigment of which remained contracted. 



Two homed toads, the melanophore pigment of which was ex- 

 panded, were shielded in a similar fashion and exposed to sunlight. 

 The melanophore pigment of all the scales remained expanded excepting 

 those shaded by the day, which cojitracted. 



From this experiment, which has been amply confirmed, it is 

 evident that if a small part of the surface of a homed toad be 

 shielded from light, the melanophore pigment of this part will 

 remain contracted while that of the rest of the body expands, 

 and if the shaded melanophore pigment be expanded, it will 

 contract without reference to the illumination of the rest of the 

 body. 



It might be objected that this effect is due to the tactile stimu- 

 lation of the clay or to the exclusion of air from the surface of the 

 skin. That this objection is invalid is indicated by the fact that 

 if the clay is applied so as not to touch the lateral scales, but only 

 the surface of the skin on both sides of it, the reaction follows in 

 the same manner. Moreover, if the scales are painted with cel- 

 loidin, which should provide a similar tactile stimulation and 

 exclude the air from the skin, but which does not keep out the 

 light, the pigment is able to expand as readily as on those scales 

 which are not covered. 



The converse experiment consists in illuminating a few scales 

 of the body when the remaining parts are covered. This experi- 



