MELANOPHORES OF THE HORNED TOAD 



315 



either directly in the pigment cells or in the adrenal glands or 

 both. The presence of such impulses delivered directly to the 

 melanophores causes an increase in the tone of these cells and a 

 consequent paling of the skin. Impulses delivered to the adrenal 

 glands cause a secretion of adrenin, which is carried by the circu- 

 lation to the pigment cells and affects them in a way identical 

 with direct nervous stimulation. 



Such nervous impulses may be set up through stimulation of 

 the eyes by the color of the environment. In this way the 

 adaptation of the color of the horned toad to the color of the 

 substratum may be explained. These nervous impulses may also 



cd. spi 



5"-^ viela'pk. ; i 



va. mela'ph. 



Fig. H Diagram of the mechanism coordinating the melanophores of the 

 horned toad. Noxious stimuli are carried along sensory neurones, n. sns., and 

 photic stimuli along othpr sensory neurones, n. sns'., to a region of the spinal cord 

 between the eighth and thirteenth vertebrae, cd. spi. From there impulses pass 

 out along motor neurones, n.mot'. to the adrenal glands, gl. adren. The secretion 

 of adrenin so induced is carried by the blood-vessels, va., to the melanophores, 

 mela'ph. Other impulses travel along other motor neurones, n. mot., which pass 

 through the sympathetic nervous system, sy., directly to the melanophores. The 

 melanophores are affected in addition by illumination and temperature, the action 

 of which is independent of the coordinating systems of the body. 



originate from the nervous excitation brought about by noxious 

 stimuh and produce a contraction of the melanophore pigment. 

 The tonic state estabhshed through the action of nerves 

 and the adrenal glands may be varied through the direct action of 

 certain stimuh upon the melanophores or upon tissues closely 

 associated with them. Thus, light and cold tend to decrease the 

 tone so established. Darkness and heat, on the other hand, tend 

 to increase the tone of the pigment cells. By this means the 



