284 ALFRED C. REDFIELD 



ment may be produced. The current used for this purpose was 

 just strong enough to cause an unpleasant sensation when appHed 

 to the human tongue. 



Summary 



The responses of the melanophores of the horned toad to exter- 

 nal stimuli are the following: 



1. Light produces an expansion; its absence a contraction of 

 the melanophore pigment. 



2. High temperatures produce a contraction ; low temperatures 

 an expansion of the melanophore pigment. 



3. Light and heat interact in such a way that the heat effect 

 dominates at extremes of temperature, the light effect dominates 

 at mean temperatures. 



4. Light coming from a dark substratum produces an expan- 

 sion of the pigment; light coming from a light substratum pro- 

 duces a contraction of the pigment. 



5. Mild mechanical stimuli do not affect the melanophores. 



6. Mild faradic stimuli cause a contraction of the melanophore 

 pigment. 



7. Noxious stimuli, such as violent mechanical or faradic 

 stimuli, produce a contraction of the melanophore pigment. 



IV. RECEPTORS INVOLVED IN MELANOPHORE REACTIONS 



1 . Direct action of stimuli upon melanophores 



It has been pointed out by Spaeth ('13) that the direct action 

 of stimuli cannot be studied satisfactorily by experiments upon 

 living animals, owing to the complications introduced by the 

 presence of the nervous system, circulation, etc. He recommends 

 the study of bits of tissue separated from the body of the animal. 

 Unfortunately, the horned toad does not lend itself easily to this 

 method of investigation. Pieces of skin placed in Ringer's 

 solution show no melanophore reactions, except that the pigment 

 is contracted very soon. This contraction may be attributed to 

 anemia (p. 292). 



