288 ALFRED C. REDFIELD 



To summarize the effect of local photic and thermal stimuli 

 upon the melanophores of the horned toad, the following points 

 should be brought out: 



1. The local exclusion of light and the local application of heat, 

 both of which produce a contraction of the melanophore pigment 

 in the parts so treated, do not affect the melanophores in other 

 parts of the body. 



2. The local application of light, which produces an expansion 

 of the exposed melanophore pigment, does not affect the melano- 

 phores of other parts of the body. 



3. The local application of a low temperature fails to produce 

 any expansion of melanophore pigment . An expansion previously 

 established may be maintained locally by this stimulus. 



Fig. A For explanation see text (p. 288). 



These experiments do not prove that the melanophores are 

 stimulated directly by light and heat, because the same results 

 might be obtained by a different mechanism. If the melano- 

 phores were under the control of nerves so arranged that the 

 stimulation of a point on the skin sets up impulses which passed 

 over a reflex arc terminating in the pigment cells of the same 

 point and no other, then a local stimulus would also produce a 

 local response. 

 ^ An experiment designed to test the presence of such a system of 



reflex arcs consisted in making cuts through the body wall of a 

 number of horned toads in such positions that had all the cuts 

 been made upon a single animal a part of the skin would have 

 been completely severed from the rest of the body. Thus one 

 animal received a cut in a position indicated by the line a-a, in 

 figure A ; a second, third and fourth animal in positions indicated 



