202 REYNOLD A. SPAETH 



mechanical stimulation (gentle pressure) produces a reversible 

 contraction of the melanophores. 



As a result of local pressure or pinching with forceps, and along 

 the margins of an incision, the melanophores in the frog contract 

 (v. Wittich '54, 71, Hering 1. c, Lister '58 a, '58 b, Fuchs '06, 

 Eternod and Robert '08). The darkening of the skin in Poly- 

 pedates Reinwardtii, ''nach leichtem Kratzen mit einer Nadel," 

 observed by Siedlecki ('09), is doubtless another case of ruptured 

 melanophores.^ 



The above observations warrant the conclusion that in the 

 radial smooth muscles of the chromatophores of cephalopods, in 

 the smooth muscle of the digestive tract in certain teleosts and 

 amphibia, as well as in the melanophores of these two vertebrate 

 groups, mechanical stimulation (gentle pressure) is followed by a 

 reversible contraction. 



V. Effects of Chemical Stimulation 



A great many experiments have been carried out upon the 

 effects of various widely differing chemical stimuli upon the 

 melanophores of vertebrates. The method of procedure has 

 been, in most cases, as follows; 1) the substances were brought 

 directly upon the skin of the normal or operated (pithed, etc.) 

 animal, the color-change being considered the criterion of the 

 action of the chemicals; 2) the substances were injected into the 

 circulation, the body cavity, or subcutaneously, the color-change 

 again serving as an indicator of chemical stimulation ; 3) the sub- 

 stances were added to the water of the environment, in aquatic 

 forms. In relatively few instances have excised pieces of skin 

 been immersed in the fluid to be tested. As I have repeatedly 

 emphasized, this is the only satisfactory method of determining 

 the direct effect of any stimulus upon the melanophores, for it 

 is only in this way that the circulation and all central nervous 



^ Observations upon the response to pressure in reptilian melanophores are 

 contradictory and unsatisfactory. Most of the experiments have been made 

 with living animals and the few trials with excised bits of skin were all carried 

 out without regard for the possibility of a darkening resulting from the destruc- 

 tion of the melanophores (Milne-Edwards '34 a, '34 b, Briicke '52, Carlton '04). 



