MELANOPHORES OF THE HORNED TOAD 289 



by the lines b-b, c-c, and d-d respectively. Whatever the course 

 of any nerves connected with the melanophores within the region 

 inclosed by these four lines, one of the cuts must be expected to 

 interfere with the passage of nervous impulses to and froiji the 

 region. The animals upon which these operations were performed 

 recovered rapidly. Four days after the operation, when the 

 melanophore pigment was fully expanded, they were placed in the 

 dark. Within two hours the melanophore pigment of all parts 

 of the skin in every one of the four cases had contracted. Since 

 this contraction cannot be attributed to a nervous reflex, the con- 

 clusion must be drawn that light acts directly upon the melano- 

 phores of the horned toad.^ In a similar way it was found that a 

 high temperature would cause a contraction of the melanophore 

 pigment of all parts of the skin of these animals. 



It is clear that a local response to local stimuli cannot be 

 explained by postulating the action of hormones or of a diffuse 

 nervous mechanism, such as a nerve net. It has been shown that 

 these responses cannot be attributed to a nicely arranged system 

 of reflex arcs. The conclusion seems unavoidable, therefore, that 

 the action of light and temperature upon the melanophores of the 

 horned toad is a local one, which does not involve the coordinat- 

 ing systems of the body. 



2. Receptors of noxious stimuli 



Noxious stimuli can bring about melanophore reactions when- 

 ever they affect sensory nerves with sufficient intensity. It has 



* The exact point upon which these stimuli act is still a subject for speculation. 

 The simplest conception is that light and cold stimulate the melanophores directly 

 and so produce their characteristic effects. If this be true, the melanophores 

 may then be considered to serve as receptors for these stimuli. This conception, 

 however, may be naive. Light and heat may act not directly upon the melano- 

 phores, but upon some closely associated tissue and still bring about the observed 

 results. Thus it will be shown (page 306) that the melanophores are acted upon 

 directly by nervous impulses, which maintain a contraction of their pigment. 

 Assume that cold acts upon the motor nerve endings which connect with the 

 melanophores so as to block the passage of nerve impulses to the pigment cells, 

 and the expansion of the melanophores by cold may be explained as an inhibition 

 of the nervous control, rather than a direct positive effect of the stimulus upon the 

 pigment cells. The present investigation has offered no grounds for choice 

 between the two possible mechanisms. 



