314 ALFRED C. REDFIELD 



be reached and the melanophore pigment will receive a different 

 distribution within the cell. In other words, by changing the 

 balance of factors which affect the melanophore, its tone is 

 changed. 



The view that reaction of the melanophores of vertebrates is of 

 a tonic nature has been held by Lister ('58), Pouchet ('76), von 

 Frisch ('11), Krukenberg ('80), Keller ('95), and others. These 

 authors had in mind, however, a tonus established and maintained 

 by the central nervous system, and although it was believed that 

 this tonus might be altered by certain stimuli acting upon the 

 melanophores, the conception was quite different from the view 

 here presented, that the tonus of the melanophores is an inherent 

 quality of these cells. 



If this view of the nature of the melanophore be taken, it is 

 obviously futile to speak of the 'active state' of the cell, a con- 

 ception which has doubtless arisen from comparing the melano- 

 phore's reaction with the ' twitch ' phenomenon of muscle, rather 

 than the 'tonus' phenomenon. The contracted condition is no 

 more active than the expanded, or at least there is no experimental 

 evidence to that effect. Activity is involved only in changing 

 from one state to the other and in either direction. One may 

 better speak of states of increased and decreased tone in describ- 

 ing the conditions of melanophores. Since Spaeth ('16a) has 

 shown that many of the physiological reagents which produce a 

 contraction of smooth muscle also produce a contraction of the 

 melanophore pigment, the contracted state may be called arbi- 

 trarily that of increased tone, the expanded state that of decreased 

 tone in the case of melanophores as well as of smooth muscle. 



2. Resume of the reactions of the melanophores of the horned toad 



If the conclusions drawn from the foregoing experiments are 

 correct, the tone of the melanophores of the horned toad is varied 

 in the following ways: 



The tonic state of the melanophores is conditioned by the action 

 of nervous impulses, spreading perhaps from a center in he 

 thoracic cord, passing along sympathetic fibers and terminating 



