276 ALFRED C. REDFIELD 



VI. Discussion '. . 312 



1. The nature of the contraction of melanophore pigment 312 



2. Resume of the reactions of the melanophores of the horned toad. 314 



3. Comparative physiology of the melanophores of vertebrates 316 



a. The influence of hormones upon melanophores 316 



b. The coordination of melanophores and smooth muscle 317 



c. The physiological basis of the emotions. 319 



VII. Summary 319 



VIII. Literature cited 321 



IX. Plates 323 



I. INTRODUCTION 



The results of an investigation of the reactions, coordination, 

 and function of the melanophores of the horned toad Phyrxiosoma 

 cornutum Harlan are described in the following paper. The 

 facts which are presented possess, in addition to their intrinsic 

 interest, a bearing upon several aspects of comparative physiology. 



Earlier studies upon the melanophores of lizards indicated that 

 in the chameleon (Briicke, '52; Keller, '95) the color changes are 

 produced under the influence of nerves which cause the melano- 

 phore pigment to contract, while in Anolis (Carlton, '03) the 

 nerve impulse appears to cause the opposite effect, an expansion 

 of the melanophore pigment. The present investigation was 

 initiated in the hope of gaining more light upon the relation of 

 the nervous system to the melanophores. It very soon became 

 evident that the melanophores are not only under the control of 

 nerves, but that certain of their reactions result from the direct 

 effect of stimuli upon them, while other reactions indicate clearly 

 that some coordinating mechanism other than the nervous system 

 is involved. The discovery that these reactions are due to a 

 hormone (Redfield, '16) and the identification of the hormone as 

 adrenin form the most novel result of the investigation. 



A few observations have already been made upon the color 

 changes of this lizard by de Grijs ('99) and upon the closely related 

 species Phrynosoma orbiculare, by Wiedersheim (Hoffmann, 

 '90, p. 1353), and Phrynosoma modestum, by Weese ('17). 

 Parker ('06) has made a more intensive study of the reactions 

 of the melanophores of Phrynosoma blainvillei to illumination 



