280 ALFRED C. REDFIELD 



may occur in as short a time as three minutes and never requires 

 longer than ten minutes. This reaction is also a dominant one, 

 occurring irrespective of the condition which the cycle of day and 

 night and the color of the environment has imposed upon the 

 melanophores. 



III. EXTERNAL STIMULI INVOLVED IN MELANOPHORE REACTIONS 



1. Illumination and temperature 



The series of color changes which accompanies the cycle of day 

 and night is dependent upon the accompanying changes in light 

 and temperature. The rhythm of the color changes may be 

 broken by a disturbance of the normal rhythm of the changes in 

 light and temperature. Thus on cool days the midday con- 

 traction of the melanophore pigment does not occur; on cloudy 

 days or when the animals are buried in the sand the skin may 

 remain in the pale condition throughout the entire day. 



If a horned toad is exposed to bright dayUght or to sunlight 

 which is not too warm, after a short time an expansion of the 

 melanophore pigment occurs. If the animal is placed in the 

 dark, the melanophore pigment is contracted. These changes 

 may be followed most clearly by observing the appearance and 

 disappearance of the dark areas upon the bases of the lateral 

 scales. The rapidity with which these changes occur in either 

 direction varies from ten minutes to one-half hour, depending 

 upon the intensity of the illumination, the temperature, and the 

 idiosyncrasy of the individual. 



In order to determine whether the effect of light upon the 

 melanophores is due to the heat developed by its action, the 

 following experiment was performed. An apparatus was devised 

 in which the temperature could be kept quite constant and to 

 which light could be admitted at will. This apparatus consisted 

 of a glass aquarium 30 cm. in diameter and of equal height. 

 Into this was inserted a second glass vessel 15 cm. in diameter. 

 Between the walls of these two jars ice or water of any desired 

 temperature could be introduced. By this means the air in the 



