194 THE BIOLOGY OF THE FROG chap. 



The green is produced, according to Biedermann, ,when 

 the black chromatophores are expanded beneath the yellow. 

 Then most of the light is reflected from the granules. When 

 the black chromatophores are contracted so that the yellow 

 cells have a lighter background, light may be reflected from 

 other elements than the blue granules, and a yellow or golden 

 color may predominate. The role of the contraction and 

 expansion of the golden pigments is not accurately deter- 

 mined. It is probable that the gray or grayish blue color 

 which is sometimes assumed may be produced by the simul- 

 taneous contraction of both the black and the golden pig- 

 ments, since frogs with the black pigment spots contracted 

 often exhibit these hues when the golden pigment has been 

 dissolved out in alcohol. Von Wittich found in the tree 

 frog that a gray color was associated with the contraction 

 of both kinds of pigment. In the ordinary color changes 

 variations in the concentration of the golden pigment are 

 much less important, however, than the changes in the black 

 cells. 



The color changes in the skin are produced by numerous 

 agencies which act upon the pigment ceUs either directly or 

 through the central nervous system. The chromatophores 

 of the frog form a very delicate and responsive system which 

 is constantly undergoing changes in response both to stimuli 

 from the environment and the varying internal states of the 

 animal. One of the most important of the external stimuli 

 affecting the skin is light. It is a well-known fact that frogs 

 exposed to a bright light become light in color, while if 

 they are kept some time in the dark, the skin turns much 

 darker. These changes are much more pronounced in tree 

 frogs (Hyla) than in the species of Rana, and they bring 

 about an adaptation of the color of the animal to that of 

 its environment which is often very close. The question 



