192 THE BIOLOGY OF THE FROG chap. 



fusca by Von Wittich. They occur in the corium, and were 

 observed to undergo changes in the distribution of their 

 pigment like those of the black chromatophores. 



Nearly all of the color changes which the skin of the frog 

 undergoes depend upon the differences in the distribution 

 of two elements, the black and the yellow pigment. When 

 the pigment of the black chromatophores is expanded, the 

 skin becomes dark in color, owing to the fact that the black 

 pigment is spread over a greater amount of surface. When 

 the skin is light in color, the black pigment becomes con- 

 tracted into small masses, thus allowing the light to be 

 reflected from the other pigment cells. These facts may 

 easily be demonstrated by comparing the skin of a dark 

 frog with that of a light one, when great differences in the 

 chromatophores will almost certainly be observed. Although 

 the black chromatophores lie mainly below the golden cells, 

 their branches cover the latter to a greater or less extent, 

 and when the black pigment is fully expanded, it cuts off 

 much of the light which would otherwise be reflected from 

 them. 



The golden color that appears in the frog's skin is due 

 directly to the pigment in the golden cells, but the green is 

 not produced in so simple a manner. There is no green 

 pigment in the frog's skin, and various explanations have 

 been offered as to how this color comes to appear. The 

 subject has been investigated by Briicke, Harless, Von Wit- 

 tich, Eberth, Biedermann, and Ehrmann, each of whom 

 disagrees in certain particulars with the others. Briicke 

 regarded the green color as a simple interference phenome- 

 non caused by the granules of guanin ; but that the golden 

 pigment is necessary to the production of green was subse- 

 quently shown by the fact that when the golden pigment is 

 dissolved out of the cells the green color disappears although 



