T90 THE BIOLOGY OF THE FROG CHAP. 



invisible except when containing pigment. Virchow, Von 

 Wittich, and Biedermann think that the changes in the 

 chromatophores may involve both of these factors. The 

 question is still not certainly decided. 



The cells which give the skin its golden and green colors 

 form a layer immediately beneath the epidermis. Unlike the 

 black chromatophores they are usually rounded or polygo- 

 nal in form, and they lie a little above the black cells, which 

 constitute a sort of dark background. Their golden color is 

 due to a fatty pigment or lipochrome, which is sometimes 

 diffused throughout the cell and at other times aggregated 

 into large drops (Biedermann). This pigment is soluble in 

 alcohol, chloroform, and ether, giving a golden yellow solu- 

 tion which turns to a yellowish green when very dilute. 

 The same substance, according to Kiihne, produces the 

 yellow color of the fat body. In frogs which have been pre- 

 served for some time in alcohol this pigment disappears, 

 and consequently the specimens lose their golden and green 

 coloration. 



The golden cells usually contain an additional source of 

 color in the form of the so-called interference granules, or 

 the iridescent pigment ofLeydig. These granules, according 

 to Ewald and Krukenberg, are composed of guanin. They 

 are soluble in caustic soda or potash and present an appear- 

 ance of cross striation (Biedermann). By transmitted light 

 they are brown or gray, but in reflected light they are 

 usually blue. 



The interference cells are stellate chromatophores which 

 are mainly confined to the subcutaneous tissue of the ventral 

 side of the body, where they produce the light color charac- 

 teristic of that region. They contain guanin granules like 

 those in most of the golden cells. 



Red stellate pigment cells have been described in Rana 



