. The biology of the frog . same substance, according to Ktihne, produces theyellow 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 greencoloration. The golden cells usually contain an additional source ofcolor in the form of the so-called interference granules, orthe iridescent pigment of Leydig. These granules, accordingto Ewald and Krukenberg, are composed of guanin. Theyare soluble in caustic soda or potash and present an appear-ance of cross striation (Biedermann). By
. The biology of the frog . same substance, according to Ktihne, produces theyellow 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 greencoloration. The golden cells usually contain an additional source ofcolor in the form of the so-called interference granules, orthe iridescent pigment of Leydig. These granules, accordingto Ewald and Krukenberg, are composed of guanin. Theyare soluble in caustic soda or potash and present an appear-ance of cross striation (Biedermann). By transmitted lightthey are brown or gray, but in reflected light they areusually blue. The interference cells are stellate chromatophores whichare mainly confined to the subcutaneous tissue of the ventralside of the body, where they produce the light color charac-teristic of that region. They contain guanin granules likethose in most of the golden cells. Red stellate pigment cells have been described in Rana IX THE SKIN 191. 192 THE BIOLOGY OF THE FROG chap. fusca by Von Wittich. They occur in the corium, and wereobserved to undergo changes in the distribution of theirpigment like those of the black chromatophores. Nearly all of the color changes which the skin of the frogundergoes depend upon the differences in the distributionof two elements, the black and the yellow pigment. Whenthe pigment of the black chromatophores is expanded, theskin becomes dark in color, owing to the fact that the blackpigment is spread over a greater amount of surface. Whenthe skin is light in color, the black pigment becomes con-tracted into small masses, thus allowing the light to bereflected from the other pigment cells. These facts mayeasily be demonstrated by comparing the skin of a darkfrog with that of a light one, when great differences in thechromatophores will almost certainly be observed. Althoughthe black chromatophores lie mainly below the golden cells,their branches cover the latter to
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