630 THE POPULAR SCIENCE MONTHLY. 



confirmed by direct experiment, namely, by Westhoff, that several 

 fresh- water and marine fishes change their color from white to 

 dark as soon as they have been transferred from a medium with 

 a light-colored bottom to another medium the bottom of which is 

 dark. Fishermen, we are told by Mr. Poulton, even keep their 

 bait in white-colored vessels in order to make it assume a lighter 

 color. The common frog also can change its color to some extent 

 in harmony with its surroundings, while the green tree-frog of 

 southern Europe was long since known for this capacity. It is 

 bright green among green leaves, and dark green when seated on 

 the earth or among brown leaves.* Like changes are also known 

 in the chameleon and in some South American Jizards. The 

 causes of these changes have already been investigated by Pouchet 

 in 1848 and Briicke in 1852, but now we have a more elaborate 

 research by JSiedermann f upon the same subject. He has dis- 

 covered three different layers of cells which contribute to give 

 the frog its varying colors. There is first, deeply seated in the 

 skin, a layer of pigment-cells which contain black pigment both 

 in their interior and in their ramified processes, spreading within 

 the skin. These cells are covered by a second layer of " interfer- 

 ence-cells " containing bright yellow granules as well as granules 

 of a pigment which sometimes appear blue or purple, and some- 

 times gray the whole being covered with a transparent outer 

 skin. The normal green color of the frog is produced by a com- 

 bination of blue and yellow interference-cells appearing on a 

 black background ; but if the black pigment of the deepest layer 

 is protruded into its ramifications, the color of the animal becomes 

 darker ; and if it retires deeper, the yellow granules of the middle 

 layer become more apparent, and the frog assumes its lemon- 

 yellow color. Finally, when the yellow pigment gathers into 

 round drops between the bluish interference-cells not above 

 them the skin acquires a whitish-gray tint. The same arrange- 

 ments exist in other reptiles and amphibia. 



Now, how is it that the cells change their position in various 

 lights ? Is it some reflex action in the nervous system, as it 

 appears in fishes, which cease to change their color when they 

 become blind ? Or have we to .deal with some direct action of 

 light ? Facts are in favor of the second explanation. The slight- 

 est change of temperature affects the mutual disposition of the 

 pigment- cells, and consequently the color of the frog ; it is enough 

 to keep the animal in the hand to provoke a contraction of its 

 black cells. The amount of blood-supply also has a definite effect ; 



* E. B. Poulton, Colors of Animals, London, 1890, p. 82 et seq. 



f W. Biedermann, Ueber den Farbenwechsel der Frosche, in Pfluger's Archiv fur Physi- 

 ologie, 1892, Bd. li, p. 455. 



