630 THE POPULAR SCIENCE MONTHLY.
confirmed by direct experiment, namely, by Westboff, that several fresb-water and marine fisbes cbange tbeir color from wbite to dark as soon as tbey bave been transferred from a medium witb a ligbt-colored bottom to anotber medium the bottom of wbicb is dark. Fisbermen, we are told by Mr. Poulton, even keep their bait in wbite-colored vessels in order to make it assume a ligbter color. The common frog also can cbange its color to some extent in barmony witb its surroundings, wbile the green tree-frog of southern Europe was long since known for tbis 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 lizards. The causes of these changes have already been investigated by Pouchet in 18-48 and Briicke in 1852, but now we have a more elaborate research by Biedermann 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 cbange tbeir 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 tbey 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 Pfliigei's Arcbiv f iir Physi- ologic, 1892, Bd. li, p. 455.