The mechanism by which the change of colour is produced in frogs has been recently studied by Biedermann.[15] If we examine the green skin of the common Tree-frog, _Hyla arborea_, under a low power and direct light, we see a mosaic of green, polygonal areas, separated by dark lines and interrupted by the openings of the skin-glands. Seen from below the skin appears black. Under a stronger power the black layer is seen to be composed of anastomosing and ramified black pigment-cells. Where the light shines through, the skin appears yellow. The epidermis itself is quite colourless. The mosaic layer is composed of polygonal interference-cells, each of which consists of a basal half which is granular and colourless, while the upper half is made up of yellow drops. Sometimes the tree-frog appears blackish, and if then the black pigment-cells are induced to contract, for instance, by warming the frog, it appears silver-grey; in this case the pigment in the yellow drops is no longer diffuse, but is concentrated into a round lump lodged between the interstices of the granular portions; the black pigment-cells are likewise balled together. These black chromatophores send out numerous fine branches, which occasionally stretch between and round the polygonal cells. When each of these is quite surrounded and covered by the black processes, the frog appears black. On the other hand, when the black pigment-cells withdraw their processes, shrink up, and, so to speak, retire, then the light which passes through the yellow drops is, by interference, broken into green. Stoppage of the circulation of the blood in the skin causes the black chromatophores to contract. Carbon dioxide paralyses them and causes them to dilate. This is direct influence without the action of nerves. But stimulation of the central nerve-centres makes the skin turn pale. Low temperature causes expansion, high temperature contraction, of the chromatophores. Hence hibernating frogs are much darker than they are in the summer. Frogs kept in dry moss, or such as have escaped into the room and dry up, turn pale, regardless of light or darkness, probably owing to a central, reflex, nerve-stimulus.