II CHANGES OF COLOUR 35 



Most Amphibia are capable of changing colour, the Urodela, 

 however, far less than the Anura, some of which exhibit an 

 extraordinary range and adaptability in their changes. 



The mechanism by which the change of colom' is produced in 

 frogs has been recently studied by Biedermann.^ If we examine 

 the green skin of the common Tree-frog, Hyla arhorea, under a 

 low power and direct light, we see a mosaic of green, polygonal 

 areas, separated by dark lines and interrupted liy 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 pig- 

 ment 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 Ijetween 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 withcbaw 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 chrom- 

 atophores. 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. 



Tree-frogs turn green as the result of the contact with leaves. 

 1 Arch. (jes. Phijsiol. li. 1892, p. 455. 



