1901. ] Colour in Amphibia. 587 



WEEKLY EVENING MEETING, 

 Friday, April 26, 1901. 



His Grace the Duke of Northumberland, K.G. D.C.L. F.E.S., 



President, in the Chair. 



Hans Gadow, Esq., M.A. Ph.D. F.R.S. 



Colour in Amphibia. 



The colours of the skin of the Amphibia are of two kinds, pigment 

 or chemical, and structural. The pigments are either diffuse or 

 granular ; they are usually black, red, or yellow, and they remain 

 the same whether examined under direct or under transmitted light, 

 although they may change in extent and intensity. The pigment 

 granules are stored up in cells, chromatophores, which send out 

 branched processes and are restricted to the cutis, or leathery part of 

 the skin, mostly to its upper layers. The superimposed epiderm is 

 thin and, as a rule, colourless. Contraction of the chromatophores 

 withdraws the pigment from the surface and causes the skin to appear 

 paler ; expansion distributes the pigment more or less equally near the 

 surface, which consequently assumes a darker, more saturated, tint. 



Secondly, there are colours which are due to structure. This 

 applies always to green, blue and violet. These colours are produced 

 by interference of the light, with an underlying stratum of black or 

 yellow. iSuch colours are highly changeable. 



A peculiar kind of colouring matter is the white pigment, which 

 may, perhaps, be classified in a third category. It is unchangeable, 

 and consists apparently of tiny crystals of guanine, a product allied 

 to uric acid, and is, as a rule, deposited within cells. 



One of the most interesting colours is green. It is not only 

 very common in many frogs, especially such as lead an arboreal 

 life, but it strikes us by its vivid, saturated tints, and last, not least, 

 by the changes which creatures thus endowed can exhibit in their 

 coloration. If you examine a piece of the skin of a green Tree-frog 

 under transmitted light, by holding it against the light, it does not 

 look green at all, but more or less black-brown. The same piece 

 when examined under low power and direct light, shows a mosaic of 

 green polygonal areas, separated by black lines and interrupted by 

 the openings of the skin glands. Seen from the under surface the 

 skin is black. Under strong power the black layer is seen to be 

 composed of ramified black pigment cells, and where the light shines 

 through, the skin appears yellow. The mosaic layer is composed of 

 polygonal cells, each of which consists of a basal half, which is 



