148 THE STORY OF REPTILE LIFE. 



chiefly to the very different nature of the covering 

 of the body. In the feathers of the bird, changes 

 of colour can take place only with extreme slow- 

 ness ; whereas, in the scaly skin of the Reptile, 

 or fish, or the smooth skin of the frog, a fleeting 

 play of colour is possible. It is brought 

 about in this wise. The horny outermost layer 

 of the skin is colourless; in the layer beneath 

 this are embedded iridescent cells with striated 

 surfaces. Below this, in the deepest layer of the 

 skin, cutis, are a large number of cells filled 

 with highly refractive granules, chiefly guanin- 

 crystals. These cause white colour by diffuse 

 reflection of direct light. Nearer the surface are 

 cells filled with oil-drops, and these give a yellow 

 colour. In the granular mass are embedded 

 numerous colour-bearing branching sacks or 

 chromatophores containing, for the most part, 

 blackish-brown or reddish pigment. The branches 

 of these sacks being contractile, the contained 

 granules of colour are drawn away from or 

 towards the surface of the skin, and thus, 

 combining with the stationary colour, effect a 

 corresponding change in the coloration of the 

 animal. 



The marvellously .vivid hues which bedeck so 

 many Reptiles are all produced by a very limited 

 palette black, red, yellow, and white, with the 

 combinations grey and brown. The white pig- 

 ment, as we have already remarked, is due to 

 guanin-crystals ; blue and green are structural or 

 optical colours. The former appears, at least in 

 birds, to be associated with a dark-coloured pig- 

 ment; the latter with a yellow pigment, in 



