Colour Change 



do not only with the physiological condition, but also concern the 

 primitive sensations and emotions of the frog. Some scientists 

 go as far as to say that changes of colour in Batrachia " can be 

 inhibited by the will and can be produced at will." 



A frog that possesses a layer of black pigment cells under 

 the epidermis will, as indicated, change from light to dark, but 

 if there are no other pigments or other structures, the shades 

 passed through will be dull browns and greys. If yellow pigment 

 is possessed also, the changes will be from black through bright 

 browns to light yellowish shades. Green colour is not produced 

 directly by a pigment, but as the result of black and yellow pig- 

 ments and a structure, namely, the so-called interference layer. 

 This is a single layer of polygonal cells between the epidermis 

 and the black pigment layer. These cells contain minute yellow 

 particles, which crowd to the sides of the cells next the epidermis. 

 If these cells were empty, the result would be simply that the 

 black pigment layer would be farther from the surface and 

 would be seen through the interference layer. This would make 

 the colour appear blue instead of black, giving the ordinary 

 colour phenomenon of dense media, as seen in the colour of 

 the sky (i. e. all the light waves of great wave lengths are 

 absorbed and only those of short lengths are ' reflected) . When 

 the interference cells contain yellow, the blue is seen through the 

 yellow and the resultant colour is green. 



The green colour thus produced may be more blue or more 

 yellow, and may change to brown when the black pigment cells 

 expand greatly and press close about the cells of the interference 

 layer, thus diminishing the density. A fragment of the outer 

 skin of a green tree frog examined from underneath, of course 

 appears black, because we are looking directly at the black pig- 

 ment layer. The same piece looked at from above and against 

 the light appears brown, but examined from above with light 

 from above, it appears green and shows the polygonal cells of 

 the interference layer. 



Therefore frogs that have the interference layer and black 

 and yellow pigments appear green or brown and can change from 

 one to the other with great rapidity. Sometimes to these pig- 

 ments and the interference layer are added pigments of other 

 colours, red, white, and metallic, or iridescent. Consideration 



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