588 



Mr, Hans Gadow 



[April 26, 



granular and colourless, while the upper half is made up of drops of 

 yellow oil interspersed in the cell plasma. Since these cells inter- 

 fere with the light, they are called the interference layer. 



A vertical section through the skin shows the following arrange- 

 ment. Below the black chromatophores, looking rather like the roots 

 of a tree-stump turned upside down. More black is distributed in 

 the upper strata of the cutis, near the interference layer. The super- 

 imposed protective epiderm may be left out of account, although its 

 condition somewhat influences the whole phenomenon. 



The presence of the layer of interference cells upon the under- 

 lying black background produces the well-known phenomenon of the 

 colour of dense media — namely, blue. Only the rays of short wave- 



Epiderm 



Oil ) Inter ferenct 



Prxsm, 



Cdls 



QuomaioDhores 



Diagram of a vertical section through the skin of a green Tree-frog, showing 

 the chromatophores in various stages of contraction and expansion. The layer 

 of Oil in the interference cells is indicated by dots, except where the Oil has 

 coagulated into a few big drops. 



At A the skin would appear green, at B dark brown, at C leaden grey, at 

 D yellow. 



lengths are reflected, while those of greater length are absorbed by 

 the black background. Consequently the skin would appear blue, 

 just as the sky is blue, or diluted milk becomes bluish, and the veins 

 of thin and white-skinned people appear bluish. But in our Frog's 

 skin the blue light is mixed with the yellow coming from the upper 

 half of the cells, from the yellow oil, and the result of blue and 

 yellow is green. 



This can be shown experimentally. Application of a drop of 

 caustic potash dissolves the guanine crystals, and the skin at once 

 appears black. On the other hand, remove the yellow oil by 

 extracting it in alcohol, then the green Frog looks blue. An instance 

 of this is the scientific name of the grass-green Australian Tree-frog, 

 Hyla coertdea, so-called because it is always blue in our spirit collec- 

 tions. A scratch in the skin of a living green Tree-frog, just deep 



