A Turn-Coat of the Woods 



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and colorless, while the upper half is made up 

 of yellow drops. Sometimes the tree-frog ap- 

 pears blackish, and if then the black pigment- 

 cells are induced to contract, for instance by 

 warming the frog, it appears silver-gray; in 

 this case the pigment in the yellow drops is no 

 longer diffuse, but is concentrated into a round 

 lump lodged between the interstices of the gran- 

 ular portions; the black pigment-cells are like- 

 wise balled together. These black chromato- 

 phores send out numerous fine branches, which 

 occasionally stretch between and around 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 withdraw 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 paralyzes them and 

 causes them to dilate. This is direct influence 

 without the action of nerves. But stimulation 

 of the nerve-centers makes the skin turn pale. 

 Low temperature causes expansion, high tem- 

 perature contraction, of the chromatophores. 

 Hence hibernating frogs are much darker than 



+ 221 &o 



