PINEAL GLAND FUNCTION 209 



NATURE OF PIGMENTATION 



The color phenomena observable in many animal life forms 

 are due to the absorption or reflection of light rays by chemical 

 substances in the integument of the animal. These materials 

 are usually found as granules of pigment lying in specialized 

 cells, the chromatophores. Chromatophores are divisible into 

 several types, but of these only two are found in frog skin. 

 First, the melanophores, lying in the skin, peritoneum, etc., 

 contain granules of dark-brown or black melanin and are often 

 contractile. Second, the xanthophores, which are found only 

 in the adult skin, contain granules of light yellow xanthin and 

 are never contractile. 



The relationships between pigmentation and environment are 

 perfectly obvious but little understood. The color changes 

 of the common tree toad (Hyla arborea) have been accounted 

 for in various ways. One writer holds that the pale condition 

 is the result of the stimulative effect of light upon the chromato- 

 phores and that the diirk phase is due to the absence of that 

 stimulant (1). Another observer (2) claims that light alone 

 has very little effect, but that changes of temperature control 

 the coloration. The problem is complicated by the fact that 

 the same individual may not react in the same way under exactly 

 similar conditions. Certain it is, however, that animals do 

 respond to environmental changes by alterations in their color- 

 ation. It has been shown by numerous experimenters that these 

 changes are to a great extent, under the control of the eyes. 

 A change of environmental light is caught by the eye and the 

 resulting stimulus transmitted to the chromatophores by the 

 central nervous system. That this is the usual method of 

 procedure is indicated by the atypical reactions of blinded 

 individuals. 



The mechanism of these changes is entirely dependent upon 

 the contraction or expansion of the melanophores. Both xan- 

 thophores and melanophores enter into the color effects but the 

 former play a passive role and always present the same appear- 

 ance. The melanophores of frog tadpoles are of two distinct 



