HISTOLOGICAL BASIS OF ADAPTIVB COLORS IN PARALICHTHYS ALBIGUTTUS. 1 7 



the radial processes, giving the pigmented area a very irregular outline. In some instances 

 the aggregates of pigment granules extended farther peripherally in one or two of the 

 radial processes, while the others were entirely free from pigment. Relatively few 

 xanthophores were expanded to a moderate degree and showed a more symmetrical dis- 

 tribution of the pigment granules. The ratio of the surface area occupied by xanthine 

 pigment to that occupied by melanin pigment was appreciably greater in these speci- 

 mens than in those adapted to a black background. However, the xanthophores were 

 more effectively obscured by overlying guanophores in the former than in the latter. 



Guanophores were apparently more numerous in the layer just beneath the epi- 

 dermis in the skin of these specimens than in the skin of specimens newly taken. They 

 occupied nearly all the surface area and were closely aggregated about many of the 

 melanophores. As indicated in an earUer section of this paper, whether or not a real 

 increase in the number of guanophores in the superficial layer of the skin occurs in speci- 

 mens which become thoroughly adapted to a white background could not be definitely 

 determined. Doubtless, the more complete occupation of the surface area by the guano- 

 phores in the skin of these specimens is 

 due, at least in part, to the expansion 

 of these bodies. Nevertheless, the 

 changes which bring about the more 

 complete occupation of the surface area 

 by the guanophores constitute an im- 

 portant factor in the production of a 

 still lighter shade following the initial 

 response of the fish to a white back- 

 ground. 



Both melanophores and xantho- 

 phores lying superficial to the proximal 

 areas of the scales and in the deeper 

 layers of the skin were contracted to a 

 somewhat lesser degree, respectively, than those in the layer just beneath the epi- 

 dermis. In many instances, while the radial processes could not be definitely out- 

 lined, the peripheral portions of many of them which contained no pigment granules 

 still assumed a slight yellowish tinge. 



The accompanying figure (text fig. 7) is an attempt to illustrate the vertical dis- 

 tribution of chromatophores and guanophores in the layer just beneath the epidermis 

 and in the layer overlying the proximal area of a scale in the skin of a specimen which 

 was thoroughly adapted to a white background. The solid areas represent melanin 

 pigment, the stippled areas include both xanthophores and guanophores. Figure 4, 

 plate I, represents camera lucida drawings of chromatophores and guanophores in 

 the layer just beneath the epidermis in a lighter and a darker area of the skin of a 

 specimen 22 cm. in length which had been kept on a white background for a period of 

 25 days. 



All the specimens which were kept in white aquaria became adapted to the white 

 background very rapidly. The initial response to a white background consists in a 

 marked concentration of the pigment granules in the majority of the melanophores and 

 the depression of the central area of many of those in which the pigment does not 



Fig. 7. — Camera lucida drawing illustrating the vertical distribu- 

 tion of chromatophores and guanophores just beneath the epidermis 

 and superficial to the proximal area of a scale in the skin of a sped- 

 men of Paralichthys albigutius adapted to a white background . Solid 

 color represents melanin pigment, stippling includes both xantho- 

 phores and guanophores. 



