HISTOLOGICAI, BASIS OF ADAPTIVE COLORS IN PARAUCHTHYS ALBIGUTTUS. 1 9 



In these specimens the majority of the melanophores in the layer just beneath 

 the epidermis were contracted only to a moderate degree. In the lighter areas the 

 melanophores were contracted to a greater degree, but rarely was the pigment all 

 concentrated in the central area. The melanophores lay immediately beneath the 

 epidermis and were not obscured to an appreciable extent by overlying guanophores. 

 However, the central area of those most fully expanded was not infrequently somewhat 

 depressed. 



Many of the xanthophores in the layer just beneath the epidermis were apparently 

 maximally expanded, while others were expanded only to a moderate degree. In 

 those which were not fully expanded the major portion of the pigment was concentrated 

 in the central area, while masses of pigment granules extended toward the periphery 

 but a short distance in some of the radial processes and farther in others. The pig- 

 mented area in these chromatophores was usually very irregular in outline. Rarely 

 was a xanthophore observed in which all the pigment was concentrated in the central 

 area. The ratio of the surface area occupied by xanthine granules to that occupied 

 by melanin granules was relatively large. 

 Because of the wider distribution of the pig- 

 ment granules the color of individual xan- 

 thophores was less intense than in the skin 

 of specimens not adapted to a yellow back- 

 ground. 



The guanophores in the layer just be- 

 neath the epidermis occupied a relatively 

 large part of the surface area, rarely over- 



, . , Fig. 8. — Camera lucida drawing illustrating the vertical dis- 



lymg melanophores, but very commonly tribuUon of chromatophores and guanophores just beneath 



overlying xanthophores, at least in part. the epidermis and superficial to the proximal area of a scale 



, in the skin of a specimem of Paralichtkys albiguttus adapted 



However, the xanthine pigment was not to a yeUow background. SoUd color represents melanin 



largely obscured by them. Guanophores pigment, stipphng includes both xanthophores and 



which lay above or in immediate contact 



with xanthophores assumed a greenish-yellow tinge. This phenomenon was more 

 apparent in specimens adapted to a yellow background than in others, because the 

 xanthine pigment in the former was more widely distributed. Guanophores not in 

 close proximity with xanthine or melanin pigment in the skin of specimens adapted 

 to a yellow background did not differ in shade or color from corresponding guanophores 

 in the skin of specimens newly taken or those adapted to a dark background. The 

 yellow color of the background had no direct effect upon the color of these bodies. 



Both melanophores and xanthophores lying superficial to the proximal areas of 

 the scales and in the deeper layers of the skin were well expanded, the xanthophores 

 to a somewhat greater degree than the melanophores. Relatively few guanophores 

 occur in these layers. 



The accompanying figure (text fig. 8) is an attempt to illustrate as nearly as pos- 

 sible the vertical distribution of chromatophores and guanophores in the layer just 

 beneath the epidermis and in the layer overlying the proximal area of the scales in the 

 skin of a specimen which was thoroughly adapted to a yellow background. The solid 

 areas represent melanin pigment, the stippled areas include both xanthophores and 

 guanophores. Figure 5, plate i, represents a camera lucida drawing of chromato- 



