1 8 BULLETIN OF THE BUREAU OF FISHERIES. 



become concentrated. This initial response may require but a few seconds and results 

 in a distinctly whitish shade. However, a fish which has not previously been adapted 

 to a white background does not become maximally white for a week or longer. Com- 

 plete adaptation to a white background involves a rearrangement and probably an 

 increase in the number of the guanophores and a material reduction in the number of 

 melanophores in the layer just beneath the epidermis. On the other hand, a specimen 

 which has recently been thoroughly adapted to a white background, but allowed to 

 assume a darker shade for a short time, resumes a maximally white shade very promptly 

 when again placed on a white background. The degree of concentration of the pigment 

 granules in the chromatophores and the relationships of the guanophores with these 

 bodies are essentially the same after the initial response in a specimen which has pre- 

 viously been thoroughly adapted to a white background as in one which has been on 

 a white background continuously for a prolonged period. 



The effect of a thorough adaptation to a white background remains apparent for 

 a considerable length of time after the fish is placed on a dark background. This is 

 shown by the following observations: A specimen 17 cm. in length which had been 

 kept in a white aquarium for four weeks was placed, on August 2, in an aquarium 

 painted black. This specimen assumed a somewhat darker shade almost immediately 

 and gradually became darker. On August 4 it still remained somewhat lighter than 

 the darker specimens newly taken. Many of the melanophores in the layer just beneath 

 the epidermis were well expanded, but the central area still remained depressed and in 

 many instances covered by guanophores. In some areas many of the melanophores 

 remained almost maximally contracted while guanophores remained closely aggregated 

 about them. In general, the melanophores were obscured to a lesser degree than when 

 the fish was transferred from the white aquarium. On August 7 it had become very 

 much darker, but not maximally black. In many areas the melanophores were maxi- 

 mally expanded, and their central areas were no longer depressed. In other areas 

 they were expanded only to a moderate degree, while many of them still remained 

 somewhat obscured by guanophores. The xanthophores remained in approximately 

 the same condition as they were when the fish was transferred from the white aquarium. 

 Guanophores still remained apparently more numerous in the superficial layer of the 

 skin than in specimens which had not been adapted to a light background, but were 

 somewhat contracted. Consequently, they occupied less of the surface area than 

 when the fish still assumed a lighter shade. On August 12 this specimen appeared 

 nearly as dark as other specimens in the same aquarium which were thoroughly adapted 

 to the black background. The melanophores in the superficial layer of the skin now 

 showed approximately the same degree of expansion as the corresponding melanophores 

 in the skin of a specimen which was maximally black. Neither were they any longer 

 obscured by overlying guanophores. These observations, in so far as they refer to the 

 rate of adaptation, are in harmony with those presented by Mast." 



FISH ON YELLOW BACKGROUND. 



The specimens used in this study were kept on a gamboge-yellow background for 

 intervals varying from 5 to 26 days. When thoroughly adapted to this background, 

 they were grayish yellow in color, mottled with areas of darker and lighter shades. 



a Bureau of Fisheries, document no. 821, p. 195-199. 



