HISTOI^OGICAL BASIS OF ADAPTIVE COLORS IN PARAUCHTHYS ALBIGUTTUS. 



33 



30 



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Z6 



following this period the number of melanophores in the superficial layer of the skin 

 remained fairly constant while the fish remained on the white background. 



The averages of counts made on specimens which had been kept on a white back- 

 ground for a period of two months show, approximately, the same number of melano- 

 phores per unit area in the layer just beneath the epidermis as was shown by the counts 

 made on the specimen used in the above experiment after it had been on the white 

 background for a period of 1 1 days. It is probable, therefore, that when a fish becomes 

 adapted to a background of a very light shade a maximum reduction in the number 

 of melanophores in the superficial layer of the skin takes place in a relatively short 

 time, and that little or no further reduction occurs even though a light shade is main- 

 tained for a prolonged period. This conclusion is supported by observations made 

 by H. S. Willis on specimens which had been kept on given backgrounds for six months 

 or longer." This observer made camera lucida drawings under low magnification 

 including all the chromatophores in the layer just beneath the epidermis on detached 

 scales taken from various parts of the body. The averages of actual counts of the 

 number of melanophpres per unit area in these drawings indicate that melanophores 

 are approximately equally abundant in 

 the superficial layer of the skin of speci- 

 mensadapted to black, blue, and red back- 

 grounds and also of specimens adapted to 

 white and yellow backgrounds. However, 

 the average number per unit area in the 

 drawings taken from specimens adapted 

 to white and yellow backgrounds is only, 

 approximately, 70 per cent of the average 

 number per unit area in those taken from 

 specimens adapted to black, blue, and red 

 backgrounds. The difference here indi- 

 cated in the number of melanophores per 

 unit area in the superficial layer of the skin of specimens which had assumed dark and 

 light shades, respectively, for a relatively long period is not appreciably greater than the 

 corresponding difference indicated by the observations recorded above on specimens which 

 had assumed dark and light shades, respectively, for relatively short periods. 



When a fish in which the number of melanophores in the superficial layer of the 

 skin has become reduced in response to the continued stimulus of a light background 

 is transferred to a dark background, the number of melanophores in the superficial 

 layer of the skin is gradually restored. The rate at which this restoration took place 

 in a single specimen is indicated by the curve Cd in the accompanying figure (text fig. 

 3). This specimen was kept on a white background for four weeks and then trans- 

 ferred to a black background. The curve is based on the averages of 10 actual counts 

 of the melanophores in a given unit area on detached scales taken from various parts 

 of the body. Counts were made when the fish was transferred from the white back- 

 ground and at intervals of two or three days following this date. At the end of two 

 weeks, when the experiment was discontinued, the number of melanophores per unit 

 area was stUl somewhat less than the average for dark-colored specimens. 



a These observations were made under the direction of Dr. S. O. Mast and are used here with his consent. 



S4 



2 



m \% 14- 



IS 20 



Fig. 3. — Curves showing the rate of change in the number of 

 melanophores in the layer just beneath the epidermis. (For 

 explanation, see text.) 



