112 



north side of the Laboratory at some distance from the windows in a position where it 

 was partly shaded. Then the result shown in Plate II was produced. But the 

 drawing for this plate had to be finished from the sole in the position I have described. 

 When the sole in the tub at its maximum darkness was carried to a table in front of 

 the window, the colours immediately began to get lighter. I found that the white 

 spots were exceedingly curious in their behaviour. As I have mentioned, on white 

 porcelain with plenty of light the white spots disappear as such and are changed into 

 bluish spots which sometimes become quite dark and conspicuous. The white spots 

 also disappeared in the sole which was kept on coal with very little light, reappearing 

 in a few seconds when more light was admitted. Thus the white spots are generally 

 visible except on white ground with a great deal of light, or black ground with very 



little. 



As to the rate of change it is usually quite rapid. A sole placed on the white dish 

 begins to get lighter almost immediately ; when it is disturbed with the hand the colours 

 become darker again, but when left alone it continues to grow paler. However, the 

 full effect is not seen till the fish has been some hours on the white ground. 



A sole kept on coarse yellow gravel in front of a window is very inconstant in its 

 colouring : it sometimes exhibits its markings quite distinctly for some hours, and then 

 begins to grow pale, its black blotches almost vanishing. 



On May ^2, I took a sole in which the markings were well expressed and the 

 colour moderately dark, and placed it on the white porcelain dish ; in a short time the 

 colours had become pale, and the dark blotches had become yellow. Then I placed 

 some fine shingle in the dish, intending to bring the markings out again, but to my 

 surprise the black blotches had not returned when I examined the fish the next 

 morning, and did not return completely when I placed it in an oaken tub nine inches 

 high, although the ground colour became somewhat darker. 



All the changes are evidently due to the action of light and depend on the quantity 

 of light acting on the sole, not on the tint or texture of the ground on which it' rests. 

 The behaviour of the white spots I cannot yet explain, but all the rest seems to me 

 easily intelligible when regarded in this way. The colour and markings of the skin are 

 due to a vast number of chromatophores situated in the skin beneath the epidermis : 

 these are of two colours, the black and the yellow, and in some places there are others 

 containing a somewhat iridescent pigment. The last are particularly abundant in the 

 white spots, the black especially abundant in the dark markings. Light causes these 

 chromatophores to contract. When expanded the chromatophores or pigment-cells 

 are stellate, giving out ramified processes on all sides. When they are contracted all 

 these processes are withdrawn and each cell becomes a mere minute speck of pigment. 

 The position of these chromatophores is fixed, and therefore when they are expanded, 

 in the absence of light, the markings always reappear 'in exactly the same positions, in 

 fact the markings never entirely disappear. When a very strong light falls upon the 

 sole all the chromatophores are contracted, and all the parts become proportionally 



