AN EXPERIMENT. II 9 



The following are the notes I made from my examination of the 

 tish immediately alter their death : — 



1. 3.2 cm in length : black and yellow chromatophores on the 

 lower surface of the longitudinal fins and in a broad band on each side 

 of the lower surface of the body ; also on the edges of the lower side of 

 the head. 



2. 3.7 cm in length. Normal pigment all over the same band at 

 the edges of the body on the lower side : also in the angle behind the 

 operculum and on the lower pectoral. 



3. 3.2 cm in length. Pigmentation on the lower sides as in 1 and 

 2 but not quite so much of it. 



4. 6.3 cm in length. A small patch of chromatophores both black 

 and yellow in the area covered by the lower pectoral, and extending 

 beyond that area. 



5. 4.2 cm in length. Little pigment on the low. r side ; a little on 

 the pectoral, on the edges of the head, and near the ventral edge behind 

 the operculum. 



6. 5.7 cm in length. Pigment on the rays of the lower pectoral, 

 a,nd on the dorsal edge of the hea d. 



7. 5.3 cm in length. Pigment on lower side of head near edges ; 

 on lower branchiostegal membrane a good deal. 



8. 4.3 cm in length. Scattered black chromatophores behind body 

 cavity. 



9. 5.8 cm in length. A few black chromatophores near dorsal 

 edge of lower side of the head. 



10. 5.5 cm in length. A few scattered black chromatophores over 

 the lower side, especially behind the body cavity on the ventral half. 



11. 5.3 cm in length. No pigment on lower side except on lower 

 surface of the tail. 



12. 5.8 cm in length. No pigment on lower side. 



13. 3.3 cm in length. No pigment on the under side. 



At the same time I examined 4 of the specimens which had been 

 kept during the same time on a sandy bottom in the aquarium and 

 found no pigment on the lower sides of either. I have also frequently 

 had occasion to examine other of these specimens of the young flounders 

 of the same age kept in the tanks since last May, and have never seen 

 aDy pigment on the lower sides of any. 



To show the significance of this experiment it must be mentioned 

 that the colours of flat-fishes always " depend on three and only three 

 kinds of cellular elements, namely the black chromatophores, yellow or 

 orange-yellow chromatophores, both of which are capable of expansion 

 and contraction, and thirdly the iridocytes which are strongly reflecting 

 and white or slightly iridescent, and which are 6xed in shape and size. 

 The iridocytes are alone present on the lower sides of normal flat-fishes, 

 and give them their opaque white appearance. 



Of the above 13 specimens whose lower sides had been exposed to 

 light for less than 4 months only three had failed to devolope black and 



