30 



8) L-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 floun- 

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

 seen any pigment on the lower sides of any. 



To show the significance of this experiment it must be mentioned 

 that the colors 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 ex- 

 pansion and contraction, and thirdly the iridocytes which are strongly 

 reflecting and white or slightly iridescent, and which are fixed in shape 

 and size. Ihe 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 develope black 

 and yellow chromatophores in the skin of those sides. Three showed 

 very well developed bands of pigment quite similar to that of the upper- 

 side over the area occupied by the muscles of the longitudinal fins. 

 'Ihe other 7 specimens possessed a less quantity of pigment on the lower 

 it is true, but chromatophores were present in one part or another 

 where they are not present in the specimens living in the ordinary 

 way on sand. 



The question of course arises, how are these pigment cells deve- 

 loped, by migration from the upper side/ from wandering lymphatic 

 cells? or from unpigmented cells already present in the same position 

 before? These questions I cannot at present answer, but am now endea- 

 vouring to find replies to them. I think the third suggestion the most 

 probable. The chromatophores in flat-fishes are situated in the derma 

 between the surface of the scales and the epidermis. 



