COLORATION OF THE SKINS OF FLAT-FISHES. 113 
shallow. After this experiment had lasted six months I observed a 
commencement of pigmentation on one of the four fish. At the end 
of June, 1891, one of the four died. I had placed bricks in the 
tank to keep the fish in the centre of the glass bottom, and this 
specimen had got fixed between a brick and the side of the tank. 
In September one of the three survivors had developed pigment 
all over the external regions of the lower side; in the other two 
pigment could not be detected with certainty. In this month another 
specimen, fortunately not the pigmented one, died. The remaining 
two lived on till July, 1892, when another died. ‘This one was 23 cm. 
(about 9 inches) long, and had a large number of separate spots 
of pigment on the lower side. These spots were of considerable size 
and dark. Under the microscope they were found to consist of 
chromatophores exactly similar to those which constitute the 
pigmentation of the upper side. The fourth specimen is still alive at 
the present time. It is now three years old, and has lived in the 
apparatus since September, 1891. It is now deeply pigmented all 
over the lower side with the exception of a very small area. 
There can be no doubt that the pigmentation in this experiment 
was due to the exposure of the lower sides of the fish to light. 
There were only four fish used, and two of them which lived long 
enough developed pigment which continually increased in extent. 
This is 50 per cent., and although pigment occurs on the lower sides 
of flounders living under natural conditions as an occasional abnor- 
mality, the percentage of such specimens is nothing like 50 per cent. 
It is important to point out that these four specimens were taken 
from a number reared in the aquarium in tanks with sand at the 
bottom, and subjected to no artificial conditions except captivity. 
In this Journal, vol. ii, No. 38, I have given the result of the 
examination of all such specimens reared from the brood of 1890. 
There were ninety specimens altogether, and one of these had a few 
small patches of pigment on the lower side. ‘These were two years 
old when examined, and a more rigid control experiment could 
scarcely be required. 
In another experiment I took one of this same brood (not one of 
the ninety just mentioned, but one taken before the examination 
referred to) which had one small spot of pigment close to the 
‘pectoral fin, and placed it in the apparatus, where its lower side was 
exposed to the light. In a few months the pigmentation of the 
lower side had extended over the greater part of that side. 
Other similar experiments are described in greater detail in the 
full memoir by Dr. MacMunn and myself recently communicated to 
the Royal Society. Other experiments are now in progress, and a 
figure of the apparatus in use, prepared from a photograph, is here 
NEW SERIES.—VOL, Ill, NO. I. 
