MUSEUM OF COMPARATIVE ZOOLOGY. 191 
developed into young with well marked black pigment cells of the type 
of those of Figures 2 and 5 of the same plate. 
With Platessa, the young kept on dark tiles passed into stages with 
well marked black pigment cells, while in the specimens kept on 
white tiles the pigment cells were reduced to a minimum, — to mere 
circular dots. 
In the case of Hemitripterus americanus, in which the reddish pig- 
ment cells play a more important part, the young kept upon white tiles 
will assume the appearance of such types as are figured on Plate III. 
Figures 8, 9, of Agassiz and Whitman’s Development of Osseous Fishes, 
Part I.,1 while in those kept upon black tiles the black pigment spots 
develop in a marked manner, and form an upper layer of large regularly 
dendritic chromatophores on the flanks of the body. See Figures 10, 
11, of the same plate. The yellowish-red colored cells are not as much 
affected by the color of the ground as was found to be the case in the 
colored cells of the young of Ctenolabrus. 
The color of the fish is due to differently colored chromatophores 
placed on a lower level than the black pigment cells. These may be 
red, yellow, brown, blue, or other color, and by their combination and 
the greater or less prominence of a special set of chromatophores will 
give to the young fish its prevailing tint, which may be distributed in a 
general tone, or in patches and bands on the side of the body or head. 
On keeping selected lots of young specimens of Ctenolabrus, prob- 
ably two months old, of various coloring, upon black and white tiles 
(Figs. 8-16), the results were found to be similar to those obtained with 
the older Gasterosteus. Only the faculty of recovering their original 
color was evidently not so easily lost as in the case of the older Gas- 
terosteus. This would seem to indicate that to retain a condition of 
coloring brought about or modified by surrounding influences, the 
young fish must remain exposed to them for a considerable time, and 
the modification will be more or less permanent, or of a greater or less 
degree, according to the age of the fish. 
The young fishes placed upon black tiles retained their brilliant col- 
oring, no matter whether red, yellow, brown, or blue chromatophores 
were present ; and neither they nor the black pigment cells were modi- 
fied by the light reflected from the black tiles (Fig. 16). On the other 
hand, the young fishes kept for ten days upon white tiles had lost, in pro- 
portion to their original dark tint, much of their dark color, the black pig- 
ment cells having become reduced in some of them to mere dots, in others 
1 Mem. Mus. Comp. Zoöl., Vol. XIV. No. 1, Part I. 1886. 
