116 COLORATION OF THE SKINS OF FLAT-FISHES. 
is not opaque white, but bluish and translucent. The chromatophores, 
both black and coloured, are the first elements of coloration to 
develop, appearing in the skin of the embryo even before it is 
hatched. The external iridocytes appear next, and are found in the 
flounder during its transformation when it is + to 4 of an inch in 
length. The internal reflecting layer of the lower side develops late 
and very gradually. It first appears in streaks along the lines of 
the intermuscular septa, when the flounder is about 14 inches long 
and extends gradually. When the flounder is 3 to 4 inches long, 
the white opacity is usually fully developed. ‘The peritoneum contains 
the same elements of coloration as the skin, namely, chromatophores 
and reflecting substance, and it is an important and significant fact 
that in the normal flat-fish the chromatophores are present only in the 
peritoneum of the upper side, while in that of the lower side they are 
absent or very scarce, and on this side the reflecting tissue is more 
largely developed. In the flounder the reflecting substance appears 
in the peritoneum of the lower side earlier than in the skin, and as 
it is visible through the walls of the body the abdominal region in 
the young flounder is marked out as a white area, while the rest of 
the lower side is bluish and translucent. 
As might be expected, the elements of coloration in other fishes 
are not essentially different from those of flat-fishes. But it is a 
striking peculiarity in the flat-fishes that they are destitute of the 
silveriness and iridescence which is so characteristic of many fishes, 
especially those like the mackerel and herring which are migratory 
or pelagic. Investigation shows that the silveriness of such fishes 
depends almost entirely on a thick subcutaneous or internal layer of 
reflecting substance corresponding to that of the flat-fish. These 
layers, in fact, are homologous, the difference in appearance being 
due to a difference in the structure of the reflecting layer, which in 
the flat-fish is granular, in the silvery fish is composed of minute 
parallel rods or needles. Both layers may be conveniently called 
the argenteum. In other fishes as in flat-fishes, chromatophores, 
black and coloured, occur in an external layer and an internal, 
and where the chromatophores are most developed the argen- 
teum is evanescent, and vice versa. In other fishes there are 
also bodies corresponding to iridocytes, but they vary in form and 
arrangement. In fishes whose skins are iridescent, as the herring, 
this quality is due to a layer of parallel rods or prisms of reflecting 
substance, which in the herring line the inner surface of each scale, 
or more accurately are present between those parts of the scales 
which overlap one another. ‘The scales themselves are never irides- 
cent. This iridescent substance is obtained from the scales of 
certain fishes, especially the fresh-water bleak (Albunus lucidus), and 
