158 
BULLETIN OF THE BUREAU OF FISHERIES 
DEVELOPMENT OF THE LARVA 
DIGESTIVE TRACT 
The yolk sac is present for but six to eight days, when the larva is 11 millimeters 
in length. A few complete the absorption of the yolk sac somewhat earlier. As 
the sac disappears the gut rapidly expands and enlarges from a size smaller than the 
notochord to that of the yolk sac itself. The mouth becomes open at about 9 milli- 
meters and the branchial apparatus is completed. Food may be taken as early as 
the 9-millimeter stage, though this is rare. Up to 17 millimeters the gut is a straight 
tube from mouth to anus. The subsequent changes in the structure of the intestine 
will be described hereafter. 
Fig. 21— 9-millimeter fry showing the terminal mouth, the exposed gills, and early condition of the fins and of the pig- 
mentation 
PIGMENTATION 
After the appearance of pigment in the eyes (see p. 157), the next to form occurs 
as a double row of large chromatophores along the mid-dorsal region and about 30 
smaller ones on the top of the head. (Fig. 21; 9 millimeters.) A row of smaller 
ones appears along the mid-lateral line and a double row along the mid-ventral region. 
By the time the embryo is 12 millimeters long (fig. 22) chromatophores appear in the 
caudal fin on each side of each ray. At 14 millimeters (fig. 24) the same is true of the 
dorsal, at 18 millimeters of the pectoral (fig. 25), and at 20 millimeters all fins contain 
pigment. 
Fig. 22. — Specimen of 12 millimeters. The taudal fin is notched; the operculum covers more of the gills 
At 20 millimeters scattered pigment is seen all over the body, which later (at 
25 millimeters) begins to follow that of the scales by outlining the area over each 
scale. At this stage young white suckers begin to appear mottled, as the pigment is 
intensified in several laterally located spots or blotches. A yellow color appears on 
the sides, also, giving a distinct golden iridescence. 
FINS 
Upon hatching, only the homocercal caudal fin and the pectoral fins are present. 
The caudal is but the terminal portion of the median fin fold, which passes from the 
middle of the back around to the vent below. By 9 millimeters the dorsal is defined, 
