WHITE SUCKER 
165 
thrown into high narrow longitudinal ridges. The duct to the air bladder leaves 
the gut in the mid-dorsal line, while the latter is surrounded by the liver. At the 
same level we see the larger duct from the liver (and pancreas?) entering in the mid- 
ventral region. Both are lined with columnar epithelium, the latter having the 
greater caliber and ending in a rounded gall bladder embedded in the liver sub- 
stance. The air duct curves to the left and then returns to enter the air bladder 
in the median line. The bile duct curves to the opposite side and upward. 
Back of the liver there are no folds in the intestinal epithelium. The diameter 
of the intestine rapidly increases posterior to the liver and occupies the entire body 
Fig. 40. — Sac fry, 8 millimeters in length, rendered transparent. The yolk sac has been drawn in solid black (camera 
lucida) 
cavity. Its wall is of simple columnar epithelium, the cross section of the organ 
being an almost perfect circle. 
At 14 millimeters the longitudinal folds of the epithelium are found much farther 
back; that is, to the posterior end of the swim bladder. The latter has grown so 
rapidly as to occupy twice the cross-section area occupied by the intestine and one- 
third the entire cross-section area of the fish. This may be a potent factor in 
inducing the top-swimming habit of fry under 17 millimeters. Back of the swim 
bladder the intestinal epithelium is not yet elevated into folds. 
The intestine shows the folds, or rugse, throughout its entire course at 17 milli- 
meters. Also the first intestinal loop is formed and the epithelial cells of the region 
Fig. 41. — Fry of 11 millimeters, showing the straight digestive tract, the development of the caudal fin, and the simple 
swim bladder 
involved appear much more closely packed as if division were proceeding very 
rapidly. This, for several reasons, including the change in form of the intestine, I 
have called the critical period in the life history. 
ADULT CONDITION 
The digestive tract, as in the Cyprinidse, has no true stomach; that is, no region 
containing special gastric glands. To be sure, the sucker appears to have a stomach 
the first 4)^ inches of the gut of a 12-inch specimen being of wider diameter and 
44699—27 12 
