WHITE SUCKER 
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the remainder of the dorsal median fin fold having disappeared. (Fig 21.) The 
yolk sac, having rapidly reduced in size, appears as only a thin ridge along the mid- 
ventral line. In the same mid-ventral line the anal fin is seen back of the vent. 
The caudal fin begins to show its two lobes (homocercal type) at 12 millimeters. 
(Fig. 22.) It is not difficult to render larva- of this length transparent and to see the 
upturned end of the notochord — a reminder of the heterocercal nature of this fin in 
the ancestor. In the adult male the lower lobe of the caudal is larger than the upper 
lobe. The pectoral fins are present at hatching, but are so transparent as easily to 
escape detection except in the examination of stained sections. They function for 
some time before the relatively smaller ventral fins appear at 15 millimeters. (See 
Figs. 23 and 24.) 
PECULIAR CHANGE IN LOCATION OF MOUTH 
At hatching (8 millimeters) the mouth is not yet open, the fore-gut being nearly 
solid. (Fig. 20.) At 9 millimeters, however, the terminal mouth, characteristic of 
the early top-swimming stage, is completely formed and open. (Fig. 21.) The 
facial expression of the larva, viewed from directly in front, is as interesting as it is 
grotesque. The horseshoe-shaped mouth lies in an almost vertical plane, arching 
up between the very large eyes so that its opening is actually as high as the center of 
Fig. 24.— Specimen 14 millimeters in length rendered transparent. The mouth is becoming inferior, the pelvic fin 
buds are evident, and the upturned notochord is seen at the base of the caudal fin 
the eyes themselves. The eyes, it may be recalled, are contained but three times in 
the length of the head at this time, whereas they are contained seven times in the head 
of a 10-inch adult. The nares are each a single oval depression located in the angle 
between the eye, laterally, and the upper lip below. From its high terminal position 
the mouth cavity slopes steeply downward and backward to the pharynx. Hyaline 
cartilage already forms the skeleton of the head. 
