WHITE SUCKER 
157 
NEWLY HATCHED LARVA 
The newly hatched fish is about 8 millimeters long but only one-sixth as deep. 
(Fig. 20.) The slender, whitish body extends along the cylindrical yolk sac, and 
the decurved head projects 0.8 millimeter in front and the tail 1.5 millimeters behind. 
The proportionate lengths of head, body, and tail are 1 : 6 : 1 3^, using the vent as 
the posterior limit of the body proper. The eyes are large and slightly elliptical, 
the long axis being parallel to the axis of the embryo. The otocysts are as yet simple 
ovoid sacs. The dome-shaped pericardial cavity containing the small S-shaped 
heart fits like a cap over the anterior end of the yolk sac. 
The mouth is still in the oral pit stage. Myotomes are visible along the back 
and tail. The anus lies immediately posterior to the yolk sac. A low median fin 
fold arises in the mid-dorsal region and sweeps around the tail to end at the vent. 
In the portion that will persist as the caudal fin, delicate rays or actinotrichia are 
seen in some of the specimens. The pectoral fins begin at this stage as rather thick 
flaps of undifferentiated mesoderm overlaid with ectoderm. Pigment is present 
only in the retinse. 
Fio. 20.— Sac fry at the time of hatching, rendered partly transparent. The oral pit is ventral, the caudal fin homocercal. 
8 millimeters 
Serial sections add much to our study of this stage. (See figs. 13 to 19.) The 
epidermis is seen to be but two cells thick. The olfactory placodes are but circular 
thickenings of the epidermis, five or six cells in thickness. No mesodermic coats 
of the eye are yet present, and the orbital musculature is barely outlined. The 
cornea resembles the general epidermis, while the lens itself still shows the vestige 
of its cavity and is relatively so large as nearly to fill the optic cup. The' heart 
wall (myocardium) is but four or five cells thick, the heart occupying only the dorsal 
quarter of the pericardial cavity. The vitelline veins, the aorta and its arches, 
may be traced. Alongside the large notochord the slender nephric ducts maintain 
an even caliber as they pass back to join the digestive tube close to the vent. 
The intestine itself is considerably smaller in section than the notochord, being 
a'" tube of columnar epithelium the cells of which contain very large, clear vacuoles. 
Just back of the heart is the junction of the anterior end of the yolk sac with the 
digestive tube, which is flattened. The fore-gut is mostly solid, though the bran- 
chial evaginations are well marked. The mouth as yet is not open. In this condi- 
tion the larva lies on the bottom or swims upward, to sink again to the bottom of 
the hatching jar. Placed in an outdoor pond, they swam at the surface at the end 
of the second day after hatching. 
44699—27 11 
