WHITE SUCKER 
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Investment of the yolk by the blastoderm up to 92 hours. — Twenty-one hours. 
(Fig. 6.) The blastodisc now consists of three layers of rounded cells, and its margin 
has begun to extend slightly farther over the yolk. In each of its 100 or more cells 
the cytoplasm shows a disposition to radiate from the nucleus, and yolk spherules 
and oil droplets are seen within the cells. The cells measure 0. 1 millimeter in diameter. 
Forty-four hours. (Fig. 7.) The disk of cells is becoming flat on top. 
Fifty-four hours. (Fig. 8.) At this stage in development the blastoderm 
begins to thin out at its edge, which has reached nearly the equator of the egg. It 
is perhaps one-third as thick as in the four-celled stage. The whole egg is again a 
sphere. 
Seventy hours. The yolk is now half covered. The advancing rim of the 
blastoderm may stand slightly above the surface level of the yolk. 
Ninety-two hours. (Fig. 9.) The yolk is entirely invested. Sections show 
that while the blastoderm is six to eight cells thick at the animal pole, it is but one 
or two cells thick at the vegetative pole. 
Embryo . — One hundred and twenty-six and one-half hours. Surface views give 
no indication of the embryonic axis. 
One hundred and forty-nine hours. (Fig. 10.) The body of the embryo fish 
is visible as a straight white ridge, narrow and rather high, nearly encircling the egg. 
At one end (the anterior) it terminates as aflat, rounded expansion. No open neural 
groove was seen, and Balfour (1881) reported the omission of that stage in the embry- 
ology of many teleosts, which has been well confirmed. 
One hundred and seventy-four hours. (Fig. 11.) Marked changes, visible in 
surface view, have occurred in the anterior axial region. The brain is clearly seen. 
The optic evaginations are pear shaped and extend outward and backward from the 
primary cerebral vesicle. The other two cerebral vesicles are present as hollow 
expansions of the neural tube, the second being about as wide as long, while the third 
(myelencephalon) is more elongate and has a diamond-shaped ventricle. Frag- 
mentary series of sections indicate that the neural tube is the only well-differentiated 
tissue present. It is almost solid, but otherwise resembles that of a chick embryo 
of 48 hours. The blastoderm abruptly passes from the thick ridge, which will 
become the body of the fish, to the portion, but one or two cells thick, that spreads 
out over the yolk. 
One hundred and ninety-eight hours. (Fig. 12.) The lateral boundaries of the 
head and body are seen now. The tail process almost reaches around to the ante- 
rior limit of the head. The limits of the yolk sac are indicated caudally. 
Two hundred and twenty-four hours. The yolk, which has been spherical, is now 
reduced to two regions. The anterior is spherical but the posterior portion, arch- 
ing around the former, is cylindrical. The notochord is visible as a clear central 
core in the tail process, which process extends well beyond the yolk sac. 
Two hundred and forty-six hours. The embryo forms more than a circle; its tail 
lies either to right or left of the head or is bent at a right angle across the front of the 
head. Pigment has appeared in the eyes. Five per cent of the eggs hatched at 
this time, though the majority were delayed for some days. The latter gained 1 
nillimeter in length over those hatching at this time, but were otherwise like them. 
