DEVELOPMENT OF TELEOST 209 
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V. The Development of 7. eleost 
The mode of development of bony fishes differs in 
many and apparently important’ regards from that of 
their nearest kindred, the Ganoids. In their eggs a large 
amount of yolk is present, and its relations to the embryo 
have become widely specialized. 
As a rule, the egg of a Teleost is small, perfectly spheri- 
‘eal, and enclosed in delicate but greatly distended mem- 
branes (Fig. 269). The germ disc, GD, is especially 
small, appearing on the surface as an almost transparent 
fleck ; it may occupy the same position as in the other 
fishes, or, as in the figure, it may occur at the lowermost 
pole. Among the fishes whose eggs float at the surface 
during development, as of many pelagic Teleosts, e.g. the 
Sea-bass, Serranus atrarius,—to which all the accom- 
panying figures refer,—the yolk is lighter in specific 
_ gravity than the germ; it is of fluid-like consistency, 
almost transparent. In the yolk at the upper pole of 
the egg an oil globule, OG, usually occurs; this serves 
to lighten the gravity of the entire egg, and from its 
position must aid materially in keeping this pole of the 
egg uppermost. 
The early segmentation of the germ is seen in Figs. 
270, 271. In the former, the first cleavage plane is estab- 
lished, and the nuclear divisions have taken place for the 
second; in the latter, the third cleavage has been com- 
pleted. As in other fishes these cleavages are vertical, 
the third parallel to the first. A segmentation cavity, 
SC, occurs as a central space between the blastomeres, 
as it does in the sturgeon and gar-pike. 
Stages of late segmentation are seen in section in Figs. 
272, 273. In both the segmentation cavity, SC, is greatly 
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