DEVELOPMENT OF TELEOST 



V. The Development of Teleost 



209 



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- 

 cal, 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 



