Embryology and Growth of Fishes 135 



come to differ more and more as they pass through later and 

 later developmental stages. 



Development of the Bony Fishes.* The mode of develop- 

 ment 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 spherical, and enclosed 

 in delicate but greatly distended membranes. The germ disc is 

 especially small, appearing on the surface as an almost trans- 

 parent fleck. Among the fishes whose eggs float at the sur- 



FIG. 97. Development of Sea-bass, Centropristes striatus (Linnaeus), a, egg 

 prior to germination ; b, germ-disk after first cleavage ; c, germ-disk after third 

 cleavage; d, embryo just before hatching. (After H. V. Wilson.) 



face during development, as of many pelagic Teleosts, e.g., the 

 sea-bass, Centropristes striatus, 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 usually occurs; this serves to lighten the relative weight 

 of the entire egg, and from its position must aid in keeping 

 this pole of the egg uppermost. 



In the early segmentation of the germ the first cleavage 

 plane is established, 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 occurs as a 

 central space between the blastomeres, as it does in the sturgeon 

 and garpike. 



In stages of late segmentation the segmentation cavity is 



* This account of the normal development of the Teleost fishes is condensed 

 from Dr. Dean's "Fishes Living and Fossil," in which work the details of 

 growth in the Teleost are contrasted with those of other types of fishes. 



