

382 



MANUAL OF ZOOLOGY 



SECT. 



Dog-fishes (Acantkias, Mustelus) a mere vestige of the 

 egg-shell is formed, and the eggs undergo the whole of 

 their development in the oviducts, the young being eventually 



born alive with the form and 

 proportions of the adult. 



The great size of the egg is 

 due to the immense quantity 

 of yolk which it contains : its 

 protoplasm is almost entirely 

 aggregated at one pole in the 

 form of a small disc. When 

 segmentation of the oosperm 

 takes place it affects the proto- 

 plasm alone, the inactive yolk 

 taking no part in the process. 

 The disc of protoplasm divides 

 to form a little heap of cells, 

 the blastoderm, situated at one 

 pole of the undivided sphere of 

 yolk. The blastoderm subse- 

 quently spreads out as a sheet 

 of cells over the yolk which it 

 ultimately completely encloses. 

 While this extension of the 

 blastoderm is taking place, its 

 middle part becomes raised up 

 into a ridge-like thickening, which is moulded, step by 

 step, into the form of the embryo fish. The head, trunk, 

 and tail acquire distinctness, and become more and more 

 clearly separated off from the bulk of the egg, the latter 

 taking the form of & yolk-sac attached by a narrow stalk to 

 the ventral surface of the embryo (Fig. 214). 



In this condition the various parts of the adult fish can 



FIG. 213. Dog-fish, egg-case 

 (After Dean.) 



