PISCES 



241 



The egg strings often hang on sticks or stones in shallow- 

 water. 



The development of a perch egg after fertilization follows 

 the usual course for metazoans (Fig. 64) ; but appears to be 

 different because the embryo forms on one end of the egg. 

 This restriction of cell-division and embryo-formation to 

 the germinal disc is due to the large amount of yolk which 

 fills the rest of the egg and which, though contained in the 

 cytoplasm, is too inert to permit cell-division. The "fer- 

 tilized" egg (Fig. 95, B) is a zygote formed by the fusion of 

 an ovum (A) and a sperm cell. The nucleus of the egg cell 



F G 



FIG. 95. Embryonic development of fish. A, unfertilized egg; gd, germinal 

 disc; y, yolk; B, zygote formed by union of ovum and spermatozoon; C, D, 

 cleavage; E, young embryo showing neural groove at left; F, showing yolk nearly 

 nearly overgrown by the vascular membrane (blastoderm) growing out from 

 embryo; G, embryo with "yolk sac;" H, young fish, just hatched, with yolk sac 

 not yet absorbed. 



lies at one end in an area of protoplasm called the germinal 

 disc (gd). The rest of the egg cell is filled with yolk 

 material (y). The sperm cell enters the germinal disc, and 

 when cleavage follows fertilization, it is confined to that 

 area. As the embryo grows the yolk is used to nourish 

 it (E, F, G). When the little perch is ready to hatch 

 (G), the yolk containing part of the egg is completely 

 invested by a vascular membrane and forms an appendage, 

 known as the yolk sac, on the belly of the young fish. After 

 leaving the egg envelopes, the young perch (H) takes no 

 food until the egg sac is completely absorbed. 



