138 



ANIMAL BIOLOGY 



nucleus 



A, sperm-cell entering an egg-cell 



nucleus of 



sperm-cell 



egg-nucleus 



B, sperm-nucleus approaching 

 the egg-nucleus 



- -fertilized 

 egg-nucleus 



C, sperm-nucleus and 

 egg-nucleus uniting 



FIG. 102. Fertilization of an egg. 



D, fertilized egg-nucleus 



developing -- ~ 

 embryo ^ 



A, four-celled stage of 

 .embryo 



developing 

 embryo 



B, many-celled stage of 

 embryo 



embryo 



^ supply 



C, embryo more fully de- 

 veloped 



yolk sac 



D, young fish with yolk still attached 

 FIG. 103. Development of a fish egg. 



