138 



ANIMAL BIOLOGY 



nucleus 



A, sperm-cell entering an egg-cell 



nucleus of 

 sperm-cell 



egg-nucleus 



B, sperm-nucleus approaching 

 the egg-nucleus 



C, sperm-nucleus and 

 egg-nucleus uniting 



D, fertilized egg-nucleus 

 Fig. 102. — Fertilization of an egg. 



developing 

 embryo 



A, four-celled stage of 

 .embrvo 



developing 



embryo 



B, many-celled stage oE 

 embryo 



embryo 



^ yolk, food 

 supply 



C embryo more fully de- 

 veloped 



yolk sac 

 D, young^h with yolk still attached 

 Fig. 103. — Development of a fish egg. 



