138 



ANIMAL BIOLOGY 



nucleus 



A, sperm-cell entering an egg-cell 



C, sperm-nucleus and 

 egg-nucleus uniting 



nucleus of 



sperm-cell 



egg-nucleus 



B, sperm-nucleus approaching 

 the egg-nucleus 



--fertilized 

 egg-nucleus 



D, fertilized egg-nucleus 

 FIG. 102. Fertilization of an egg. 



developing 

 embryo 



A, four-celled stage of 

 .embryo 



embryo 



C, embryo more fully de- 

 veloped 



developing 

 embryo 



B, many-celled stage of 

 embryo 





yolk, food \^ v 



supply 



yolk sac 



D, young fish with yolk still attached 

 FIG. 103. Development of a fish egg. 



