98 PHYSIOLOGY 



dissimilar and we call the process fertilization. The two 

 cells are usually formed in different individuals, though 

 they may be formed in the same individual. The egg 

 cell is large, round, quiet ; the sperm cell is small, irregu- 

 lar, motile. In aquatic forms the cells pass into the 

 water and unite there, but in animals that live on land 

 the union takes place in the tube leading from the ovary 

 to the outside. The egg becomes better and better pro- 

 tected as development becomes more complex; finally it 

 remains in the body until the whole development has 

 taken place. 



