282 



A YEAR IN SCIENCE 



only one season, while elephants are known to live 

 two hundred years. Sooner or later, however, each 

 animal dies. Frequently death is due to some external 

 cause, such as disease or injury. If not, the cells of the 

 body gradually lose the power to perform the functions 

 of growth and repair, and finally all the life processes 

 cease. Since animals persist on earth, evidently new 

 ones are constantly being formed. This process of pro- 

 ducing new individuals is known as reproduction. 



In the one celled animals this process is very simple. 

 After an ameba, for example, reaches a certain size, 



Fig. 136. Four stages in 

 division of an ameba. 



the 



Fig. 137. Hydra show- 

 ing 1 a bud. Budding is 

 one method by which ani- 

 mals may reproduce. 



the whole cell divides into two nearly equal parts, each 

 of which is a perfect but smaller individual. Some- 

 times, as in sponges, hydras, and corals, reproduction 

 takes place by budding. A small knob-like outgrowth 

 appears on the side of a sponge. This gradually 



