LIFE PROCESSES IN ANIMALS 283 



increases in size, and finally a complete new sponge is 

 formed. 



In all higher animals certain organs of the body are 

 used for reproduction, just as certain organs are used 

 for digestion or for respiration. In these organs two 

 kinds of cells, egg and sperm cells, are produced. The 

 egg cells are formed in the ovary which is a structure 

 present in the female, and the sperm cells are formed 

 in the spermary of the male. One of these egg cells 

 unites with one of the sperm cells, and a fertilized egg 

 is formed. The single large cell then divides a great 

 many times, and gradually a new animal is formed, 

 which is like the parents which produced it, except 

 smaller and younger. 



All higher animals commence life as a single cell 

 formed by the union of two cells, one from the male and 

 one from the female. The development of the animal 

 from this cell varies in different animals. In many 

 animals, such as the frog, the fish, the reptile, and the 

 bird, there is often attached to this cell a large amount of 

 food material forming what is known as an egg. As 

 the young animal develops in this egg it lives upon this 

 food. In all mammals the egg develops into a young 

 animal within the body of the mother. The new indi- 

 viduals are then born with the various structures of 

 the body well formed. 



