THE GROWTH Or ANIMALS 21 



number then passes to one end of the spindle and the 

 other half to the opposite end of the spindle. The spindle 

 then contracts in the middle and eventually breaks in 

 two, as it were, and after a few more changes two com- 

 plete new nuclei are formed, after which the body of the 

 cell divides in two, thus forming two new cells. Figure 7 

 shows these, changes in detail. 



Growth of animals. — Growth takes place in an animal 

 mainly by an increase in the number and size of the cells. 

 This means, in general, that a tiny kitten does not possess 

 as many cells as a large kitten and that a large kitten does 

 not possess as many cells as a full-grown cat. 



Before the egg cell, from which most animals originate, 

 begins to form mother cells, it must be fertilized by the 

 male sperm cell. After fertilization it is known as the 

 fertilized egg cell, or oosperm. The oosperm may begin at 

 once to divide and start the development of the animal. 

 The character of the division varies in different cases, but, 

 in general, the oosperm, or a part of it, divides into two cells, 

 these into four, these into eight, these into sixteen, and so on 

 until a globular ball of similar cells is formed. At this point 

 in the development of the embryo, a change occurs and the 

 cells cease dividing so regularly in multiples of two. They 

 now begin to differ from each other in shape, size, and func- 

 tion, and some of them go to form skin, others to make bone, 

 others to build up muscles, and so on until every part of the 

 animal is formed. While this activity among the cells is 

 going on, food is needed to furnish energy and materials 

 out of which the cells may be built. Even after the animal 

 is formed, a constant supply of food is demanded for build- 

 ing new cells to take the places of those worn out and cast 

 off. 



