THE GENERAL STRUCTURE OF THE HUMAN BODY 7 



At cell before divi- 

 sion. 



B, cell with divided C, single cell divided 

 nucleus. into two cells. 



does not occur ; for when a cell reaches its normal size, the 

 nucleus divides (Fig. 4), and the halves separate from each 

 other to form two nuclei. The cell body now divides into 

 two parts, and cell walls are formed between the two cells. 

 Thus are produced two cells, each having its own nucleus, 

 and these in 

 turn assimilate 

 and divide. In 

 this way the 

 number of cells 

 increases with 

 the growth of 

 the body. 



7. Cells of the 

 blood. — If we 

 were to examine 

 with the com- 

 pound micro- 

 scope a drop of fresh blood,' we should find that it is not the 

 simple red liquid it seems to be ; it consists of solid particles, 

 called blood corpuscles, floating in a watery liquid known as 

 blood plasma. These corpuscles are single cells. Two kinds 

 can be distinguished, which from their color are known as 

 red corpuscles and white corpuscles (Fig. 5). 



There are three hundred to seven hundred times as many 

 red corpuscles as white. We shall first consider the white 

 corpuscles. Each consists of a minute bit of protoplasm 

 in which is imbedded a nucleus. These cells of the blood 



1 The blood may be easily obtained by tying a cord tightly about 

 the finger and then pricking it with a needle cleaned by an antiseptic 

 like peroxid of hydrogen or by heating it in a flame. A drop of 

 blood is squeezed out upon a glass slide and covered with a thin 

 cover glass. 



Fig. 4. — Cell division. 



