6 



HUMAN BIOLOGY 



a. 



foot. 77 can put out any part of its body as an arm, and take in a 

 speck of food ; or, if the food happens to be near, the ameba can make 

 a mouth in any part of its body, and swallow the food by closing around 

 it (Animal Biology, Fig. 12). The ameba has no lungs, but breathes 

 with all the surface of its body. Any part of its body can do anything 

 that another part can do. When the ameba grows to a certain size, Lt 

 multiplies by squeezing together near the middle (Animal Biology, Fig. 

 13) and dividing into two parts. Amebas have not been observed to 

 die of old age ; starvation and accident aside, they are immortal. 



The Ameba and Man Compared.- -The microscope shows us that the 

 skin, the muscles, the blood, in fact, all parts of the body, contain 



numberless small 

 Parts called cells. 

 These cells are 

 continually chang- 

 ing with the activi- 

 ties of the body. 

 One of the most 

 interesting kinds 



of cells we shall find to be the white blood cells, or corpuscles. One is 

 shown in Fig. 6, with the changes that it had undergone at intervals 

 of one minute. The thought readily occurs that these cells, although 

 part of mail's body, resemble tJie ameba that lives an independent life. 

 A man or a horse or a fish in fact any animal not a protozoan has 

 something of the nature of a colony, or collection, of one-celled ani- 

 mals. We are now prepared to understand a little as to how the body 

 grows, and how a cut in the skin is re- 

 paired. The cells take the nourishment 

 brought by the blood, use it, and grow 

 and multiply like the ameba. Thus new 

 tissue is formed. All animals and vege- 

 tables --that is to say, all living things 

 are made of cells. 



FIG. 6. A WHITE BLOOD CELL, magnified ; forms 

 noticed at intervals of one minute. 



A living cell always contains a 

 still smaller body called a nucleus 

 (Fig. 7). There is sometimes a 

 small dot in the nucleus, called 

 the nucleohis. TJie main body of 

 the cell consists of the living substance called protoplasm, con- 

 taining nitrogen. Usually, but not always, there is a wall 



FIG. 7. DIAGRAM OF A 

 CELL. 



protoplasm; n, nucleus; ', nu- 

 cleolus. 



