INTRODUCTION 



about the rounded cells, or corpuscles, which float in the 

 blood, and of the flattened cells which can be scraped from 

 the tongue. 



13. Variety of Work done by Cells. Some cells can 

 change their forms, as those of muscle ; other cells make 

 fluids which help us to digest our food. The outer layer 

 of cells, known as the skin, forms a protective coat to the 

 body. The liver cells manufacture 



or secrete the bile, and the bone 

 cells help to make the bones. 



Millions of blood cells do their 

 work and perish every day, while 

 the brain cells act in some mysteri- 

 ous way to help us to think. 



In short, our very life exists in 

 the cell. 



14. Cell Life as Shown by the 



Amoeba. The work done by cells 

 is well shown in the life history of the amoeba, a tiny creature 

 not more than ^-^ of an inch in diameter and found in 

 water containing decaying matter. 



If we use a microscope powerful enough to magnify a 

 pin's head into the size of a bicycle wheel, we find that 

 this bit of life appears as a mass of jelly like substance 

 with little grains within it. This jelly is known as proto- 

 plasm, meaning the primitive or first stuff. Within the 

 amoeba may sometimes be seen a round spot known as 

 the nucleus. 



With the aid of the microscope the life of the amoeba, 

 as it is lived in a drop of water, can be studied. It creeps 

 about, it changes its shape, it selects and digests its food, 

 it breathes in oxygen from the air in the water and 



FIG. i. Diagram of a Cell. 



4, cell wall ; B, nucleus ; C, nucle- 

 olus ; D, protoplasm of cell body. 



