MATTER AND CELLS 11 



analyzed after it has ceased to live, to be highly complex. 

 A large part of its weight is water, while its solid portion 

 is chiefly composed of proteids. These are substances 

 found in many foods, white of egg being a familiar example. 

 They contain carbon, hydrogen, oxygen, and nitrogen-. 



4. The Cell (Fig. 2). All living bodies, both plants 

 and animals, are found to consist of cells. Cells are 

 the ultimate units of which living be- 

 ings are made up, just as bricks are 



the units of which a brick wall is com- 

 posed. A cell is a microscopic bit of 

 protoplasm, with or without an inclos- 

 ing wall, having suspended within it a 

 rounded body of denser material called Fig. 2. Diagram of 



, , IT* u v the P arts of a cel1 - 



the nucleus. It may be living apart, or 



. - nucleus. 



may form one of the units of an organ- & cell body or proto- 

 ism. Plant cells have usually the cell v}* sm - 



J c cell wall. 



wall, but an animal cell may be only a 

 naked speck of living matter. Free cells tend to assume 

 a round shape, but under pressure they may take almost 

 any form. 



5. The cells of the human body vary in size from -g^- 

 to -g-flVfr of an inch in diameter. All animals begin their 

 existence as single cells, and the life of any animal is the 

 sum of the activities of all its separate cells, while its 

 physical structure consists of the cells themselves and the 

 intercellular matter which they produce, together with 

 the various lifeless substances which they deposit within 

 themselves. 



6. Essential Properties of Cells. All living things pos- 

 sess two properties without which they cannot exist. 



One of these properties is nutrition, using the word in 

 its broader sense to include the double process of taking 



