14 CELL STRUCTURE 



3. Although the different cells composing the many tissues 

 of the body are all distinct in their respective characteristics, 

 they are yet all dependent upon each other for the proper per- 

 formance of their work, just as the individual persons compos- 

 ing the population of a large city lead independent lives, yet 

 are all dependent upon each other for their respective exist- 

 ences. Each cell has a life of its own ; it eats, digests, moves, 

 and reproduces its own kinjd, and yet it is dependent for its 

 health and life upon the healthy performance of the functions 

 of all the other cells of the body. 



4. Structure of the Cell. As the type of the simplest cell 

 structure, we have the best illustration in a microscopical ani- 

 mal, found in stagnant water, and called the Amoeba. It is 

 composed of transparent, jelly-like material, enclosed in a very 

 thin membrane, and having a more or less irregular shape. 

 In its interior we see that a small portion of its jelly-like 

 material is more opaque than the rest, as if composed of a 

 number of granules. If we observe this Amoeba under the 

 microscope, we shall notice that it has life, as manifested by 

 its power to contract in places, and in other places to project a 

 part of its body. It feeds by enclosing within its substance 

 any particles which it may take for food. It retains them in 

 its interior until it has absorbed therefrom what it can use, 

 when it again partly opens itself at some point near the par- 

 ticle, and rejects or moves away from what it does not want. 

 It propagates itself by a simple process of subdivision ; that 

 is, it divides itself into two unequal portions, each portion 

 retaining some of the granular matter, which seems to be 

 essential to its existence. 



5. All the cells of the human body have the following 

 essential "structure : First, a thin membrane enclosing the cell, 

 called the cell wall. Second, a jelly-like or granular material, 

 composing its body, called protoplasm, and embedded in this 

 body a more or less round or oval body of firmer consistency 

 than the cell mass, called the nucleus, which, in many cases, 



8. Describe cell life. 4. The structure of the cell. 5. In what does the essential struc- 

 ture of the cell consist ? 



