THE CELL AS A FUNDAMENTAL UNIT OF LIFE 23 



of which contains one of the new nuclei. We commonly say that the 

 amoeba has divided, but it has not divided in the sense that an apple 

 is divided when it is cut into two equal parts. These are not two half- 

 cells — these are two entire cells. Hence, it would be more accurate to 

 think of this process as cell duplication rather than cell division. It 

 is true that each of the cells formed will be only about one half as 

 large as the original cell, but each of these will be a complete organism 

 bearing all of the potentialities of the original amoeba. Through 

 growth each of these organisms can attain the size of the original, for 

 each has all of the important units of heredity possessed by the original 

 one. Each of these will in turn duplicate itself in the manner just 

 described unless some misfortune should intervene. 



For the amoeba, cell duplication is always reproduction, for two 

 animals result whenever a cell is duplicated. For the animals with 

 many cells in their bodies, cell duplication becomes a vital part of bodv 

 growth as well as reproduction. Each of us, as a human being, began 

 life as a single cell. It has been through the process of repeated cell 

 duplication that the billions of cells which now form your body have 

 been produced. Furthermore, the process continues in your body 

 throughout life to replace certain injured and worn-out body tissues. 

 Your skin is constantly being shed from the surface and being replaced 

 by new cells which are formed beneath. Billions of new blood cells 

 are poured into your blood stream each day to replace those which 

 wear out in their daily duties or which may be lost by bleeding. When 

 the time comes for reproduction, again this process furnishes a plentiful 

 supply of cells which can be used to produce new individuals. Thus it 

 becomes apparent that cell duplication is an extremely important part 

 of the life of any animal that exists. 



The Process of Mitosis 



The normal process by means of which cells become duplicated is 

 known as mitosis. The most spectacular phase of this process consists 

 of the splitting of the cell into two parts, but this represents the finale — 

 the climax of an extensive series of prepatory events which insures the 

 production of two complete cells by this splitting. To get at the begin- 

 ning of mitosis let us first consider those submicroscopic units of hered- 

 ity known as genes. These tiny units are found within the nucleus of 

 cells. They are the directors of the activities within the cell as well as 

 supervisors of the final role which the cell shall assume in the formation 

 of a body. For instance, if you have brown eyes, curly hair, or even 

 great aptitude in music, it is because your cells contain genes which give 



