CHAPTER XXV 

 CELL-DIVISION 



The Growth of Organisms. — In earlier chapters it 

 was shown that living things grow by the transformation 

 of certain sorts of food into their living substance, and 

 that complex organisms are composed of very numerous 

 cells highly specialized to perform their particular func- 

 tions. And furthermore it was indicated that all the dif- 

 ferent sorts of cells in a complex organism arise from 

 a single original one by division. Growth of a higher ani- 

 mal or plant thus consists of three things: (a) the mak- 

 ing of new protoplasm whereby the individual cell be- 

 comes larger, (b) the division of the cell when it attains 

 its appropriate size, and (c) the gradual speciahzation 

 of different cells to perform their individual functions. 



Cell Growth and Division. — Assuming the ability 

 of protoplasm to take food of a suitable sort and assimi- 

 late it into its own substance and thereby increase the 

 amount of living cell contents, it is now desired to direct 

 attention to the fact that cells have a definite limitation 

 to their increase in size. When a cell reaches the size 

 characteristic of the particular plant or animal of which 

 it is a part, it either ceases to grow or it divides. Very 

 little is known respecting the mechanism which deter- 

 mines what this size must be. On the contrary, the ad- 

 vantages of size limitation are obvious. Since cells take 

 in food and excrete their waste through their surfaces, 

 it follows that the rate of this exchange will be deter- 

 mined by the extent of the surface. The surface of a cube 

 or ball increases as the square of the diameter (Fig. 65). 

 The volume increases with the cube. Thus doubling the 



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