MICROORGANISMS AND HUMAN WELFARE 13 



place. Suppose, then, we start at ten o'clock some morning 

 with a single healthy bacterium. If conditions are favorable, 

 there would be two cells at eleven o'clock, and by twelve 

 o'clock each of these two daughter cells would form two 

 granddaughter cells; the colony would then number four 

 individuals. Should this process continue for twenty- 

 four hours or until ten o'clock on the day after the single 

 bacterium began its race, the colony would number 16,777,- 

 216 bacteria. " It has been calculated by an eminent 

 biologist," says Dr. Prudden,' " that if the proper conditions 

 could be maintained, a rodlike bacterium, which would 

 measure about a thousandth of an inch in length, multiply- 

 ing in this way, would in less than five days make a mass 

 which would completely fill as much space as is occupied 

 by all the oceans on the earth's surface, supposing them to 

 have an average depth of one mile." 



12. Spore formation in bacteria. — Such startling possi- 

 bilities as those suggested in the preceding section fortunately 

 can never become realities, for favorable conditions soon 

 cease to exist and the cells either die or cease to multiply. 

 Sometimes, when food or moisture begins to fail, the pro- 

 toplasm within each cell rolls itself into a ball and covers 

 itself with a much thickened wall. This protects it until 

 it again meets with conditions favorable for growth. The 

 process we have been describing is known as spore formation ; 

 the tiny protoplasmic sphere is called a spore, and its dense 

 covering a spore wall (Fig. 7). In this condition bacteria 

 may be blown hither and yon as a part of the dust. They 

 may be heated even above the temperature of boiling water 

 without being killed. When at length they settle dowh on 



'"The Story of the Bacteria," by Dr. T. Mitchell Prudden. 

 G. P. Putnam's Sons, New York. 



