BACTERIOLOGY. 43 



the four divide into eight, the eight into sixteen, the 

 sixteen into thirty-two. Since it takes only thirty 

 minutes for one cell to divide, it can be computed 

 approximately how many new individuals will be 

 formed in twenty-four hours out of a single bacterium. 

 Conn gives the number as ''over 16,500,000 in one 

 day, and about 281,500,000 in two days. In three 

 days, at this rate of multiplication they would produce 

 a mass of bacteria weighing 16,000,000 pounds." 

 However, these figures have no practical significance, 

 and are given merely to convey to the reader some con- 

 ception of their astonishing rate of multiplication. In 

 nature multiplication is rapid, as may be inferred from 

 the great changes they work, but their activities are 

 limited to their usefulness. Their growth is held in 

 check by the products of their own metabolism (excre- 

 tory products), by the absence of suitable food, and by 

 natural forces acting upon them as they do upon all 

 creation. 



When examined with a microscope, indi- 

 Shape. vidual bacteria are seen to look like so 



many dots, dashes, and commas. There 

 are thousands of species of bacteria, yet each species 

 conforms more or less closely to one of these three 

 shapes. According to their form, therefore, they are 

 called, cocci, bacilli, and spirilla, and under these 

 designations all known species are classified. 



