Size Reproduction 31 



imply motility, as they may also serve to stimulate the passage 

 of currents of nutrient fluid past the organism, and so favor 

 its nutrition. The flagellate bacteria are more numerous 

 among the saprophytic bacteria found in water or other 

 fluids, than among the pathogenic forms found in the tis- 

 sues, though it may be added that parasitic bacteria not 

 habitually entering the tissues, but inhabiting the intestine, 

 as the bacillus of typhoid fever and the spirillum of cholera, 

 are actively motile, like the saprophytes. 



Bacillus megatherium has a distinct but limited ameboid 

 movement. 



The dancing movement of some of the spheric bacteria 

 seems to be the well-known Brownian movement, which is 

 a physical phenomenon. It is sometimes difficult to deter- 

 mine whether an organism viewed under the microscope is 

 really motile or whether it is only vibrating. One can usu- 

 ally determine by observing that in the latter case it does 

 not change its relative position to surrounding objects. 



In some cases the colonies of actively motile bacteria, 

 such as the proteus bacilli, show definite migratory tenden- 

 cies upon 5 per cent, gelatin. The active movement of 

 the bacteria composing the colony causes its shape con- 

 stantly to change, so that it bears a faint resemblance to an 

 ameba, and moves about from place to place upon the sur- 

 face of the gelatin. 



Size. Bacteria are so minute that a special unit has 

 been adopted for their measurement. This is the micro- 

 millimeter (p) t or one-thousandth part of a millimeter, 

 equivalent to the one-twenty-five-thousandth (-^^u) of an 

 inch. 



The size of bacteria varies from a fraction of a micro- 

 millimeter to 20 or even 40 micromillimeters. 



Reproduction. Fission. Bacteria multiply by binary 

 division (fission). A bacterium about to divide appears 

 larger than normal, and, if a spheric organism, more or less 

 ovoid. By appropriate staining karyokinetic changes may 

 be observed in the nuclei. When the conditions of nutri- 

 tion are good, fission progresses with astonishing rapidity. 

 Buchner and others have determined the length of a gener- 

 ation to be from fifteen to forty minutes. 



The results of binary division, if rapidly repeated, are 

 almost appalling. "Cohn calculated that a single germ 

 could produce by simple fission two of its kind in an hour; 

 in the second hour these would be multiplied to four; 



