32 Structure and Classification of Micro-organisms 



This arrangement is, however, less satisfactory than that of 

 Migula already given. 



Motility. — The greater number of the bacteria supplied with 

 flagella are actively motile, the locomotory power no doubt being 

 the lashing flageUa. The rod and spiral micro-organisms are most 

 plentifully supplied with flagella; only a few of the spheric forms have 

 them. 



The presence of flagella, however, does not invariably 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 than 

 the pathogenic forms. 



Bacillus megatherium has a distinct but limited ameboid move- 

 ment. 



The dancing movement 6f some of the spheric bacteria seems to be the well- 

 known Brownian movement, which is a physical phenomenon. It is some- 

 times difficult to determine whether an organism viewed under the microscope 

 is really motile or whether it is only vibrating. One can usually 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 tendencies upon 5 per cent, 

 gelatin. The active movement of the bacteria composing the 

 colony causes its shape constantly to change, so that it bears a 

 faint resemblance to an ameba, and moves about from place to 

 place upon the surface of the gelatin. 



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 nutrition are good, fission progresses with 

 astonishing rapidity. Buchner and others have determined the 

 length of a generation to be from fifteen to forty minutes. 



The results of binary division, ii 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, and in three days they would, if their 

 surroundings were ideally favorable, form a mass which can scarcely 

 be reckoned in numbers." "Fortunately for us," says Woodhead, 

 "they can seldom get food enough to carry on this appalling rate of 

 development, and a great number die both for want of food and 

 because of the presence of other conditions unfavorable to their 

 existence." 



Sporulation.— When the conditions for rapid multiplication by 

 fission are no longer good, many of the organisms guard against 

 extinction by the formation of spores. 



Endospores, or spores developed within the cells, are generally 



