12 A MANUAL OF BACTERIOLOGY 



propel the organism through the medium. A cell will, 

 however, move indifferently in either direction ; if a 

 motile organism be watched it will often be seen to 

 proceed in one direction, stop, and then return without 

 turning round. The flagella are very fragile and liable to 

 become detached, and are not visible in the living state, 

 unless dark ground illumination be used, nor by the 

 ordinary methods of staining, the use of a mordant 

 being necessary. Flagella vary considerably in number 

 and in length ; some organisms have but a single flagellum 

 at one pole (monotrichic) , e.g. Bacillus pyocyaneus , others 

 have two or more flagella forming a brush or tuft (lopho- 

 trichic), e.g. Spirillum rubrum, while others may be 

 almost entirely covered with them (peritrichic), e.g. B. 

 typhosus ; in some the flagella are short and straight, 

 and in others long and sinuous. The motility of organisms 

 does not necessarily depend directly upon the number of 

 flagella they possess, an organism with a few flagella 

 often being more active than another possessing many, 

 and some are apparently non-motile, though well-marked 

 flagella can be demonstrated. Generally speaking, how- 

 ever, an organism with several flagella will be more motile 

 than a similar form with a few. 



" In looking at Nature," says Darwin, "it is most neces- 

 sary never to forget that every single organic being may be 

 said to be striving to the utmost to increase in numbers," 

 and in no group perhaps of the animal and vegetable 

 kingdoms is this more marked than among the Bacteria. 

 Reproduction is generally considered to be always rion- 

 sexual, and takes place in two ways by simple division 

 or fission and by spore formation. Dobell considers that 

 all the evidence is definitely against the view that a 

 sexual process occurs at any stage in the life-history of 

 Bacteria. Schaudinn, however, described an apparent 

 conjugation in one species (B. fl&xilis) and Nadson states 



