12 A MANUAL OF BACTERIOLOGY 



however, move indifferently in either direction ; if a 

 motile organism be watched it will often be seen to proceed 

 rapidly in one direction, stop, and then return without 

 turning round. The flagella are not visible in the living 

 state, unless dark ground illumination be used, nor by the 

 ordinary methods of staining, unless previously treated 

 with a mordant, and are extremely liable to be broken 

 off. They 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 (lophotrichic), 

 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 

 twisted. 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 demon- 

 strated. Generally speaking, however, an organism with 

 several flagella will be more motile than a similar form 

 with a few. 



Darwin says : " In looking at Nature it is most necessary 

 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 probably always non-sexual, 

 and takes place in two ways by simple division or fission 

 and by spore formation. Schaudinn described an apparent 

 conjugation in one species (B. flexilis) and Nadson states 

 that in a few species sister cells conjugate and from this 

 conjugation a spore arises. Dobell, however, considers 

 that all the evidence is definitely against the view that a 

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



