120 STRUCTURE, MOTIONS, REPRODUCTION. 



well as a progressive one, and this is often extremely rapid. Some- 

 times bacilli spin around with a rotatory motion, as if they were an- 

 chored fast to a fixed point, as they may be by the flagellum being 

 attached to the slide or cover glass. Frequently, in a pure culture, 

 the individual bacilli may be seen to come to rest, and, after an inter- 

 val of repose, to dart forward again in the most active way. Or we 

 may find, on examining the same culture at different times, that 

 upon one occasion there is no evidence of vital movements, and on 

 another all of the bacilli are actively motile. These differences de- 

 pend upon the age of the culture, temperature conditions, etc. 



Reproduction by binary division is common to all of the bacte- 

 ria, and in many species this is the only mode of reproduction known. 

 When circumstances are favorable for rapid multiplication the indi- 

 vidual cells grow in length, and a constriction occurs in the middle 

 transverse to the long diameter. This becomes deeper, and after a 

 time the cell is completely divided into two equal portions, which 

 again divide in the same way. Separation may be complete, or the 

 cells may remain attached to each other, forming chains (strepto- 

 cocci) or articulated filaments (scheinfdden of the Germans). 



The bacilli and spirilla divide only in a direction transverse to the 

 long diameter of the cells, but among the micrococci division may 

 occur either in one direction, forming chains ; or in two directions, 

 forming tetrads ; or in three directions, forming " packets "of eight 

 or more elements. The staphylococci, in which the cells do not re- 

 main associated, divide indifferently in any direction. 



The rapidity of multiplication by binary division varies greatly in 

 different species, and in the same species depends upon conditions re- 

 lating to the culture medium, age of the culture, temperature, etc. 

 Under favorable conditions bacilli have been observed to divide in 

 twenty minutes, and it is a matter of common laboratory experience 

 that colonies of considerable size and containing millions of bacilli 

 may be developed from a single cell in twenty-four to forty-eight 

 hours. A simple calculation will show what an immense number of 

 cells may be produced in this time as a result of binary division oc- 

 curring, for example, every hour. The progeny of a single cell 

 would be at the end of twenty-four hours 16,777,220, and at the end 

 of forty-eight hours the number would be 281,500,000,000. 



Some of the earlier observers have noted the presence of oval or 

 spherical refractive bodies in cultures containing bacilli ; but that 

 these were reproductive elements, although suspected, was not de- . 

 monstrated until a comparatively recent date. Pasteur was one of 

 the first to point out the fact that certain bacteria have two modes of 

 reproduction by fission and by the formation of endogenous spores ; 

 but the first careful study of the last-mentioned method was made by 



