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 (scheinftden 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 
