MICROBES, OR BACTERIA. 89 
be seen swimming with it in the field of the microscope, 
we are struck by the difference. Infusoria come and 
go, swiftly or slowly—they go back or move to the 
right or left; in a word, their movements seem to be 
actuated in some sense by will. Nothing like this 
is observed in the bacterium. The undulatory move- 
ment by which it is animated is always the same, and 
impels it straightforward, like a stone sent from a 
sling; it never voluntarily goes back nor out of its 
course, but only under the influence of a foreign im- 
pulse, such as contact with another bacterium, when it 
rebounds, just as a projectile may rebound from a wall. 
On encountering an obstacle, the bacterium remains 
indefinitely undulating before it, without ever pausing 
or showing signs of fatigue, until some external cause 
comes to release and send it to the right or left. We 
may often see a tangled mass of bacteria, perhaps 
adhering by their cilia or by some other substance, in 
which all the individuals continue to undulate until 
the rupture of the mass permits them to depart in all 
directions. These organisms are therefore plants in the 
character of their movements, as well as in the rest of 
their organization. 
In bacteria each cell consists of a cellulose wall 
containing protoplasm, as we saw was the case in fer- 
ments. The multiplication by fission is effected in 
precisely the same way in bacteria and ferments, and 
so also is the formation of spores. Under certain 
circumstances, when the liquid on which they subsist 
