ISABEL MULFORD. . 995 
forms swarm in liquids, some develop a film or mycoder- 
ma, such as the ‘‘mother”’ of vinegar, and some de- 
velop a gelatinous mass or zoogloea, in which the sepa- 
rate cells are imbedded. 
The movements of bacteria are thus described by M. 
Cohn : 
‘‘In certain conditions, they are excessively mobile ; 
and when they swarm ina drop of water, they present 
an attractive spectacle, similar to that of a swarm of 
enats, oran ant-hill. The bacteria advance, swimming, 
then retreat without turning about, or even describe cir- 
cular lines. At onetime they advance with the rapidity 
of an arrow, at another, they turn upon themselves like 
a top; sometimes they remain motionless for a long 
time, and then dart off like a flash. The long rod-bac- 
teria twist their bodies in swimming, sometimes slowly, 
sometimes with address and agility, as if they tried to 
force for themselves a passage through obstacles. Itis 
thus that a fish seeks its way through aquatic plants. 
They remain sometimes quiet, as if to repose an instant ; 
suddenly the little rod commences to oscillate and then 
to swim briskly backwards to again throw itself forward 
some instants after. All of these movements are ac- 
companied by a second movement analogous to that of 
a screw which moves in a nut. When the Vibr7os in 
the shape of a gimlet turn rapidly round their axis, 
they produce a singular illusion; one would believe 
that they twisted like an eel, although they are ex- 
tremely rigid.”’ 
The bacteria are multiplied by fission. <A cell elon- 
gates, forms a partition wall in the centre, and the parts 
finally separate and thus make two distinct cells or 
plants. Under favoring conditions of temperature and 
moisture, together with abundance of nutriment, this 
multiplication is exceedingly rapid. Cohn has calen- 
lated that a bacterium which divides into two in the space 
109 
