210 
HAHOLD’S DISCUSSIONS. 
Their motions would soon convince the most skep¬ 
tical observer that they are alive. 
Bacteria differ from yeast germs chiefly in this, 
that they multiply by fission. A bacterium begins to 
contract at the mid¬ 
dle, or at several 
points, until it 
vides into two 
di- 
more new organisms, 
each as perfect as the 
parent. Each grows 
and divides again, 
and generation fol¬ 
lows generation with 
great rapidity. 
One species of 
bacteria divides into 
two, and each new 
one into two again, 
which again divide. 
In half an hour there 
will be three in the 
place of one, in an hour six, in two hours twenty- 
four, and so on. It has been computed that in three 
days there would be 
so many that we could 
not comprehend their 
number, and that in 
weight they would ex¬ 
ceed seven thousand 
pounds. IN'ot only are Fig. 104.—Diptheria bacillus. 
Fig. 103. — Various types of bacteria 
“ colonies,” cultivated in gelatin. 
