224 BACTERIA. 
of years, will be found entirely free from microscopic 
life. 
We have not the slightest ground for believing that 
any form of life exists which has not had its origin in 
some pre-existing organism of like nature. 
These minute organisms lie on the border-land between 
the animal and vegetable kingdoms, but the great weight 
of authority now classes them with plant life, because 
their cells are coated with a membrane of cellulose and 
because they are able to assimilate nitrogen from. in- 
organic compounds. 
There is still great diversity of opinion as to whether 
they belong to the alge or to the fungi. They are 
doubtless allied to the Oscillatoria,. among the alge, 
but as they do not develop chlorophyl, the tendency is 
to class them with the fungi. 
With reference to their physiological effects, the bac- 
teria are divided into three groups. 
1. Chromogenes, producing pigments. 
2. Zymogenes, producing fermentations and putrefac- 
tions. 
3. Pathogenes, those forms observed in connection 
with disease. 
The generic distinctions are based chiefly upon the 
form of the cells, which may be spherical, -oblong, cyl- 
indrical, spiral, etc. Those bacteria whose cells are 
spherical or nearly so, belong to the genus Micro- 
coccus, those whose cells are short, cylindrical rods, be- 
long to the’ genus Bacterium, and those whose rods are 
united in long filaments producing endogenous spoies, 
belong to the genus Bacillus. 
Bacteria are the smallest living organisms of which we 
have any knowledge. Many are with difficulty detected 
even with our highest magnifying powers. A single 
cell constitutes an individual. Some forms occur singly, 
some are slightly united in chains or filaments, some 
108 : 
