THE MICROBES OF HUMAN DISEASES. 157 
or Schizophyta, there is a very remarkable dimorphism 
of mode and habitat. In Beggiatoa of sulphurous 
waters, for instance, and in Cladothria, which forms a 
whitish pellicle on the surface of putrefying liquids, 
Zopf has found, under certain conditions, all the forms 
designated as Micrococcus, Bacillus, Leptothrix, and 
Bacterium; that is, microbes strictly so called, in- 
cluding those which are the producing agents of 
contagious diseases. 
Where these algee are found in water or on a damp 
soil, conditions of existence favourable to their develop- 
ment, there they live and multiply. But when the 
soil dries up, when a river returns to its bed after 
a flood, or a marsh disappears in consequence of the 
evaporation of its waters, all these alge give forth 
dormant spores, destined to ensure their propagation. 
We have described the formation of these spores by 
concentration of the protoplasm in the interior of 
each cell; in this form their volume is very small, and 
they are extremely light, so that as soon as they are 
desiccated, and then only, these spores are carried 
away by the slightest breeze and borne to great dis- 
tances, These are termed air-germs. 
When these moving germs encounter a favourable 
medium, at once moist and warm, such as the human 
mouth or lungs, they fasten there and are immediately 
developed, first in the form of Micrococeus, then of 
that of Bacterium, Bacillus, or Leptothria, according 
to the species to which the spore in question belongs. 
