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MORPUOLOOY AND BIOLOGF OF BACTERIA. 9

identical with those from which thej sprang. Bj some these
cells are called cocci, by others spores (arthrospores).
The spores (Fig. 3) are spherical or ovoidal bodies, with sharp
contours and strongly refractive contents. They are formed
chiefly in media of which the available nourishment for bacteria
has been already, for the most part, consumed. Considered from
a hygienic stand-point they are of great interest, since they
DfFer more resistance to influences of every nature (chemical,
thermal, etc.) than the vegetative cells (as the active growino- cells
may be designated in contradistinction to the inactive, restins;,
spores). Formation of spores has been observed in various rod-
and some screw-forms, and recently also in the case of a micro-
coccus. Bacteria with endogenous spore-formation are called
endospore; those without endogenous spore-formation are called
arthrospore.

4. The Origix of Bacteria.

It has long been known that various kinds of bacteria are
found in the atmosphere, water, soil, etc. This fact was first
established by Ehrenberg'' (1828), although Leeuwenhoek" had
observed microscopic organisms in the human mouth as early
as 1675. Regarding their origin, a long and violent dispute
was carried on for many years between those who claimed
that they were produced spontaneously in solutions of organic
matter (^gencrafio sjwntanea, sive CBqw'roca, abiogencsis), and those
who believed that they could originate only from jire-existing
germs. The exact methods of sterilization practiced in the last
ten years have alone made it possible to establish the fact beyond
all .question, that the proposition " omnis ccllula e celhdo" is
quite as a[iplicable to these low forms of life as to the higher
animal organism. The appearance of bacteria in solutions sup-
posed to have been sterilized, simply proves that the latter were
either not thoroughly sterilized in the beginniug, or that they
were afterwards infected from without.
The question whether the number of species of bacteria re-
mains constant, or whether, in the course ot^ centuries, through
the propagation of varieties in the Darwinian sense, new species