the mother
cell-wall having adapted itself to the outline of the spore (fig. 4, F).
The ripe spores of Schizomycetes are spherical, ovoid or long-ovoid in
shape and extremely minute (_e.g._ those of _Bacillus subtilis_ measure
0.0012 mm. long by 0.0006 mm. broad according to Zopf), highly refractive
and colourless (or very dark, probably owing to the high index of
refraction and minute size). The membrane may be relatively thick, and even
exhibit shells or strata.

The germination of the spores has now been observed in several forms with
care. The spores are capable of germination at once, or they may be kept
for months and even years, and are very resistant against desiccation, heat
and cold, &c. In a suitable medium and at a proper temperature the
germination is completed in a few hours. The spore swells and elongates and
the contents grow forth to a cell like that which produced it, in some
cases clearly breaking through the membrane, the remains of which may be
seen attached to the young germinal rodlet (figs. 5, 9 and 11); in other
cases the surrounding membrane of the spore swells and dissolves. The
germinal cell then grows forth into the forms typical for the particular
Schizomycete concerned.

The conditions for spore-formation differ. Anaerobic species usually
require little oxygen, but aerobic species a free supply. Each species has
an optimum temperature and many are known to require very special
food-media. The systematic interference with these conditions has enabled
bacteriologists to induce the development of so-called asporogenous races,
in which the formation of spores is indefinitely postponed, changes in
vigour, virulence and other properties being also involved, in some cases
at any rate. The addition of minute traces of acids, poisons, &c., leads to
this change in some forms; high temperature has also been used
successfully.

[Illustration: FIG. 11.--Stages in the development of spores of _Bacillus
ramosus_ (Fraenkel), in the order and at the times given, in a hanging drop
culture, under a very high power. The process begins with the formation of
brilliant granules (A, B); these increase, and the brilliant substance
gradually balls together (C) and forms the spores (D), one in each segment,
which soon acquire a membrane and ripen (E). (H. M. W.)]

[Sidenote: Classification.]

The difficult subject of the classification[4] of bacteria dates from the
year 1872, when Cohn published his system, wh