splanted to suitable media and placed under
favourable conditions, the spores germinate, usually within twenty-four
to thirty-six hours, and successively undergo the following changes
which may be followed in hanging-drop cultures on a warm stage:

1. Swell up slowly and enlarge, through the absorption of water.

2. Lose their refrangibility.

3. At this stage one of three processes (but the particular process is
always constant for the same species) may be observed:

(a) The spore grows out into the new bacillus without discarding the
spore membrane (which in this case now becomes the cell membrane); _e.
g._, B. leptosporus.

(b) It loses its spore membrane by solution; e. g., B. anthracis.

(c) It loses its spore membrane by rupture.

In this process the rupture may be either polar (at one pole only _e.
g._, B. butyricus), or bipolar (e. g., B. sessile), or equatorial;
(e. g., B. subtilis).

In those cases where the spore membrane is discarded the cell membrane
of the new bacillus may either be formed from--

(a) The inner layer of the spore membrane, which has undergone a
preliminary splitting into parietal and visceral layers; e. g., B.
butyricus.

(b) The outer layers of the cell protoplasm, which become
differentiated for that purpose; e. g., B. megatherium.

The new bacillus now increases in size, elongates, and takes on a
vegetative growth--i. e., undergoes fission--the bacilli resulting
from which may in their turn give rise to spores.

[Illustration: FIG. 92. Simple.]

[Illustration: FIG. 93. Solution.]

[Illustration: FIG. 94. Polar.]

[Illustration: FIG. 95. Bipolar.]

[Illustration: FIG. 96. Equatorial.]

~Food Stuffs.~--1. _Organic Foods._--

(a) The pure parasites (e. g., B. leprae) will not live outside the
living body.

(b) Both saprophytic and facultative parasitic bacteria agree in
requiring non-concentrated food.

(c) The facultative parasites need highly organised foods; e. g.,
proteids or other sources of nitrogen and carbon, and salts.

(d) The saprophytic bacteria are more easily cultivated; e. g.,

1. Some bacteria will grow in almost pure distilled water.

2. Some bacteria will grow in pure solutions of the carbohydrates.

3. _Water_ is absolutely essential to the _growth_ of bacteria.

Food of a definite reaction is needed for the growth of bacteria. As a
general rule growth is most active in media which react slightly acid to
phenolphthalein--that is, neutral