ur granules.

3. Crenothrix. }
4. Cladothrix. } These forms do not contain
5. Leptothrix. } sulphur granules.

6. Streptothrix. A group which exhibits true but
not dichotomous branching, and contains some pathogenic
species.

The morphology of the same bacterium may vary greatly under different
conditions.

For example, under one set of conditions the examination of a pure
cultivation of a bacillus may show a short oval rod as the predominant
form, whilst another culture of the same bacillus, but grown under
different conditions, may consist almost entirely of long filaments or
threads. This variation in morphology is known as "pleomorphism."

Some of the factors influencing pleomorphism are:

1. The composition, reaction, etc., of the _nutrient medium_ in which
the organism is growing.

2. _The atmosphere_ in which it is cultivated.

3. _The temperature_ at which it is incubated.

4. Exposure to or protection from _light_.

The various points in the anatomy morphology and physiology of bacteria
upon which stress is laid in the following pages should be studied as
closely as is possible in preparations of the micro-organisms named in
connection with each.

~ANATOMY.~

1. _Capsule_ (Fig. 85, b).--A gelatinous envelope (probably akin to
mucin in composition) surrounding each individual organism, and
preventing absolute contact between any two. In some species the capsule
(e. g., B. pneumoniae) is well marked, but it cannot be demonstrated in
all. In very well marked cases of gelatinisation of the cell wall, the
individual cells are cemented together in a coherent mass, to which the
term "zoogloea" is applied (e. g., Streptococcus mesenteroides). In
some species colouring matter or ferric oxide is stored in the capsule.

2. _Cell Wall_ (Fig. 85, c).--A protective differentiation of the
outer layer of the cell protoplasm; difficult to demonstrate, but
treatment with iodine or salt solution sometimes causes shrinkage of the
cell contents--"plasmolysis"--and so renders the cell wall apparent (_e.
g._, B. megatherium) in the manner shown in figure 85. Stained bacilli,
when examined with the polarising microscope, often show a doubly
refractile cell wall (e. g., B. tuberculosis and B. anthracis).

In some of the higher bacteria the cell wall exhibits this
differentiation to a marked degree and forms a hard sheath within which
the cell protoplasm is freely movable; and during