ing flat, or
spheroidal, or irregular colonies, the size of which is practically
unlimited. The bacterial [v.03 p.0159] cell is always clothed by a definite
cell-membrane, as was shown by the plasmolysing experiments of Fischer and
others. Unlike the cell-wall of the higher plants, it gives usually no
reactions of cellulose, nor is chitin present as in the fungi, but it
consists of a proteid substance and is apparently a modification of the
general protoplasm. In some cases, however, as in _B. tuberculosis_,
analysis of the cell shows a large amount of cellulose. The cell-walls in
some forms swell up into a gelatinous mass so that the cell appears to be
surrounded in the unstained condition by a clear, transparent space. When
the swollen wall is dense and regular in appearance the term "capsule" is
applied to the sheath as in _Leuconostoc_. Secreted pigments (red, yellow,
green and blue) are sometimes deposited in the wall, and some of the
iron-bacteria have deposits of oxide of iron in the membranes.

[Illustration: FIG. 3.--Types of Zoogloea. (After Zopf.)

A. Mixed zoogloea found as a pellicle on the surface of vegetable
infusions, &c.; it consists of various forms, and contains cocci (a) and
rodlets, in series (b and c), &c.

B. Egg-shaped mass of zoogloea of _Beggiatoa roseo-persicina_ (_Bacterium
rubescens_ of Lankester); the gelatinous swollen walls of the large crowded
cocci are fused into a common gelatinous envelope.

C. Reticulate zoogloea of the same.

D, E, H. Colonies of _Myconostoc_ enveloped in diffluent matrix.

F. Branched fruticose zoogloea of _Cladothrix_ (slightly magnified).

G. Zoogloea of _Bacterium merismopedioides_, Zopf, containing cocci
arranged in tablets.]

[Sidenote: Cell-contents.]

The substance of the bacterial cell when suitably prepared and stained
shows in the larger forms a mass of homogeneous protoplasm containing
irregular spaces, the vacuoles, which enclose a watery fluid. Scattered in
the protoplasm arc usually one or more deeply-staining granules. The
protoplasm itself may be tinged with colouring matter, bright red, yellow,
&c., and may occasionally contain substances other than the deeply-staining
granules. The occurrence of a starch-like substance which stains deep blue
with iodine has been clearly shown in some forms even where the bacterium
is growing on a medium containing no starch, as shown by Ward and others.
In other forms a substance (probably glycogen or am