escent, etc.--during the course of their life and growth.
The colouring matter usually exists as an intercellular excrementitious
substance. Occasionally, however, it appears to be stored actually
within the bodies of the bacteria. The chromogenic bacteria are
therefore classified, in accordance with the final destination of the
colouring matter they elaborate, into--

_Chromoparous_ Bacteria: in which the pigment is diffused out upon and
into the surrounding medium.

_Chromophorous_ Bacteria: in which the pigment is stored in the cell
protoplasm of the organism.

_Parachromophorous_ Bacteria: in which the pigment is stored in the cell
wall of the organism.

Different species of chromogenic bacteria differ in their requirements
as to environment, for the production of their characteristic pigments;
e. g., some need oxygen, light, or high temperature; others again
favor the converse of these conditions.

_Light Production._--Some bacteria, and usually those originally derived
from water, whether fresh or salt, exhibit marked phosphorescence when
cultivated under suitable conditions. These are classed as "photogenic."

_Enzyme Production._--Many bacteria produce soluble ferments or enzymes
during the course of their growth, as evidenced by the liquefaction of
gelatine, the clotting of milk, etc. These ferments may belong to either
of the following well-recognised classes: proteolytic, diastatic,
invertin, rennet.

_Toxin Production._--A large number, especially of the pathogenic
bacteria, elaborate or secrete poisonous substances concerning which but
little exact knowledge is available, although many would appear to be
enzymic in their action.

These toxins are usually differentiated into--

_Extracellular_ (or Soluble) Toxins: those which are diffused into, and
held in solution by, the surrounding medium.

_Intracellular_ (or Inseparate) Toxins: those which are so closely bound
up with the cell protoplasm of the bacteria elaborating them that up to
the present time no means has been devised for their separation or
extraction.

_End-products of Metabolism._--Under this heading are included--

Organic Acids (e. g., lactic, butyric, etc.).

Alkalies (e. g., ammonia).

Aromatic Compounds (e. g., indol, phenol).

Reducing Substances (e. g., those reducing nitrates to nitrites).

Gases (e. g., sulphuretted hydrogen, carbon dioxide, etc.).

And while the discussion of their formation, etc., is beyond the sc