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52 THE MICRO-ORGANISMS OF THE HUMAN MOUTH. :

fuchsine, methylene blue, methyl violet, gentian violet, vesuvine
(in concentrated alcoholic solutions, to be diluted on using), bis-
marck brown (aqueous solution).
In preparing tissues for examination, naturally the various
hardening, fixing, decalcifying, imbedding, and mounting solu-
tions may be required which are used in general histological
work, although most tissues may be cut and stained wdth excel-
lent results in the simple manner described in connection with
the preparation of decayed dentine. For staining tissues, picro-
carmine and picro-lithio-carmine receive the preference.

Pure Culture.
By a pure culture we understand a cryptogamous growth in
which only one kind of micro-organism is represented. For ex-
ample, a culture containing only cells which belong in the develop-
mental cyclus of the tubercle bacillus, is a pure culture of that
bacillus. As soon, however, as a single cell of another species finds
its way into the culture, it becomes an impure culture. Passing
over the various methods wliich were formerly employed for
obtaining pure cultures in liquids (fractional and dilution cul-
tures of Klebs, Xiigeli, etc.), we will describe in brief only the
methods of cultivating micro-organisms on solid media, intro-
duced by Vittadini and Brefeld, and perfected by Koch.
I will confine myself to the two methods most commonly
employed for cultivating on gelatine or agar-agar. These are
1, line cultures; 2, dilution cultures.
Line Cultures.

The principle of line cultures may be made clear by the fol-
lowing illustration : Suppose we were to dip a wet glass rod into
a bag containing a mixture of timoth}' and clover seeds, so that
a vast number of seeds of both kinds adhere to the rod, and
then draw it over prepared ground; at first, great numbers of
seeds' will be left in the wake of the rod ; gradually, however,
they will become less and less, until at last only single seeds may
be deposited. When these seeds grow, we will have in the first
part of the track a mixture of timothy and clover, but in the