ater-bath (Fig. 121).

2. Inoculate the liquefied medium and label it, etc., precisely as if
dealing with a tube of bouillon.

3. Place the newly planted tube in the upright position (e. g., in a
test-tube rack) and allow it to solidify.

4. Label the tube; when solid, incubate.

_Esmarch's Roll Cultivation._--

1. Liquefy three tubes of gelatine by heat.

2. Prepare three dilutions of the inoculum (as described for
plate cultivations, page 228, steps 4 to 7).

3. Roll the tubes, held almost horizontally, in a groove
made in a block of ice, until the gelatine has set in a thin
film on the inner surface of tube (Fig. 120); or under the
cold-water tap.

[Illustration: FIG. 120. Esmarch's roll culture on block of
ice.]

In order that the medium may adhere firmly to the glass, the
agar used for roll cultivation should have 1 per cent.
gelatine or 1 per cent. gum arabic added to it before
sterilisation.

Roll cultivations, which served a most important purpose in
the days before the introduction of Petri dishes for plate
cultivations, are now obsolete in modern laboratories and
are merely mentioned for the benefit of students, since
examiners who are interested in the academic and historical
aspects of bacteriology sometimes expect candidates to be
acquainted with the method of preparing them.

The Preparation of Plate Cultures.

If a small number of bacteria are suspended in liquefied gelatine, agar,
or other similar medium, and the infected medium spread out in an even
layer over a flat surface and allowed to solidify, each individual
micro-organism becomes fixed to a certain spot and its further
development is restricted to the vicinity of this spot. After a variable
interval the growth of this organism becomes visible to the naked eye
as a "colony." This is the principle upon which the method of plate
cultivation is based and its practice enables the bacteriologist to
study the particular manner of development affected by each species of
microbe when growing (a) unrestricted upon the surface of the medium,
(b) in the depths of the medium. The method itself is as follows:

~Apparatus Required.~--

1. Tripod levelling stand.

2. Large shallow glass dish, with a square sheet of plate
glass to cover it.

3. Spirit level.

4. Case of sterile Petri dishes.

5. T