ium) is
essential to the accurate study of the formation of colonies under
various conditions, but when the main object of the separation of the
bacteria is to obtain subcultivations from a number of individual
bacteria, "surface" plates must be prepared, since here colony formation
is restricted to the surface of the medium. The method adopted varies
slightly according to whether the medium employed is gelatine or agar,
or one of the derivatives or variants of the latter.

(a) ~Gelatine Surface Plates.~--

1. Liquefy three tubes of nutrient gelatine.

2. Pour each tube into a separate Petri dish and allow it to solidify.
Then turn each plate and its cover upside down.

[Illustration: FIG. 126.--Surface plate spreader.]

3. When quite cold raise the bottom of plate 1, revert it and deposit a
drop of the inoculum (whether a fluid culture or an emulsion from solid
culture) upon the surface of the gelatine with a platinum loop--close to
one side of the plate; replace the bottom half of the Petri dish in its
cover.

4. Take a piece of thin glass rod, stout platinum wire or best of all a
piece of aluminium wire (say 2 mm. diameter) about 28 cm. long. Bend the
terminal 4 cm. at right angles to the remainder, making an L-shaped rod
(Fig. 126). Sterilise the short arm and adjacent portion of the long
arm, in the Bunsen flame, and allow it to cool.

5. Now raise the bottom of the Petri dish in the left hand, leaving the
cover on the laboratory bench, and holding it vertically, smear the drop
of inoculum all over the surface of the gelatine with the short arm of
the spreader by a rotatory motion, (Fig. 127). Replace the dish in its
cover.

6. Raise the bottom of plate 2 and rub the infected spreader all over
the surface of the gelatine--then go on in like manner to the third
plate in the series.

7. Sterilise the spreader.

8. Label and incubate the plates.

[Illustration: FIG. 127.--Spreading surface plate.]

After incubation, plate No. 1 will probably yield an enormous number of
colonies; plate 2 will show fewer colonies, since only those bacteria
adhering to the rod after rubbing over plate 1 would be deposited on its
surface, and by the time the rod reached plate 3 but very few organisms
should remain upon it. So that the third plate as a rule will only show
a very few scattered colonies, eminently suitable for detailed study.

(b) ~Agar Surface Plates.~--

1. Liquefy three tubes of nutrient agar--nutros