CULTURES, AND THEIR STUDY. 143

Petri's Dishes.—These small dishes (Fig. 26), about
4 inches in diameter and ]/2 inch deep, with accurately
fitting lids, are about as convenient as anything that has
been devised in bacteriological technique. They dis-

FlG. 26.—Petri dish for making plate-cultures.

pense with plates and plate-boxes, with moist chambers
and benches, and usually with the levelling apparatus,
though this is still employed in connection with the
Petri dishes in some laboratories.

The method of the employment of Petri dishes is very
simple. The dishes are carefully cleaned, polished, and
sterilized by hot air, care being taken that they are placed
in the hot-air closet right side up, and after sterilization
are kept covered and in that position. The dilution of
the material under examination is made with gelatin or
agar-agar tubes in the manner described above, the plugs
are removed, the mouth of the tube is cautiously held
for a moment in the flame, then the contents of each
tube are poured into one of the sterile dishes, whose top
is elevated just sufficiently to allow the mouth of the
tube to enter. The gelatin is spread over the bottom
of the dish in an even layer, is allowed to solidify,
labelled, and then stood away for the colonies to develop.

Esmarch Tubes.—This method, devised by Esmafch,
converts the walls of the test-tube into the plate and dis-
penses with all other apparatus. The tubes, which are
inoculated and in which the dilutions are made, should
contain less than, half the usual amount of gelatin or
agar-agar. After inoculation the cotton plugs are pushed
into the tubes until even with their mouths, and then
covered with a rubber cap, which protects them from
wetting. A groove is next cut in a block of ice, and