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the tube, held almost horizontally, is rolled in this until
the entire surface of the glass is covered with a thin
layer of the solid medium (Fig. 27). Thus the tube
becomes the plate upon which the colonies develop.

FIG. 27.—Esmarch tube on block of ice (redrawn after Abbott).

Several little points need to be observed in carrying-
out Esmarch's method. The tube must not contain too
much culture-medium, or it cannot be rolled into an even
layer. In rolling the contents should not touch the cotton
plug, lest it be glued to the glass and its subsequent use-
fulness be injured. No water must be admitted from the
melted ice.

The offspring of each bacterium growing upon the
film of gelatin constituting a plate-culture form a mass
which has already been pointed out as a colony. These
small bacterial families may be seen through a micro-
scope when still much too small for detection by the
naked eye, and because of their minuteness should always
be studied with the microscope.

The original plates of Koch are very inconvenient for
such examination, because it is impossible to remove
them from the moist chamber and lay them upon the
stage of the microscope without exposing them to the
danger of contamination by the atmosphere, so that the
advantages of Petri dishes and Esmarch tubes, where
the examination may be made through the glass tube or