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METHODS OF BACTERIOLOGICAL INVESTIGATION. 51
between different species (Fig. 18, and plate, Figs. 1-4), although
sometimes two ditFerent organisms grow so nearly alike that
they cannot be distiusj-uished bv this means alone.
FiQ. 18.
la. 2«. 3 a.
Test-Tcbe CiLTURES OF Three Differest Bacteria.
All of same age.
Pure cultures in bouillon are obtained in the same manner,
thoufifh in this case we naturally cannot invert the tube, and
particular caution is requisite to prevent impurification during
the process of inoculation.
Other Solid Culture Media.
It sometimes happens that two dilierent micro-organisms, culti-
vated on gelatine or agar-agar, present characteristics of growth
so similar to each other that it is not possible to distinguish be-
tween them, or that they do not grow at all on these media. In
such cases we have recourse to other media, such as coagulated
blood-serum, boiled potato, boiled egg, starch-paste, etc. I can-
not enter into a description of the method of preparing blood-
serum. Any one wishing to undertake this by no means very
easy task, must refer to the works on this subject. I would
certainly advise any one, unless he needs a very large quantity
of the material, to buy it, if possible, already prepared. It is
congealed, after sterilization, in shallow glass vessels, or in
