The colonies which develop at 24° C. upon 15 per
cent, gelatin plates are described as small, round, cir-
cumscribed, finely granular white points which grow
slowly, never attain any considerable size, and do not
liquefy the gelatin (Fig. 99).
If, instead of gelatin, agar-agar be used and the plates
kept at the temperature of the body, the colonies which
develop upon the plates appear as transparent, delicate,
drop-like accumulations, scarcely visible to the naked
eye, but under the microscope distinctly granular, the
central darker portion being frequently surrounded by a
paler marginal zone.
In gelatin puncture-cultures, made with 15 instead of
the usual 10 per cent, of gelatin, the growth takes place
along the entire path of the wire in the form of little
whitish granules distinctly separated from each other.
The growth in gelatin is always very limited.
Upon agar-agar and" blood-serum the growth consists
of minute, transparent, semi-confluent, colorless, dew-
drop-like colonies, which die before attaining a size
which permits of their being seen without careful in»
In bouillon the organisms grow well, clouding the
medium very slightly.
Milk is quite well adapted as a culture-medium, its
casein being coagulated.
No growth can be secured upon potato at any tem-
perature or by any manipulation yet known.1
When it is desired to maintain or increase the virulence
of a culture it must be very frequently passed through
the body of a rabbit. The degree to which the virtilence
can be raised in this way is remarkable. C. W. Lincoln
has succeeded in reducing the fatal dose for rabbits to
f a c.cm.
If a small quantity of a pure culture of the virulent
1 Ortmann asserts that the pneumococcus can be grown on potato at 37° 'C.,
but this is not generally confirmed. The usual acid reaction of the potato
would indicate that it was a very unsuitable culture-medium.