CUL 77 VA TION OF BA CTERIA. 131

Glycerin Agar-agar.—For an unknown reason certain
of the bacteria which will not grow upon the agar-agar
as prepared above will do so if 3-7 per cent, of glycerin
be added. Among these is the tubercle bacillus, which,
not growing at all upon plain agar-agar, will grow well
when glycerin is added—a fact discovered by Roux and
Nocard. The glycerin may also be added to gelatin or
any other medium.

Blood Agar-agar was recommended by R. Pfeiffer for
the cultivation of the influenza bacillus. It is ordinary
agar-agar whose surface is coated with a little blood
secured under antiseptic precautions from the finger-tip,
ear-lobule, etc., of man, or the veins of one of the lower
animals. Some bacteriologists prepare a hemoglobin
agar-agar by spreading a little powdered hemoglobin
upon the surface of the agar-agar. This has the disad-
vantage that powdered hemoglobin is not sterile, and the
medium must be sterilized after its addition.

The blood agar-agar should be kept in the incubator a
day or two before use so as to insure perfect sterility.

Blood-serum.—The great advantage possessed by this
medium is that it is itself a constituent of the body, and
hence offers opportunities for the development of the
parasitic forms of bacteria under the most natural con-
ditions possible. It is the most difficult of all the media
to prepare. The blood must be obtained from a slaughter-
house in an appropriate receptacle, the best things for the
purpose being tall narrow jars of about i liter capacity,
with a tightly-fitting lid. The jars are sterilized by heat
or by washing with alcohol and ether, are carefully dried,
closed, and carried to the slaughter-house where the blood
is to be obtained. As the blood flows from the severed
vessels of the animal the jars are filled one by one. It
seems advisable to allow the first blood to escape, as it is
likely to become contaminated from the hair. By waiting
until a coagulum forms upon the hair the danger of con-
tamination is obviated. The jars when full are allowed
to stand undisturbed until quite firm coagula form within