152 PATHOGENIC BACTERIA.

or may not accompany it, and the subsequent gelatiniza-
tion or digestion of the coagulum.

Blood-serum is liquefied by some bacteria. The ma-
jority of organisms are not very characteristic in their
development upon it. Others, as the bacillus of diph-
theria, are, however, characterized by their shape, color,
and rapidity of development at given temperatures.

While most of the saprophytic bacteria will grow well
at the ordinary temperature of a well-warmed room, the
important pathogenic forms require to be kept at the
temperature of the body. To do this accurately an in-
cubating oven becomes a necessity. Various forms, of
wood and metal, are in the market, the one shown in the
illustration (Fig. 34) being one of the newest and best.

It scarcely need be pointed out that gelatin cultures
cannot be grown in the incubating oven, as the medium
will not remain solid at temperatures above 20-22° C.