572 BACTERIOLOGY. 
dark-yellow or brown in color, and forms occur which 
are absolutely unlike the typical cholera colonies. In 
gelatin stick cultures the growth is at first thread-like 
and uncharacteristic. At the end of twenty-four to 
thirty-six hours a small, funnel-shaped depression ap-— 
pears on the surface of the gelatin, which soon spreads 
out in the form of an air-bubble above, while below 
- this is a whitish, viscid mass. Later, the funnel in- 
creases in depth and diameter, and at the end of from 
four to six days may reach the edge of the test-tube; 
Fie. 78. 
Cholera colony in gelatin. > 30 diameters. (DUNHAM.) 
in from eight to fourteen days the upper two-thirds of 
the gelatin is completely liquefied. (See Fig. 79 and 
Fig. 31, page 230.) Freshly isolated cholera vibrios 
liquefy gelatin more rapidly than old laboratory cul- 
tures; a certain variation in the characteristic liquefac- 
tion of the gelatin even in fresh cultures under some 
circumstances should be borne in mind in making a 
diagnosis. Such variations in cultural peculiarities 
occur also with other bacteria, and only the sum of all 
the characteristics taken together enables a positive 
diagnosis to be established. 
Upon the surface of agar the comma bacillus develops 
a moist, shining, grayish-yellow layer. Blood-serum 
is rapidly liquefied at the temperature of the incu- 
