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; 



PIG. 78. 



Cholera colony in gelatin. X 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 ofagar the comma bacillus develops 

 a moist, shining, grayish-yellow layer. Blood-serum 

 is rapidly liquefied at the temperature of the incu- 



