212 



PATHOGENIC BACTERIA. 



Certain bacteria, as the tubercle bacillus, will not grow 

 at all upon gelatin. Some forms which are rigidly ae- 

 robic will only grow upon or near the surface ; others, 

 anaerobic, only in the deeper parts. The majority, how- 

 ever, grow both upon the surface and in the puncture 

 made by the wire. Sometimes the consistence of the 

 gelatin is unaltered ; sometimes it is liquefied throughout, 

 sometimes only at the surface. Sometimes offshoots ex- 

 tend from the colonies into the gelatin, giving the culture 



Fig. 33. — Various forms of gelatin puncture-cultures : a, Bacillus typhi ab- 

 dominalis ; b, B. anthracis ; c, B. mycoides ; d, B. mesentericus vulgatus ; 

 e, B. of malignant edema ; f, B. radiatis. 



a bristling appearance. Figure 33 will serve to illustrate 

 different varieties of gelatin growth. 



The growth in gelatin is generally so far removed from 

 the walls of the tube (a central puncture nearly always 

 being made in the culture-medium, in order that the 

 growth be symmetrical) that it is next to impossible to 

 make a microscopical examination of it with any power 

 beyond that given by a hand-lens. 



Much attention has been given of late to the preparation 

 of microtome sections of the gelatin growth. To accom- 

 plish this the glass is warmed sufficiently to allow the 

 gelatin to be removed and placed in Miiller's fluid (bi- 



