126 



PATHOGENIC BACTERIA. 



distinct membranous pellicle called a mycoderma. In 

 such a growth multitudes of degenerated bacteria and 

 laVge numbers of spores are to be observed. On the 

 other hand, it occasionally happens that the growth 

 occurs chiefly below the surface, and may produce gelat- 

 inous masses which are known as zooglea. 



In gelatin the bacteria exhibit a great variety of ap- 

 pearances, many of which are beautiful and interesting. 

 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. 29. 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 t B. radiatis. 



a bristling appearance. Figure 29 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 



