230 



A MANUAL OF BACTERIOLOGY 



These gradually increase in size, and, where near 

 the surface of the gelatine, a small depression forms 

 over them, so that, on looking from the side at the 

 surface of such a cultivation, it presents numerous 

 little depressions instead of the original smooth 

 surface of the gelatine, each depression correspond- 

 ing to a colony of these bacilli. As the colony in- 

 creases in size it becomes less compact, and the 



gelatine in the immedi- 

 ate vicinity becomes 

 fluid. 1 At this stage 

 zooglcea are formed. 

 The colony goes on in- 

 creasing in size for a 

 few days, but ultimately 

 ceases to extend, or 

 extends only very slow- 

 ly. Tube-cultivations 

 are also characteristic. 

 In twenty-four hours, 

 at a temperature of 18 



C " g r0wth is evident 



along the needle- track 

 as a whitish line, 

 broader at the upper part, and gradually tapering 

 to the lower. At the upper part of the gelatine 

 there is a slight depression, and during the next 

 twenty-four hours the growth becomes more marked, 

 the depression increasing in size so as to look like 



1 Dr. Lander Brunton has shown that this liquefaction is due 

 to a [ferment (enzyme) secreted by the comma bacillus (Proc. 

 Roy. Soc., vol. xlvi. p. 542). 



FIG. 44. COLONIES OF CHOLERA-BACILLI 



ON GELATINE-PLATE. 



(X80.) 



