386 APPLICATION OF METHODS OF BACTERIOLOGY 



are not sharply circumscribed, but usually present at first 

 a fringe of delicate filaments about their periphery. (See 

 Fig. 71.) As growth progresses and liquefaction becomes 

 more advanced the central mass of the colony sinks into 



FIG. 70 



FIG. 72 





FIG. 70. Stab-culture of ps, ceruginosa in gelatin after twenty-eight 

 hours at 22 C. 



FIG. 71. Colony of ps ceruginosa after twenty-four hours on gelatin at 

 20-22 C. 



FIG. 72. Colony of ps. ceruginosa after forty-two hours on gelatin at 

 20-22 C. 



the liquid, while at the same time there is an extension of 

 the colony laterally. At this stage the colony, when slightly 

 magnified, may present various appearances, the most 

 common being that shown in Fig. 72. 

 The gelatin between the growing colonies takes on a 



