366 APPLICATION OF METHODS OF BACTERIOLOGY 



liquefaction continues, and the whole of the gelatin ulti- 

 mately becomes fluid, the green color is confined to the 

 superficial layers in contact with the air. The form taken 

 by the liquefying portion of the gelatin in the earliest stages 

 of development is somewhat that of an irregular slender 

 funnel. (See Fig. 70.) 



On gelatin plates the colonies develop rapidly; they 

 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 

 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 

 bright yellowish-green color; but as growth is comparatively 

 rapid, it is quickly entirely liquefied, and one often sees the 

 colonies floating about in the pale-green fluid. 



On agar-agar the growth is dry, sometimes with a slight 

 metallic lustre, and is of a pale gray or greenish-gray color, 

 while the surrounding agar-agar is bright green. With 

 time this bright green becomes darker, passing into blue- 

 green, and finally turns almost black. 



On potato the growth is brownish, dry, and slightly 

 elevated above the surface. In some cultures the potato 

 about the line of growth becomes green; in others this 

 change is not so noticeable. With many cultures a peculiar 

 phenomenon, consisting of a change of color from brown to 

 green, may be produced by lightly touching the growth with 

 a sterile platinum needle. The change occurs only at the 

 point touched. It is best seen in cultures that have been 



