268 BACTERIOLOGY. 



development is somewhat that of an irregular, slender 

 funnel. (See Fig. 57.) 



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. 58). As growth progresses and liquefaction 

 becomes more advanced, the central mass of the colony 

 sinks into the liquefied depression, 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. 59. 



The gelatin between the growing colonies takes on a 

 bright yellowish-green color; but as growth is compar- 

 atively 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 whitish or greenish- 

 white color, while the surrounding agar-agar is bright 

 green. With time this bright green becomes darker, 

 passing to blue-green, and finally turns almost black. 



On potato the growth is brownish, dry, and slightly 

 elevated above the surface. With some cultures the 

 potato about the growth becomes green; with others this 

 change is not so noticeable. With many cultures a pecu- 

 liar phenomenon may be produced by lightly touching 

 the growth with a sterilized platinum needle. This 

 phenomenon consists in a change of color from brown 

 to green at the point touched. It is best seen in cul- 

 tures that have been kept in the incubator for from 

 seventy-two to ninety-six hours. It occurs in from one 

 to three minutes after touching with the needle, and 



