306 BA OTERIOLOGY. 



the earliest stages of development is somewhat that of 

 an irregular, slender funnel. (See Fig. 59.) 



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. 60.) 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. 61. 



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 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 kept in the 

 incubator for from seventy-two to ninety-six hours. It 

 occurs in from one to three minutes after touching with 



