GENUS BACTERIUM 225 



the entire surface soon being covered by a grayish, tough 

 pellicle which has a tendency to come away in thin strips or 

 strands. 



Gelatin. On gelatin plates, colonies develop within 

 twenty- four to forty- eight hours as opaque, white disks, pin- 

 head in size, irregularly round and rather flat. As the col- 

 onies increase in size their outlines become less regular and 

 under the microscope they are seen to be made up of a hair-like 

 tangle of threads spreading in thin wavy layers from a more 

 compact central knot. The microscopic appearance of these 

 colonies has been aptly described as resembling a "Medusa" 

 head. Fragments of a colony examined on a slide with a 

 higher power show the individual threads to be made up of 

 parallel chains of bacteria. 



After a day or two of further growth, the gelatin about 

 the colonies becomes liquefied. 



In gelatin stab cultures the growth appears at first as a 

 thin white line along the course of the puncture. From this, 

 growth proceeds in thin spicules or filaments diverging from 

 the stab, more abundant near the top than toward the bot- 

 tom, Owing to more active growth in well oxygenated environ- 

 ment. Liquefaction begins at the top, at first a shallow de- 

 pression filled with an opaque mixture of bacteria and liquid 

 gelatin. Later the bacteria sink to the bottom of the flat de- 

 pression, leaving a clear supernatant fluid of peptonized 

 gelatin. 



Potato. On potato, growth is rapid, white, and rather 

 dry. Sporulation upon potato is rapid and marked, and the 

 medium is favorable for the study of this phase of develop- 

 ment. 



Bouillon. In bouillon the growth takes place rapidly. 

 Usually an initial pellicle forms at the top where the oxygen 

 supply is greatest. Simultaneously with this, a slimy sedi- 

 ment appears at the bottom of the tube, due to the sinking of 

 the bacteria. Apart from isolated flakes and threads the inter- 

 vening broth is clear. When gently shaken the growth in the 



