THE BA CILL US PYOCYA NUS. 249 



commonly called. The bacillus pyocyanus is a delicate 

 rod with rounded or pointed ends. It is actively motile ; 

 does not form spores. As seen in preparations made 

 from cultures it is commonly clustered together in 

 irregular masses. It does not form long filaments, 

 there being rarely more than four joined together end to 

 end, and most frequently not even two. 



It grows readily on all artificial media, and gives to 

 some of them a bright-green color that is most con- 

 spicuous where it is in contact with the air. This green 

 color is not seen in the growth itself to any extent, but 

 is diffused through the medium on which the organism 

 is developing. With time this color becomes much 

 darker, and in very old agar-agar cultures may become 

 almost black (sometimes very dark blue-green, at others 

 brownish-black). 



Its growth on gelatin in stab cultures is accompanied 

 by liquefaction, and the diffusion of a bright-green color 

 throughout the unliquefied medium. As liquefaction 

 continues, and the entire gelatin ultimately becomes 

 fluid, the green color is confined to the superficial layers 

 that are 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. 52.) 



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. 53). 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 pre- 



