320 BACTERIOLOGY. 



colony itself. As growth continues the depression 

 increases in extent and ultimately assumes an appear- 

 ance that consists in the apparent sinking of the liquefied 

 portion in such a way as to leave a perceptible air-space 

 between the top of the liquid and the surface of the solid 

 gelatin. The growth now appears to be capped by a 

 small air-bubble. The impression given by it at this 

 stage is not only that there has been a liquefaction, but 

 also a coincident evaporation of the fluid from the 

 liquefied area and a constriction of the superficial open- 

 ing of the funnel. (See a, b, c, and d, Fig. 63.) Lique- 

 faction is not especially active along the deeper portions 

 of the track made by the needle, though in stab cultures 

 in gelatin the liquefaction is much more extensive than 

 that usually seen around colonies on plates. It spreads 

 laterally at the upper portion, and after about a week 

 a large part of the gelatin in the tube may have become 

 fluid, and the growth loses its characteristic appearance. 



Stab- and smear-cultures on agar-agar present noth- 

 ing characteristic. They are usually only an exaggera- 

 tion of the appearance afforded by the single colonies 

 on this medium. 



Its growth in bouillon is luxuriant, causing a diffuse 

 clouding and the ultimate production of a delicate film 

 upon the surface. 



In sterilized milk of a neutral or amphoteric reac- 

 tion at a temperature of 36°-38°C. it develops actively, 

 and gradually produces an acid reaction with coagula- 

 tion of the casein. It retains its vitality under these 

 conditions for about three weeks or more. The blue 

 color of milk to which neutral litmus tincture has been 

 added is changed to pink after thirty-six or forty-eight 

 hours at body temperature. 



