RopiNESS IN Milk and Cream. 409 



readily in the presence of air upon all of the ordinary culture 

 media. The minimum temperature at which growth will occur, 

 is below 46° F. (8° C.) and the maximum, at about 104° F. 

 (40° C.) A freshly inoculated milk culture was frozen for 

 twenty hours without destroying its vitality. The cultures 

 which are described below were all grown at room temperature. 



Agar. — Growth appears in agar plate cultures in twenty-four 

 hours after inoculation as circular gray colonies i to 2 mm. in 

 diameter. Young colonies are opalescent and exhibit the phe- 

 nomonon of diffraction of light. Mature colonies ma}' be irreg- 

 ular in contour, flat, with sharply defined borders, and grayish 

 white in color. Under low magnification no distinctive marking 

 is apparent. The growth is viscid so that the substance of the 

 colony adheres to the platinum needle and spins out into a fine 

 thread. After several weeks, when the culture media has dried 

 perceptibly, the viscid character tends to disappear, the colony 

 becoming more pasty in consistency. In such old cultures, faint 

 concentric markings are visible near the border. 



The young growth upon the surface of slanted agar is opales- 

 cent and usually consists of numerous small confluent colonies. 

 Later, when the growth thickens, the opalescent appearance is 

 not noticeable. The condensation water becomes cloudy, viscid, 

 and after considerable evaporation has occurred, brown in color. 

 Growth in slant cultures of glycerine agar presents no distinctive 

 features. 



Fifteen per cent gelatin. — Surface colonies appear first as minute 

 gray points upon the surface of the medium. Under a low mag- 

 nification they are granular and show concentric, circular mark- 

 ings. Older colonies are approximately circular with a sharply 

 defined border, and distinct circular markings. Some have a 

 pronounced central protuberance surrounded by a thinner border, 

 while others consist of a hemispherical mass alone. The colo- 

 nies, at first whitish in color and opalescent, later become dis- 

 tinctly yellow. The growth is viscid like that of agar, but upon 

 drying somewhat, the colon}^ adheres to the platinum needle and 

 becomes detached from the surface of the medium. The gelatin 

 is not liquefied even after long standing. In ten per cent gela- 

 tin the growth is more luxuriant. Sub surface colonies appear 



