Cultivation 459 



Agar-agar. Upon agar-agar and glycerin agar-agar the 

 colonies are slower to develop, larger, more translucent, 

 without the yellowish-white or china-white color of the 

 blood-serum cultures, and are more or less distinctly divided 

 into a small elevated center and a flat surrounding zone 

 with indented edges, and a radiated appearance. When 

 transplantations are made from blood-serum to agar-agar, 

 the resulting growth is usually meager, but the oftener the 

 organism is transplanted to fresh agar-agar, the more luxu- 

 riant its growth becomes. 



Bouillon. When planted in bouillon a distinct, whitish, 



Fig. 151. Bacillus diphtheriae ; colony twenty-four hours old, upon 

 agar-agar. X 100 (Frankel and Pfeiffer). 



granular pellicle forms upon the surface of the medium. 

 The pellicle appears quite uniform when the flask is un- 

 disturbed, but it is so brittle that it at once falls to pieces if 

 the flask be moved, the minute fragments slowly sedimenting 

 and forming a miniature snow-storm in the flask or tube. 

 The organism at times also causes a diffuse cloudiness of the 

 medium, but, not being motile, soon settles to the bottom 

 in the form of a flocculent precipitate which has a tendency 

 to cling to the sides of the glass, and leave the bouillon 

 clear. 



