224 PATHOGENIC BACTERIA. 



clinical diagnosis, and must never be taken as positive 

 in itself. 



The bacillus grows similarly upon blood-serum and 

 Loffler's mixture. Upon glycerin agar-agar and agar-agar 

 the colonies are much larger, more translucent, always 



a b c 



FIG. 64. Diphtheria bacilli (from photographs taken by Prof. E. K. Dun- 

 ham, Carnegie Laboratory, New York) : a, pseudo-bacillus ; b, true bacillus ; 

 c, pseudo-bacillus. 



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

 blood-serum cultures, and generally are distinctly divided 

 into a small elevated centre and a flatter surrounding zone 

 with indented edges, sometimes with a distinctly radiated 

 appearance. It must be remarked that when sudden 

 transplantations are made from blood-serum to agar- 

 agar the growth resulting is meagre, but the oftener 

 this growth is transplanted to fresh agar-agar the more 

 luxuriant it becomes. 



