288 PATHOGENIC BACTERIA. from the throat does not necessarily prove the patient to be diseased. Virulent bacilli can often be discovered in the throats of healthy persons who have knowingly or unknowingly come in contact with the disease. The bacteriologic examination is only an adjunct to the 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 & c FIG. 78.óDiphtheria bacilli (from photographs taken by Prof. E. K. Dun- ham, Carnegie Laboratory, New York): #, pseudo-bacillus; b, true bacillus; c, pseudo-bacillus.