3o6 MICRO-ORGANISMS AND DISEASE [chap. 



of the solid medium or by first shaking up a particle of the 

 membrane in sterile salt solution and rubbing a droplet of this 

 over the culture surface. But, unfortunately, in a large per- 

 centage of doubtful cases the diphtheria bacilli arc mixed 

 up in the exudation with numerous cocci : in such cases it 

 is necessary to use serum cultures. On this medium the 

 diphtheria bacillus grows better than the cocci, and therefore 



Fig. 122- — FiLi\i Siecimen of a Gelatinp: Cultuke after several Days 

 Growth. 



after 24-36 hours its colonies can be recognised. Another 

 plan which I found useful is to melt over the flame sterile 

 nutrient Agar or Glycerine Agar and to pour it out into 

 sterile plate dishes ; after it has set herein a particle of the 

 suspected secretion or membrane is rubbed over the whole 

 surface of the solid Agar, and the plate is incubated at 37° C. 

 After twenty-four hours, by means of a magnifying glass or 

 simple microscope, the colonies are carefully examined, and 



