8 



Petri dish and sterilized in dry heat. When the Petri dish has cooled 

 .sufficiently to be handled the slides are taken out with sterile forceps and 

 hot glucose agar is poured on each to form a uniform thin film. Each 

 .slide is immediately replaced in the Petri dish and the agar allowed to 

 solidify. The exudate which contains many streptococci in the bodies 

 of the leucocytes is now gently smeared on these agar films, the watch 

 .glass is half filled with sterile water, and the large dish is placed in 

 the incubator for four to six hours. The water is used to prevent dry- 

 ing of the agar. 



Staining. The slides are now taken out of the Petri dish and dried 



Fig. 1. Photomicrograph x 1300. Streptococci in guinea-pig's leucocytes, 

 taken from the pleura t\vo hours after injection of streptococci. 



in a thermostat at 55 C. for fifteen to twenty minutes, or longer if nec- 

 essary; fixed in 40 per cent, formalin for four or five minutes, washed 

 in water, stained for only a few seconds with Loeffler's methylene blue 

 solution, again washed in water and dried in the thermostat. 



The best stains were obtained when the methylene blue solution was 

 poured over the wet slide, the slide being held so that the stain ran off 

 readily. The slide is then immediately placed in a dish of cold water 

 and gently rinsed in several changes of water. Violent agitation in the 

 water pulls off the film of agar. When the agar is perfectly dry (which 

 requires only about half an hour), a cover-glass is placed on it, either 



