300 PATHOGENIC BACTERIA. 



The colonies which develop at 24 C. upon 15 per 

 cent, gelatin plates are described as small, round, cir- 

 cumscribed, finely granular white points which grow 

 slowly, never attain any considerable size, and do not 

 liquefy the gelatin (Fig. 96). 



FIG. 96. Diplococcus pneumoniae : colony twenty-four hours old upon gelatin ; 

 x 100 (Frankel and Pfeiffer). 



If, instead of gelatin, agar-agar be used and the plates 

 kept at the temperature of the body, the colonies which 

 develop upon the plates appear as transparent, delicate, 

 drop-like accumulations, scarcely visible to the naked 

 eye, but under the microscope distinctly granular, the 

 central darker portion being frequently surrounded by a 

 paler marginal zone. 



In gelatin puncture-cultures, made with 15 instead of 

 the usual 10 per cent, of gelatin, the growth takes place 

 along the entire path of the wire in the form of little 

 whitish granules distinctly separated from each other. 

 The growth in gelatin is always very limited. 



Upon agar-agar and blood-serum the growth consists 

 of minute, transparent, semi-confluent, colorless, dew- 

 drop-like colonies, which die before attaining a size 



