246 BACTERIOLOGY. 



theriae grow so much more rapidly than other organ isms 

 usually present in the secretions and exudations in the 

 throat that at the end of twenty-four hours they are 

 often the only colonies that attract attention, and if 

 others of similar size are present, they are generally of 

 quite a different aspect Its colonies are large, round, 

 elevated, grayish-white, with a centre more opaque than 

 the slightly irregular periphery. The surface of the 

 colony is at first moist, but after a day or two rather 

 dry in appearance. 



A blood-serum tube studded over with coalescent or 

 scattered colonies of this organism is so characteristic 

 in appearance that one can anticipate with tolerable cer- 

 tainty the results of microscopic examination. 



GLYCERIN AGAR-AGAR. Upon nutrient glycerin- 

 agar-agar the colonies likewise present an appearance 

 which may readily be recognized. They are in every way 

 more delicate in their structure than when on the serum 

 mixture. They appear at first, when on the surface, as 

 very flat, almost transparent, dry, non-glistening, round, 

 points which are not elevated above the surface upon 

 which they are growing. When slightly magnified 

 they are seen to be granular, and present an irregular 

 central marking which is more dense and darker by 

 transmitted light than the thin, delicate zone which sur- 

 rounds it. The periphery of the colony is marked by an 

 irregularly notched appearance. These colonies are al- 

 ways quite dry in appearance. When deep down in the 

 agar-agar they are coarsely granular. They rarely exceed 

 3 mm. in diameter. 



GELATIN. On gelatin the colonies develop much 

 more slowly than on the other media which can be 

 retained at a higher temperature. They rarely present 



