BACTERIUM DIPHTHERIA. 405 



usually be made out : while the forms on the glycerin- 

 agar-agar cultures are throughout small, and pretty 

 regular in size, shape, and staining, those on the plain 

 agar-agar are larger, stain less uniformly, vary more in 

 shape, and when stained by Loffler's blue are not so 

 regularly marked by pale transverse lines that give to 

 them the appearance of being made up of numerous 

 short segments. 



Though the outline of this organism is more regular 

 under some circumstances than others, it is nevertheless 

 always conspicuous for its manifold variations in shape. 



GROWTH ON SERUM MIXTURE. The medium upon 

 which bacillus diphtheria grows most rapidly and lux- 

 uriantly, and which is best adapted for determining its 

 presence in diphtheritic exudates, is, as has been stated, 

 the blood-serum mixture of Loffler. (See chapter on 

 Media.) On the blood-serum mixture the colonies of 

 bacillus diphtheria? grow so much more rapidly than the 

 other organisms usually present in secretions and exuda- 

 tions 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 or yellowish, w T ith a centre more 

 opaque than the slightly irregular periphery. The sur- 

 face of the colony is at first moist, but after a day or 

 two becomes rather dry in appearance. 



A blood-serum tube studded with coalescent or scat- 

 tered colonies of this organism is so characteristic that 

 one familiar with the appearance can anticipate with tol- 

 erable certainty the results of microscopic examination. 



GLYCERIN AGAR-AGAR. Upon nutrient glycerin 

 agar-agar the colonies likewise present an appearance 



