330 BACTERIOLOGY. 



In cultures made upon two sets of nutrient agar-agar 

 tubes, differing only in the fact that one set contains 

 glycerin to the extent of 6 per cent., while the others 

 contain none, a noticeable difference in morphology can 

 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 more irregularly, vary more in 

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

 uniformly 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 it grows most rapidly and luxuriantly, and which 

 is best adapted for determining its presence in diphther- 

 itic exudation, is, as has been stated, the blood-serum 

 mixture of Lceffler. (See chapter on Media.) On the 

 blood-serum mixture the colonies of the bacillus diph- 

 therias grow so much more rapidly than the other or- 

 ganisms usually present in 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 or yellowish, with 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 ratlier dry in appearance. 



A blood-serum tube studded over with coalescent or 

 scattered colonies of this organism is so characteristic 



