PNEUMONIA AND DIPHTHERIA. 231 



guinea-pigs, and in mice ; on- the other hand, Fried- 

 lander's pneumococcus has only a slight effect on 

 guinea-pigs, and none at all on rabbits. 



The diphtheria bacillus is an organism similar in 

 many respects to these. It occurs in the form of thick 

 rodlets about •004-"006 mm. in length, which may 

 be either slightly curved or straight, and which have 

 rounded off ends. Thus we see that it has no defi- 

 nite characteristic form. It is indeed remarkably 

 polymorphous, being sometimes thickened at one 

 pole, and sometimes at both, when it looks something 

 like' a dumb-bell. Occasionally, in long bacilli, a 

 third swelling occurs in the middle, or, as in the 

 tubercle bacilli, one or more gaps are present, so that 

 it appears as though the organism were broken up into 

 several pieces. As a rule it has no gelatinous sheath, 

 but sometimes it happens that it is surrounded by a 

 colourless, not especially wide capsule. Generally the 

 bacilli are isolated, but occasionally a few cells collect 

 together to form clumps. 



It is a very difficult matter to stain these bacilli ; 

 since, if the ordinary aqueous solutions are used, as 

 a rule, only quite unsatisfactory results are produced ; 

 only certain parts, generally the poles, take up the 

 stain well, whilst the interlying portions are either 

 colourless or only very slightly coloured. Experiments 

 with Eoffler's methylene blue are somewhat more suc- 

 cessful, especially if we have to deal with young cul- 



