THE BACILLUS OF PLAGUE. 107 



so that there now exists a large amount of information, not 

 only on the etiology, but also on the modes of communica- 

 tion, means of spread, pathology, and treatment of this 

 disease. In the communication made by Metchnikoff at 

 the International Medical Congress in Moscow, August, 

 1897, is found an admirable exposition of the nature of the 

 plague (7). 



As described by Yersin the specific microbe of plague 

 is a very short bacillus with rounded ends and apparent 

 vacuolation in the centre. The organism, which is not en- 

 dowed with movement, is often surrounded by a capsule, 

 and Zettnow (8) has demonstrated this by Loffler's method 

 for staining the flagella of bacteria. He considers that the 

 capsule represents the plasma of the bacterial cell, while 

 the microbe itself is a nucleus. By van Ermengem's method 

 Gordon (9) has succeeded in staining a spiral fiagellum about 

 twice the length of the bacillus, which is attached to this 

 only at one extremity ; occasionally a second fiagellum can 

 be seen close to the other. The specimens showing this 

 were obtained from agar cultures of twenty hours' growth 

 at a temperature of $7° C. which had been made from the 

 blood of an inoculated animal. If examined in hanging 

 drops this observer states that the pest-bacilli are to a 

 slight extent motile, a view which is shared by no other 

 worker. With the usual aniline dyes the bacillus stains 

 well, especially at the ends rather than the centre, so that 

 an appearance is presented like the bacillus of chicken- 

 cholera. By the method of Gram it does not stain but be- 

 comes tinted if a contrast colour is used. Two characters 

 of the bacillus render it difficult to recognise microscopically, 

 a marked tendency to the production of polymorphic and in- 

 volution forms, but both these characteristics have been 

 utilised as aids to diagnosis. Klein (10) has pointed out 

 that gelatine cultures twenty-four to twenty-eight hours old 

 show atypical thread-like colonies which resemble those of 

 Proteus vulgaris, and the presence of these colonies which 

 have also been noted by other observers in older cultures 

 he considers may be regarded as characteristic of the pest- 

 bacillus. Hankin and Leumann (11) describe well-marked 



