ch.ii CHARACTERS OF THE B. PESTIS 15 



ends of the rods, whereas the middle portion is occupied 

 by a vacuole. Some observers (Fisher) maintain that this 

 peculiar staining is observed only in dried (film) specimens ; 

 but I cannot accept this interpretation as entirely satis- 

 factory, because I find, on careful examination of B. pestis 

 in the hanging drop, that some bacilli show a central 

 vacuole already in the living and fresh state ; moreover, 

 such a vacuole is also observed in many other bacteria in 

 the fresh condition, as also in stained and dried specimens 

 — e.g. B. coli, B. typhosus, Proteus vulgaris, — with this 

 difference, however, that B. pestis shows the bipolar 

 staining, both in dried film specimens of tissues as also of 

 culture, in a more conspicuous manner and more constant 

 than other bacteria. A further point to be mentioned in 

 this connection is that in B. pestis taken from the tissues 

 — and this applies notably to B. pestis in the bubo — a fair 

 number of individuals show an unstained vacuole either at 

 one or both ends, whereas the rest contains the stained 

 chromatic substance. Plague bacilli of this kind appear 

 at first sight to exhibit one or both ends truncated or 

 concave, the former if the vacuole is at one end only, the 

 latter if each end has a vacuole. Such forms do not occur 

 in culture. 



The bipolar condition is always marked and easily 

 demonstrated both in B. pestis of the tissues as also of 

 recent culture, provided the film specimens are well stained 

 and then well washed ; if the specimen is overstained and 

 insufficiently washed, the whole bacillus appears either 

 uniformly stained, or it shows in places slight differences 

 in depth of colour. 



The best methods of staining to show the bipolar 

 character are these : — 



