15 



from the above-given description and while not representing the 

 most characteristic type, are so frequently fonnd in smears that 

 the student of plague must thoroughly familiarize himself with 

 them. These are elliptical or egg-shaped, or almost spherical, and 

 they show only a very narrow peripheral staining, or they may not 

 stain at all; hence they look like mere empty shells, which indeed 

 they jjrobably are, since they are most commonly found in older 

 'buboes. Also, in smears from cases which have succumbed only 

 after several days' sickness, one frequently sees various involution 

 forms of the plague micro-organism, which aj)pear like yeast 

 cells or which are either quite hazy and indistinct or club shaped 

 and irregular in outline. Sata has shown that such involution 

 forms may be seen on and after the fourth day in experimental 

 plague infection in animals. In cover-glass preparations from 

 jDure cultures the bacilli are not so characteristic as we find them in 

 smears from human plague cases, except in some involution forms 

 to be more fully described below. The first generation, as a rule, 

 shows the polar staining and the unstained center fairly well, but 

 in subsequent ones these features often become more or less indis- 

 tinct, or may even entirely be lost. Plague bacilli from pure 

 cultures, particularly from the water of condensation of agar 

 tubes, or from bouillon, frequently show shorter or longer chains, 

 in which dividing lines between the individual bacilli are so 

 indistinct as to cause them to appear like filaments. Involution 

 forms are likewise liable to present themselves early even on 

 favorable media. We have occasionally seen them as soon as the 

 second or third day on agar tubes. 



The plague bacillus may be stained with any one of the watery 

 solutions of the common basic aniline dyes. The carbol-thionin 

 stain, frequently mentioned in connection with it, is not to be 

 recommended if one works in the Tropics, since for certain reasons 

 it is very liable to fade. IvosseFs modification of Eomanowsky's 

 stain has not been very satisfactory in our hands, and for general 

 practical purposes we have found dilute carbol-fuchsin used in 

 the manner above recommended the most simple and most imiformly 

 relialjle. 



The plague bacillus, if treated by Gram's method, is decolorized. 



The bacillus, if grown in the animal body, possesses a capsule, 

 which, however, is not very eas)' to demonstrate unless thin spreads 

 are prepared and fixed in alcohol with great care. There is nothing 



