THE TYPHOID BACILLUS. 209 



surface is covered witli a thick almost felt-like coat- 

 ing, whicli consists of a very vigorous growth of th^ 

 colonies of the bacteria. Unfortunately these bacilli 

 appear to be capricious in their behaviour on potato 

 slices, for they do not always show this characteristic 

 appearance. Either the colonies remain confined to 

 the inoculating stroke, without spreading all over the 

 surface, or the growth is to be perceived, appearing 

 as a whitish, or slightly yellowish grey film, which 

 may be seen without disturbing the surface. It is 

 supposed that this atypical growth of the typhoid 

 bacillus is due to the acid reaction of some potatoes, 

 and indeed, as a rule, the characteristic appearance 

 can be made to manifest itself if the potato is ren- 

 dered alkaline by the use of weak soda solution. 

 This, however, is not always suflScient, and thus the 

 phenomena cannot be fully explained. However, 

 these cases of atypical growth of the typhoid bacillus 

 on the potato are not of very frequent occurrence, so 

 that its cultivation on potato may be considered as 

 affording the surest means for its recognition. 



If samples are taken from various young cultures, 

 and are examined in the hanging drop, it is im- 

 mediately noticed that the bacilli are exceedingly 

 motile. There is nothing characteristic either in the 

 movement or in the form of these bacilli. As a rule 

 they are thick rodlets, of from xo^oo- *o to*oo '^™- '^ 

 length, with rounded ends. Occasionally two or more 



p 



