STAINING REACTIONS. 211 



arranged in a more or less parallel manner. Tubercle 

 bacilli are quite devoid of motility. 



Aberrant Forms. Though such are the characters of 

 the organism as usually met with, other appearances are 

 sometimes found. In old cultures, for example, very much 

 larger elements may occur. These may be in the form of 

 long filaments, which may be swollen or clubbed at their 

 extremities, may be irregularly beaded, and may even show 

 the appearance of branching. Such forms have been 

 studied by Metchnikoff, Maffucci, Klein, and others. Their 

 significance has been variously interpreted, for while some 

 look upon them as degenerated or involution forms, others 

 regard them as indicating a special phase in the life history 

 of the organism. This latter view, however, has many 

 facts against it, especially the circumstance that these 

 aberrant forms are chiefly met with when the organisms are 

 undergoing retrogressive change. The question, however, 

 is one which at present is not definitely settled. 



Staining Reactions. The tubercle bacillus takes up the 

 ordinary stains with great slowness, and for successful 

 staining one of the most powerful solutions ought to be em 

 ployed, e.g., gentian-violet or fuchsin, along with aniline oil 

 water, or solution of carbolic acid. Further, such staining 

 solutions require to be applied for a long time, or the 

 staining must be accelerated by heat, the solution being 

 warmed till steam arises and the specimen allowed to remain 

 in the hot stain for two or three minutes. It was at first 

 supposed that the organism could not be coloured at all by 

 a simple watery solution of a basic aniline stain. This is 

 not strictly correct, but the colour is taken up with great 

 slowness and very faintly. As stated above, Koch at first 

 used a solution of methylehe-blue with caustic potash added, 

 but even this method stains somewhat faintly, and he after- 

 wards abandoned it in favour of the combination of aniline 

 oil with gentian violet, introduced by Ehrlich. One of the 

 best and most convenient methods is the Ziehl-Neelsen 

 method (see p. 102). The bacilli present this further 

 peculiarity, however, that after staining has taken place they 



