STAINING PECULIARITIES OF B. TUBERCULOSIS. 263 



brittle in appearance, though in truth the individuals 

 adhere tenaciously together by a very glutinous sub- 

 stance. 



The tubercle bacillus does not develop on gelatin, 

 because of the low temperature at which this medium 

 must be used. 



Microscopic Appearance of the Tubbrci^e 

 Bacillus. — Microscopically the organism itself is a 

 delicate rod, usually somewhat beaded in its structure, 

 though rarely it is seen to be homogeneous. It is either 

 quite straight or somewhat curved or bent on its long 

 axis. In some preparations involution-forms, consist- 

 ing of rods a little clubbed at one extremity or slightly 

 bulging at different points, may be detected. It varies 

 in length — sometimes being seen in very short segments, 

 again much longer, though never as long threads. On 

 an average its length is seen to vary from 2 to 5 /«. It 

 is commonly described as being in length about one- 

 fourth to one-half the diameter of a red blood-corpuscle. 

 It is very slender. (Fig. 51, page 239.) 



These rods usually present, as has been said, an 

 appearance of alternate stained and colorless portions. 

 It is the latter portions which are believed to be the 

 spores of the organism, though as yet no absolute proof 

 of this opinion has been established. 



At times these colorless portions are seen to bulge 

 slightly beyond the contour of the rod, and in this way 

 give to the rods the beaded appearance so commonly 

 ascribed to them. 



Staining Peculiarities. — A peculiarity of this 

 organism is its behavior toward staining reagents, and 

 by this means alone it may be easily recognized. The 

 tubercle bacillus does not stain by the ordinary methods. 



