352 BACTERIOLOGY. 



In bouillon they grow as a thin pellicle on the sur- 

 face. This may fall to the bottom of the fluid and con- 

 tinue to develop, its place on the surface being taken by 

 a second pellicle. 



The tubercle bacillus does not develop on gelatin 

 because of the low temperature at which this medium 

 must be used. 



MICROSCOPIC APPEARANCE OF BACTERIUM TUBER- 

 CULOSIS. 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, consisting 

 of rods a little clubbed at one extremity or slightly 

 bulging at different points, may be detected. Branch- 

 ing forms of this organism have been described. It 

 varies in length sometimes being seen in very short 

 segments, again much longer, though never as long 

 threads. Usually its length varies 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. (See Fig. 62.) 



These rods usually present, as has been said, an ap- 

 pearance of alternate stained and colorless portions. 

 The latter portions are believed to be the spores of 

 the organism, though as yet no incontestable proof of 

 this opinion has been presented. 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 stain ing-reagents, and 

 by this means alone it may be easily recognized. The 



