THE TUBERCLE BACILLUS. 345 



though the development is much more limited. On 

 this medium they are of nearly the same color as the 

 potato on which they are growing. When cultivated 

 for a time on potato they are said to lose their patho- 

 genic properties. 



On milk-agar-agar they are of so nearly the same 

 color as the medium that, unless they are growing as 

 characteristic mealy-looking masses, considerably ele- 

 vated above the surface, their presence is less conspicu- 

 ous than when on other media. 



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 Bacillus TtrsEE- 

 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. 63.) 



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



