472 BACILLI IN CHRONIC INFECTIOUS DISEASES. 
about half a minute in dilute nitric acid (ten per cent); then wash 
out color in sixty-per-cent alcohol ; counter-stain for two or three 
minutesin a saturated aqueous solution of methylene blue ; dehydrate 
with absolute alcuhol or with aniline oil; clear up in oil of cedar, 
and mount in xylol balsam. If the aniline-water-methyl-violet solu- 
tion has been used for staining the bacilli a saturated solution of 
vesuvin may be used as a contrast stain. : 
Biological Characters.—A parasitic, aérobic, non-motile ba- 
cillus, which grows only at a temperature of about 37° C. Is also a 
facultative anaérobic (Frankel). 
The question as to spore formation has not been definitely deter- 
mined. It has been generally assumed that the unstained spaces 
which are frequently seen in the bacilli are spores ; and the fact that 
Fic. 116.—Section through a tuberculous nodule in the lung of a cow, showing two giant cells 
containing tubercle bacilli. x 950, (Baumgarten.) 
caseous material in which a microscopical examination has failed to 
demonstrate the presence of bacilli may produce tuberculosis, with 
bacilli, when inoculated into guinea-pigs, has been explained upon the 
supposition that this material contained spores. But a few bacilli 
present in such caseous material might easily escape detection. As 
pointed out by Frankel, the oval spaces in stained specimens have 
not the sharply defined outlines of spores. Moreover, the bacilli, when 
examined in unstained preparations, do not contain corresponding re- 
fractive bodies, recognizable as spores. And when the bacilli are 
stained by Gram’s method the protoplasm is often contracted in the 
form of little, spherical stained masses, while the unstained spaces 
are larger and no longer have the oval form presented in rods stained 
by Ehrlich’s method. The great resisting power of the bacillus to 
heat and to desiccation has been supposed to be due to the presence 
