300 -B^ GTEBIOL OGY. 



berculous subject, the tubercle bacillus will give us 

 most difficulty in our efforts at cultivation. 



It is, in the strict sense of the word, a parasite, and 

 finds conditions entirely favorable to its development 

 only in the animal body. On ordinary artificial media 

 the bacilli taken directly from the animal body grow 

 only very imperfectly, or, in many cases, not at all. 

 From this it seems probable that there is a difference 

 in the nature of individual tubercle bacilli — some 

 appearing to be capable only of growth in the animal 

 tissues, while others are apparently possessed of the 

 power to lead a limited saprophytic existence. It may 

 be, therefore, that those bacilli which we obtain as arti- 

 ficial cultures from the animal body are offsprings from 

 the more saprophytic varieties. At best, one never sees 

 with the tubercle bacillus a saprophytic condition in 

 any way comparable to that possessed by many of the 

 other organisms with which we have to deal. 



In efforts to cultivate this organism directly from the 

 tissues of the animal, the method by which one obtains 

 the best results is that recommended by Koch, viz. , cul- 

 tivation upon blood-serum. So strictly is this organism 

 a parasite that very limited alterations in the conditions 

 under which it is growing may result in failure to study 

 it successfully. It is, therefore, necessary that the 

 injunctions for obtaining it in pure culture should be 

 carefully observed. 



Preparation of Ccjltuebs from Tissues. — Under 

 strictest antiseptic precautions remove from the animal 

 the tubercular organ — the liver, spleen, or a lymphatic 

 gland being preferable. Place the tissue in a sterilized 

 Petri dish and dissect out with sterilized scissors and 

 forceps the small tubercular nodules. Place each nodule 



