TUBERCULOSIS. 



319 



of the tubercle bacillus and are stood in an incubator. In 

 the course of two weeks a slight surface growth is appar- 

 ent, which in the course of time develops into a pretty 

 firm pellicle and gradually subsides. At the end of 

 some weeks development ceases and the pellicle sinks. 



Fig. 70. — Massive culture of the tubercle bacillus upon the surface of glycerin- 

 bouillon, used in the manufacture of tuberculin. 



The contents of a number of flasks are then collected in 

 an appropriate vessel and evaporated over a water-bath to 

 one-tenth their volume, then filtered through a Pasteur- 

 Chamberland filter. This is crude tuberculin. 



When such a product is injected in doses of a fraction 



