268 BACTERIOLOGY. 
ditions which they require, tubercle bacilli cannot be 
grown in pure culture by the plate method on the 
ordinary culture media. Koch first succeeded in culti- 
vating and isolating this bacillus on coagulated blood- 
serum, which he inoculated by carefully rubbing the 
surface with sections of tuberculous tissue and then 
leaving the culture, protected from evaporation, for 
several weeks in the incubator. Roux and Nocard 
afterward showed that the bacilli from man and ani- 
mals occasionally grow on nutrient agar to which 
glycerin has been added in the proportion of 5 per 
cent. 
Growth on Coagulated Blood-serum. On this medium, 
which is regularly used to obtain the first culture, the 
growth first becomes visible at the end of ten to four- 
teen days at 37° C., and at the end of three to four 
weeks a distinct and characteristic development has 
occurred. Small, grayish-white points and scales first 
appear on the surface of the medium. As development 
progresses there is formed an irregular, membranous- 
looking layer. When a tiny piece of this is removed, 
placed on a cover-glass without rubbing, stained, and 
then observed under the microscope the surface growth 
presents a characteristic appearance, the bacilli being 
arranged in parallel rows of variously curved figures. 
Owing to the greater facility of preparing and steril- 
izing glycerin-agar, and the more rapid and abundant 
growth of the bacilli, which have become accustomed 
to growth outside the body on this medium, it is now 
usually employed in preference to blood-serum for 
preserving cultures. The development at the end of 
fourteen to twenty-one days is more abundant than 
upon blood-serum after several weeks. When numer- 
