CULTIVATION OF BACILLUS 491 



the body, though growth occurs as low as 18 C. On agar 

 and on blood serum the colonies are whitish circular discs 

 of somewhat transparent appearance and smooth, shining 

 surface. When examined with a lens, their borders appear 

 slightly wavy. In stroke cultures on agar there forms a con- 

 tinuous line of growth with the same appearance, showing partly 

 separated colonies at its margins. When agar cultures are kept 

 at the room temperature, some of the colonies may show a more 

 luxuriant growth with more opaque appearance than the rest of 

 the growth, the appearance in fact being often such as to suggest 

 the presence of impurities 

 in the cultures. In stab 

 cultures in peptone gela- 

 tin, growth takes place 

 along the needle track as *. 



a white line, composed of 

 small spherical colonies. * ** 



On the surface of the ** 



gelatin a thin, semi- t,*ij* '*' 



transparent layer may be ;'** -* ; 4 \**. 



formed, which is usually % ',*'* 



restricted to the region of ^ % /?* ^ 

 puncture, though some- ' % *^** ?&T" * 1^* 



times it may spread to 

 the wall of the tube ; 

 sometimes, however, 



there is practically no FIG. 148. Culture of the bacillus of plague 

 <mrfapp o-rnwth Thprp on 4 P er cent - salt a S ar > showing involution 



surrace growtn. ine formi f of great vari ety of size and shape, 

 is no liquefaction of the g e e also Plate IV., Pig. 17. 

 medium. In gelatin Stained with carbol-thionin-blue. x 1000. 

 plates the superficial 



colonies develop first and form slightly raised semi-transparent 

 discs with somewhat crenated margins ; the deeper colonies are 

 smaller and of spherical shape, with smooth outline. In bouillon 

 the growth usually forms a slightly granular or powdery deposit 

 at the foot and sides of the flask, somewhat resembling that of 

 a streptococcus. If oil or melted butter is added to the bouillon 

 so that drops float on the surface, then a striking mode of growth 

 may result, to which the term " stalactite " has been applied. 

 This consists in the growth starting from the under surface of 

 the fat globules and extending downwards in the form of 

 pendulous, string-like masses. These masses are exceedingly 

 delicate, and readily break off on the slightest shaking of the 

 flask; accordingly during their formation the culture must be 



