CHARACTERS OF CULTURES. 



323 



a somewhat characteristic 



appearance. On the sur- 

 face of the medium growth 



spreads outwards from the 



puncture as a thin film or 



pellicle, with irregularly 



wavy margin (Fig. 116, A). 



It is semi-transparent and 



of a bluish-white colour. 



Ultimately this surface 



growth may reach the wall 



of the tube. Not infre- 

 quently, however, the sur- 

 face growth is not well 



marked. Along the stab 



there is an opaque whitish 



line of growth, of finely 



nodose appearance. There 



is no liquefaction of the 



medium, and no formation 



of gas. In stroke-cultures 



there is a thin bluish-white 



film, but it does not spread 

 to such an extent as in the case of the surface growth of a 

 stab-culture (Fig. 116, B). In gelatin plates also the superficial 



and deep colonies present cor- 

 responding differences. The 

 former are delicate semi-trans- 

 parent films, with wavy margin, 

 and are much larger than the 

 colonies in the substance, which 

 appear as small round points 

 (Fig. 117). These appearances, 

 which are well seen on the third 

 or fourth day, resemble those 

 seen in agar plates, as already 

 described in the method of 

 FIG. n 7 . colonies of the typhoid bacii- isolation; but on gelatin the 



lus (one superficial and three deep) in a surface colonies are rather 

 gelatin plate. Three days' growth at room 



temperature, x 15. more transparent than those on 



FIG. 116. 



A. Stab-culture of the typhoid bacillus in gelatin, 

 five days' growth. 



B. Stroke-culture of the typhoid bacillus on gela- 

 tin, six days' growth. 



C. Stab-culture of the bacillus coli in gelatin, nine 

 days' growth; the gelatin is split in its lower part 

 owing to the formation of gas. 



