CULTURES IN SOLID MEDIA. 



69 



or in its absence a facultative anaerobic. In the third case the 

 microorganism is an anaerobic, which cannot grow in the presence 

 of oxygen, and consequently does not grow upon the surface of the 

 culture medium or along the upper portion of the line of puncture. 



Again, we have differences as to the character of growth upon the 

 surface or along the line of puncture. The surface growth may be 

 a little mass piled up at the point where the needle entered the gela- 

 tin ; or it may form a layer over the entire surface, and this may 

 be thin or thick, dry or moist, viscid or cream-like, and of various 

 colors green, blue, red, or yellow, of different shades or more fre- 

 quently of a milk-white color. 



The growth along the line of puncture also differs greatly with 

 different species. We may have a number of scattered spherical 

 colonies (a, Fig. 41), and these maybe translucent or opaque ; or we 

 may have little tufts, like moss, projecting from the line of puncture 

 (b, Fig. 41) ; or slender, filamentous branches may grow out into the 

 gelatin (c, Fig. 41). 



The liquefying bacilli also present different characters of growth. 

 Thus liquefaction may take place all along the line of puncture, 

 forming a long and narrow funnel of liquefied gelatin (a, Fig. 42) ; 

 or we may have a broad funnel, as at b ; or a cup-shaped cavity, as 

 at c; or the upper liquefied portion may be separated from that 

 which is not liquefied by a horizontal plane surface, as at d. 



