PURE CULTURES 53 



melted in its tube and poured out into a sterile Petri 

 dish, where it solidifies into a broad level surface. 



Gelatine or agar slants may be used for streak cultures 

 instead. These are obtained by melting the medium in 

 its tube, then placing the tube in a sloping direction, and 

 allowing it to remain in this position until the medium 

 cools and solidifies again. In this way a sloped surface 

 is obtained in the tube, on which streak-cultures can be 

 made (Fig. 20). 



Although the characters of bacteria as exhibited by 



FIG. 20. Mode of preparing gelatine or agar "slants." 



their colonies on solid media are not absolutely constant, 

 it is useful to note points of difference in this respect 

 between the various species. 



Colonies are variously coloured, some being white and 

 opaque, others white and translucent, while many kinds 

 are distinctively coloured red, yellow, green, black, and 

 other tints. 



The shape of the single colonies may be circular or 

 irregular in outline, with entire or variously indented 

 edges (Fig. 17), and the surface may be flat and smooth, 

 or raised into prominences of various form. All these 

 characters must be carefully observed. In the case of 

 stab-cultures the extent and form of the growth along 



