CULTURES IN SOLID MEDIA. 71 



needle, then, to such a colony, which is quite independent of, and 

 well separated from, all others, we may make a stick culture in gela- 

 tin or agar, and preserve the pure culture for further study. This 

 is a most important advantage which pertains to the use of solid 

 culture media. It is a singular fact that, as a rule, colonies of bac- 

 teria which lie near each other do not grow together, but each re- 

 mains distinct. If there are but few colonies, each one, haying 

 plenty of room, may grow to considerable size ; if there are many 

 and they are crowded, they remain small, but are still independent 

 colonies. 



Now, these colonies differ greatly in their appearance and char- 

 acters of growth, according to the species (Fig. 43). Some are 

 spherical, and these may be translucent or opaque, or they may have 

 an opaque nucleus surrounded by a transparent zone. Again, the 



Fro. 43. Colonies of Bacteria. 



outlines may be irregular, giving rise to amoeba-like forms, or to a 

 fringed or plaited margin, or the form may be that of a rosette, etc. ; 

 or the colony may appear to be made up of overlapping scales or 

 masses, or of tangled filaments; or it may present a branching 

 growth. In the case of liquefying bacteria, when the colonies have 

 developed in a gelatin medium they commonly do not at once cause 

 liquefaction of the gelatin, but at the end of twenty-four hours or 

 more the gelatin about them commences to liquefy and they are 

 seen in a little funnel of transparent liquefied gelatin ; or in other 

 cases little opaque drops of liquefied gelatin are seen, which, as the 

 liquefaction extends, run together. All of these characters are best 

 studied under a low-power lens, with an amplification of five to 

 twenty diameters ; and by a careful observation of the differences in 

 the form and development of colonies we are greatly assisted in the 

 differentiation of species. 



Single, isolated colonies do not always contain a single species, 

 for they are not always developed from a single cell. We may have 



