CULTURES IN SOLID MEDIA. 73 
needle, then, to such a colony, which is quite independent of, and 
well separated from, all others, we may make a stab 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. Itis 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, having 
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 
Fie. 48.—Colonies of Bacteria. 
outlines may be irregular, giving rise to amceba-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, wher 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, zsolated colonies do not always contain a single species, 
for they are not always developed from a single cell. We may have 
