72 CULTURES IN SOLID MEDIA. 
The characters of growth in agar-agar jelly are not so varied, 
but this medium possesses the advantage of not liquefying at a tem- 
perature of 35° to 38° C., which is required for the development of 
certain pathogenic bacteria. Variations in mode of growth are 
also manifested in nutrient agar similar to those referred to as pro- 
duced by non-liquefying bacteria in flesh-peptone-gelatin. These 
relate to the surface growth and to growth along the line of punc- 
ture. One character not heretofore mentioned consists in the for- 
mation of gas bubbles in stab cultures either in gelatin or agar. 
Colonies.—If we melt the gelatin or agar in a test tube, pour 
the liquid medium into a shallow glass dish previously sterilized, 
¢ d 
Fie. 42. 
and allow it to cool while properly protected by a glass cover, we 
will have a broad surface of sterile nutrient material. If now we ex- 
pose it to the air for ten or fifteen minutes, and again cover it and 
put it aside for two or three days at a favorable temperature, we can 
scarcely fail to have a number of colonies upon the surface of the 
culture medium, which have been developed from atmospheric germs 
which were deposited upon it during the exposure. Each of these 
colonies, as a rule, is developed from a single bacterium or spore, 
and consequently the little mass, visible to the naked eye, which we 
call a colony, is a pure culture of a particular species. In this ex- 
periment we are more apt to have colonies of mould fungi than of 
bacteria, but the principle is the same, viz., that a colony developed 
from a single germ isa pure culture. By touching our platinum 
