CULTURES IN SOLID MEDIA. V7 
tube. This is due to the fact that a little fluid is pressed out of the 
jelly, probably by a slight contraction while cooling. If the tubes 
are slightly inclined from the horizontal the film does not slip and 
the fluid accumulates at the bottom. After a day or two they may 
be placed in an upright position. 
These roll tubes possess several advantages. They are quickly 
made and take but little space in the incubating oven, and the film 
of jelly is protected from contamination by atmospheric germs. 
When colonies have formed we may examine them through the thin 
walls of the tube, either with a pocket lens or a low-power objective. 
In making a stab culture from a single colony in one of these roll 
tubes, we invert the tube, remove the cotton air filter, and pass the 
point of a sterilized platinum needle up to the selected colony. In 
the same way we obtain material for microscopical examination. 
Streak Cultures.—In his earlier experiments with solid culture 
media Koch made “streak cultures” by drawing the point of a plati- 
num needle, charged with bacteria, over the surface of a gelatin or 
agar plate ; and this method is still useful in certain cases. If we 
draw the needle over the moist surface several times in succession 
the greater number of bacteria will be deposited in the first streak, 
and in the second or third single cells are likely to be left at such 
intervals from each other that each will develop an independent 
colony. If the streaks were made with impure stock we may thus 
succeed in getting separate colonies of the several bacteria contained 
in it, so that this method may be employed for obtaining pure cul- 
tures. But for this purpose it is much inferior to the plate method, 
and it is chiefly used for observing the growth of bacteria on the sur- 
face of solid culture media. Thus we commonly make a streak upon 
the surface of cooked potato or solidified blood serum in studying the 
development of various bacteria on these culture media. 
Cultures upon Blood Serum.—The use of blood serum as a 
solid medium is practically restricted to stab cultures and streak 
cultures, for we cannot substitute it for the gelatin and agar media 
in making plates and roll tubes. This is because it only becomes solid 
at a temperature which would be fatal to most bacteria (70° C.), and 
when once made solid by heat cannot again be liquefied. Its use is, 
therefore, restricted mainly to the cultivation of bacteria for which 
it is an especially favorable medium. It may be used, however, in 
combination with a gelatin or agar medium. For this purpose it is 
most conveniently kept in a fluid condition in the little flasks hereto- 
fore described (‘‘ Sternberg’s bulbs ”). 
The gelatin or agar jelly in test tubes is liquefied by heat and 
cooled in a water bath to about 40° C. The desired amount of ste- 
rile blood serum is then forced into each tube by passing the slender 
