652 NOTES AND MEMORANDA. 
owing to the solid character of the nourishing support the 
germs and spores cannot get mixed as they do in a liquid, 
each remains distinct from its neighbour even though in very 
close proximity, and without any trouble from the resulting 
growth, which proceeds in a day or two from each germ— 
new and perfectly pure cultivations may be started in suitable 
sterilized fluids. 
Dr. Koch’s method consists in substituting for the potato 
slice a layer of gelatine which is so saturated with water as 
just to become solid on cooling. The gelatine liquid is 
readily sterilized by boiling, and into it can be introduced 
either Pasteur’s salts, peptones, blood-serum, or other nu- 
trient material required by one or other species of Bac- 
terium. The gelatine-medium thus prepared may be kept 
in atube and a cultivation thus carried on—on its sur- 
face, or (and this is its principal use) it may be spread when 
liquid on a microscope object-slide and allowed to cool. Then 
such a gelatine plate may be inoculated by touching its 
surface with material containing the Bacteria which it is 
desired to study. The plate is readily protected from the 
access of accidental atmospheric germs, and maintained at 
such temperature and degree of moisture (by a glass shade) 
as the experimenter may desire. The main point of advan- 
tage, however, is this—that the point of inoculation on the 
surface of the gelatine can, owing to its transparency, be 
readily examined with the highest powers of the microscope 
and the growth of the Bacteria followed—whilst further, 
owing to the fact that the medium in which the growth 
takes place is solid, no mixture of the different kinds which 
may be present occurs, but each Bacterium produces around 
it a little spherical nest of itsown kind. From these nests, 
with a sterilized needle-point, individuals can be removed to 
start new pure cultivations. 
But it is obvious that, if the original point of inoculation 
was very minute, there is no danger of any accidental con- 
tamination from atmospheric germs, for these are not likely 
to fall on the identical spot no bigger than the puncture of 
a needle’s point, where the experimental culture is going 
on. As a matter of fact, where they fall on to the gelatine 
there they remain and grow, and fifty such accidental 
spores may fall on to the gelatine plate without in the least 
interfering with the purity of the experimental culture. 
There is yet, further, a very simple device which enables 
Dr. Koch to use this gelatine surface as a means of 
“spacing” and dividing the various species in a mixture of 
Bacteria, He dips a sterilized needle into such a mixture, 
