CULTIVATION. 147 
How should test tubes be prepared before the culture medium is put 
into them ? 
Whether the tubes are new or have been used it is well to boil them in 
a solution of common washing soda. After rinsing them they may be 
washed in water containing one per cent. of commercial hydrochloric acid, 
and lastly, rinsed in clean running water and allowed to drain until per- 
fectly dry. They are then to be stopped with plugs of common cotton 
wool, non-absorbent. These plugs must be fitted evenly but not too closely. 
The cotton wool permits the access of air but filters out any organisms 
that may be floating in it. The tubes are put in a wire basket and steri- 
lized in a dry sterilizer at a temperature of about 150° C. Flasks are 
prepared in the same way. 
Describe the process of filling the tubes. 
The nutrient medium in a fluid state is best put in through a small 
funnel, in order that the mixture does not touch the sides of the tube. 
They are filled about one-third full and stopped with the ectton-wool 
plugs, and sterilized in the steam sterilizer for about fifteen minutes each 
day for three days. 
What apparatus is needed in making plate cultures ? 
Test tubes of nutrient material, an dese, levelling plate and tripod, 
glass plates and bridges and a culture dish. 
What is an dese ? 
It is a glass rod, in one end of which is fixed a piece of platinum wire. 
It is used in transferring the material to be studied from one receptacle 
to another. This platinum wire must be heated before and cj/ter using 
in order to sterilize it. 
What is a levelling plate and tripod ? 
The levelling plate is made of glass and rests upon a shallow dish 
filled with water sufficiently to allow only a single large air bubble to 
be seen under the levelling plate. The dish rests upon the tripod 
which is fitted with levelling screws in order that the plate may be 
brought to an exact level. This is determined by the position of the 
air-bubble. 
What is the principle of plate cultures ? 
The organisms are evenly distributed through the melted culture 
medium, and poured out upon the glass plates. When the mixture 
cools the organisms are-held so that they are kept separate, and each 
individual goes on and develops a colony without mingling with any 
other variety. 
