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selection, to a minimum.* The colonies present a very great 
variety of appearances on this solid medium. Some are bright 
red ; some have a red centre with a pale rim ; others are white, 
or yellow, and some have a transparent look. The red colonies 
usually cause a precipitation of bile acid in the solid medium 
producing a haze round the colony. The appearance of any 
particular colony on the medium is of little or no use for iden- 
tifying the bacillus present, very little reliance can be placed 
on the naked eye appearance. Two colonies very different in 
appearance, the one bright red with haze of precipitated bile salt 
round it, the other pale yellow, may on further investigation 
turn out to be the same organism. The reason for this is 
probably due to the fact that acid and alkali may be both pro- 
duced by colonies at different times, sometimes the one pre- 
dominates, and sometimes the other. 
Having obtained a good plate with, say, 10 or 12 discrete 
colonies on it, each separate colony is now put through the 
various sugar reactions. If the colonies are small, and are 
crowded together, or if, as sometimes happens, instead of 
getting a plate containing 10 colonies, it contains 100, it Is 
advisable to pick off 10 or 12 colonies, while they are small, 
and inoculate agar slope tubes from each one. In doing this 
care should be taken not to select 10 colonies from all over the 
plate, otherwise the tendencies to get them all of one kind is 
natural, and the ‘‘ speculative venture ’’ factor comes in, but 
it is better to mark off a segment of the plate and take all out 
of the part marked. If, on the other hand, the plates have 
been successful, and colonies are separate, it is possible to 
inoculate the 6 tubes from each colony, without passing through 
the stage of growing it separately on agar. If this latter 
method is to be carried out the first thing to do is to num- 
ber the colonies on the back of the plate with a pencil, otherwise 
confusion will occur. Six separate tubes containing the various 
materials are inoculated from each colony. The safest and 
quickest method of carrying out this operation is to scoop up 
the whole colony with a loop, put the growth in a small test 

* Houston says: ‘The picking out of coli-like colonies for studying in 
‘pure culture is, after all, even to the expert, a speculative venture.”’ 
