FOOD FROM THE SEA. 387 
in the experiments, because the length of the chains which are formed 
gives a good indication of the healthiness of the culture. 
In order to obtain a pure culture of a Plankton diatom two methods 
may be used. The first—which, however, does not give very satisfactory 
or certain results in practice—is to pick out under the microscope, by 
means of a very fine glass pipette, a single cell or short chain of the diatom 
and put it in a flask containing a suitable culture solution. 
The second method, which is the one I have more often used, is as 
follows: A glass flask, containing about 2 litres (say, half a gallon) of 
suitable culture solution, the nature of which I shall describe presently, 
is brought to the boil in order to kill off any animals or plants which may 
be present in it, and then allowed to cool. We then take some sea- 
water containing a mixture of Plankton animals and plants, which have 
been collected with a fine silk net in the way already described, and 
add just one or two drops of it to the flask containing the half-gallon of 
cold culture solution. The flask is shaken up, and the living organisms, 
which were present in the two drops of sea-water, become evenly distri- 
buted through it. The water in the flask is then divided up, by pouring 
it into, say, forty or fifty tiny flasks, and these small flasks are put to 
stand in a north light and kept at an even temperature. After a week or 
ten days a brownish growth appears in many of the flasks, and if the 
experiment is successful, that is to say if our one or two drops of water 
containing the mixture of organisms has been sufficiently divided up, 
we shali find in perhaps two or three of our fifty small flasks a pure 
culture of one of the Plankton diatoms. A culture once obtained in 
this way can be kept as long as we wish, by continually inoculating 
new flasks of sterile culture solution, transplanting to one new flask after 
another as often as may be desired. I have kept cultures alive in this 
way for six or seven years. 
Such cultures are, however, not quite perfect. They always contain in 
addition to the diatoms some bacteria, and these are very difficult to get 
rid of ; indeed I have never really succeeded in entirely eliminating them. 
They may be greatly reduced by a process of differential poisoning. By 
adding to a series of culture flasks gradually increasing doses of chlorine 
gas, it is possible to hit off a strength of the poison which will kill most 
of the bacteria without killing the diatoms. In this way a culture of the 
diatom Thalassiosira was obtained in which there remained only one kind 
of micro-organism, or at least only one kind that could be detected in the 
ordinary way by growth on agar-agar plates. The experiments to be 
described were made chiefly with this culture. Its growth was very rapid 
and healthy under favourable conditions, and very long chains of diatoms 
were formed. 
