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278 have to be used in order to obtain colonies on solid culture media. It was further recognised that the tissue would have to be crushed into a pulp, which could be spread more or less evenly over the surface of the medium, in order that any micro-organisms which might be present should have a chance of coming into contact with it. Moreover, it was clearly seen that in carrying out experiments of this kind the chances of accidental contamination were not inconsiderable. The methods which were first employed in Scotland were later somewhat modified when the investigations were subsequently continued in Cambridge.

The precise conditions under which these experiments were conducted are matters of considerable importance, since upon them depend the reliability of the results which were obtained. We have therefore no hesitation in describing the methods in detail.

Previous to beginning an experiment the room was carefully prepared. All dust was removed from the window ledges and elsewhere, and the floor and bench were flooded with a mixture of glycerine and lysol to lay the dust. All the windows and ventilation shafts were closed during the actual operation of making the cultures. As a further precaution against aerial contamination the tissues were crushed inside a glass frame which was constructed as follows (PI. XXXIX., Fig. 1): — Two sheets of plate glass, 21 x 8 inches, formed the top and bottom respectively, the former being supported on blocks of wood, which formed the sides. The back also was formed of a sheet of plate glass, and the front was closed by a curtain of linen, soaked in lysol, which could be partially turned back when required. The joints of the frame were made draught-proof by means of rubber tubing. On the floor of the frame another sheet of plate glass, which extended the whole length, but was 3 inches narrower than the bottom, was placed towards the back, so as to form a ledge near the centre of the floor, upon which the plates used for crushing the tissues could be conveniently manipulated, and yet be covered by the roof. The height of the frame from the top to this ledge was 3$1⁄2$ inches.

Before use the frame was washed out with a mixture of glycerine and lysol. In order to estimate the risk of aerial contamination agar plates were exposed on the bench and inside the frame during the whole period of time the cultures were being made.

The birds, if living, were killed by decapitation, weighed and immediately