SPONTANEOUS sarcomas of the domestic fowl are relatively common (Olson and Bullis, 1942; Campbell, 1945) while leukaemias not infrequently reach epizootic levels in commercial flocks (Olson, 1948). Both of these are considered to be of viral origin although this point is very rarely investigated. In consequence, the characterisation of the viruses is hardly ever attempted. Their viral origin therefore remains in most instances an unproven assumption. Hence any attempt to consider in a scientific manner the epidemiology of either condition founders at once in the absence of information concerning the number and mutability of any viruses present in a particular flock. The present account deals with three new sarcomata isolated at this Centre and arising from spontaneous cases submitted to the Ministry of Agriculture's diagnostic laboratory at Lasswade. The cooperation of their staff is appreciated.
(McIntosh, 1935 ;Amies, 1937 ; Claude, 1937 Claude, , 1938 Claude, , 1939 Claude, , 1940 Claude and Rothen, 1940; Pollard, 1938 Pollard, , 1939 Stern and Duran-Reynals, 1939;Dmochowski, 1948a). Claude (1938Claude ( , 1939) isolated a fraction corresponding to less than 3-5 per cent of the dry weight of the original extract which, in amounts corresponding to 4-0 X 10-10 mg. N, produced tumours in 13 to 17 days after subcutaneous inoculation into fowls. There are two difficulties in the separation and concentration of the agent bv fractional centrifugation: (a) the viscosity of the initial tumour extract-which contains a mucoprotein, and (b) the tendency of the agent to aggregate, and to adsorb to tissue components.
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