1. Nichols, J. E., Nature, 168, 932 (1951).
2. Report for 1945–47 of the Dept. of Animal Health, University College of Wales, pp. 98–118.
3. Cmd. 8266 (H.M.S.O., 1951).
4. Lincoln, F. C., Circ. U.S. Dept. Agric. No. 118 (1930).
5. Southern, H. N., Ann. App. Biol., 27, 509 (1940).