1. Baker, R. D. (1957). J. Amer. med. Ass., 163, 805.
2. Bassert, D. E., and Ferrington, E. (1957). A.M.A. Arch. Path., 63, 176.
3. Schofield, R. A., Elder, T. D., and Spoto, A. P. (1956). Fed. Proc., 15, 506.
4. Bauer, H., Ajello, L., Adams, E., and Useda Hernandez, D. (1955). Amer. J. Med., 18, 822.
5. Casagrandi, C. (1931). Riv. Biol., 13, 1.