1. Dingle, J. H., Rammelkamp, C. H., and Wannamaker, L. W. (1953). Lancet, 1, 736.
2. Hadley, P., Delves, E., and Klimek, J. (1931). J. infect. Dis., 48, 1.
3. Hayward, N. J., and Miles, A. A. (1943). Lancer, 2, 116.
4. Jackson, D. M., Lowbury. E. J. L., and Topley, E. (1951). Ibid., 2, 705.
5. Lcwbury, E. J. L., and Cason, J. S. (1954). Brit. med. J., 2, 914.