1. Ratcliffe, F. N., Myers, K., Fennessy, B. V., and Calaby, J. H., Nature, 170, 7 (1952).
2. Ratcliffe, F. K., personal communication (1953).
3. Martin, C. J., Univ. of Cambridge Inst. An. Path. Fourth Report, 16 (1934–35).
4. Bull, L. B., and Mules, E. W., J. Coun. Sci. Indust. Res., 17, 1 (1944).
5. Fenner, F., Marshall, I. D., and Woodroofe, G. M., J. Hyg., in the press (1953).