1. Ashby, W. (1919). J. exp. Med., 29, 267.
2. Berlin, R. (1951). Acta. med. scand., Suppl.
3. Brown, G. M. (1950). Canad. med. Ass. J., 62, 472.
4. Hayward, 0. C., Powell, E. 0. and Witts, L. J. (1944). J. Path Bact., 56, 81.
5. Callender, S. T., Powell, E. 0. and Witts, L. J. (1945). Ibid., 57, 129.