1. Day, M. F., Nature, 145, 264 (1940).
2. Fraser, A., Quart. J. Micro. Sci., 100, 395 (1959).
3. Geldiay, S., Biol. Bull., 117, (2), 267 (1959).
4. Panov, A. A., Dokl. Akad. Nauk USSR, 145, 1409 (1962).
5. Highnam, K. C., Quart. J. Micro. Sci., 102, 27 (1961).