1. Godwin, H., Nature, 174, 868 (1954).
2. Levi, H., Nature, 176, 727 (1955).
3. Johnson, F., Arnold, J. R., and Flint, R. F., Science, 125, 240 (1957).
4. Vries, Hl. de, Proc. Kon. Ned. Akad. van Wetenschappen, B, 61 (2), 1 (1958).
5. Vries, Hl. de, Vries, A. E. de, and Harris, A., Science, 128, 472 (1958).