1. Pak, W. L., Grossfield, J., and White, N. V., Nature, 222, 351 (1969).
2. Hotta, Y., and Benzer, S., Nature, 222, 354 (1969).
3. Cosens, D. J., and Manning, A., Nature, 224, 285 (1969).
4. Heisenberg, M., J. Exp. Biol., 55, 85 (1971).
5. Pak, W. L., in Handbook of Genetics (edit. by King, R. C.) (Van Nostrand-Reinhold, New York, in the press).