1. D. H. Elliott, Biol. Rev.40, 392 (1965).
2. J. Diamant, A. Keller, E. Baer, M. Litt andR. G. C. Arridge, Proc. R. Soc. B180, 293 (1972).
3. Support of the U.S. National Institutes of Health, under grant No. T01 HD00304-01, is gratefully acknowledged.