1. Institute of Biophysics, Czech Academy of Sciences, Kralovopolska 135, 612 65 Brno, Czech Republic (B.V.)
2. Reproduction and Developmental Biology, Ecole Normale Superieure de Lyon, 46 Allée d'Italie, 69364 Lyon cedex 07, France (I.N., N.B., S.G., F.M.); and