1. Biologie et Génétique du Développement, CNRS UPR 41, Université de Rennes I, Campus de Beaulieu, 35042 Rennes cedex, France; Cell Cycle Group, Laboratory of Electron Microscopy, A.N. Belozersky Institute, Moscow State University, 119899, Moscow, Russia; and Department of Genetics and Development, School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom