1. Present address: University of Utah School of Medicine, Department of Neurobiology and Anatomy, Salt Lake City, UT 84132-3401, USA
2. Present address: University of Illinois at Urbana-Champaign, Department of Cell and Structural Biology, Urbana-Champaign, IL 61801, USA
3. Carnegie Institution of Washington, Department of Embryology, Baltimore,MD 21210, USA