1. Department of Human Anatomy and Cell Biology, University of Liverpool, P. O. Box 147, Liverpool L69 3BX, U.K.
2. M.R.C. National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K.
3. Department of Anatomy and Developmental Biology, University College London, University Street, London WC1E 6BT, U.K.