1. Institut Max von Laue-Paul Langevin, B.P. 156, F-38042 Grenoble, France; European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble, France; Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, U.K.; and Institute for Science and Technology in Medicine, Keele University Medical School, ST4 7QB, U.K.