1. Medical Research Council Reproductive Biology Unit, Center for Reproductive Biology (N.A., C.M., M.W., K.J.T., J.S.F., M.M., M.R.M., R.M.S.), Edinburgh, Scotland EH3 9ET
2. The Institute of Experimental Morphology and Anthropology, Bulgarian Academy of Science (N.A.), 1113 Sofia, Bulgaria
3. The School of Biological and Molecular Sciences, Oxford Brookes University (N.P.G.), Headington, Oxford, United Kingdom OX3 0BP