1. Medical Research Council Human Reproductive Sciences Unit (K.A.L.T., M.W., R.M.S., P.T.K.S.), Centre for Reproductive Biology, University of Edinburgh, Edinburgh EH16 4SB, Scotland, United Kingdom
2. Department of Developmental Biology (K.D.G., E.D., G.V.), Laboratory for Experimental Medicine and Endocrinology, Catholic University of Leuven, B-3000 Leuven, Belgium
3. Institute of Experimental Morphology and Anthropology (N.A.), Bulgarian Academy of Science, 1113 Sofia, Bulgaria