D.C. Wathes1,2, S. E. F. Guldenaar1,2, R. W. Swann1,2, R. C. Webb1,2, D. G. Porter1,2, B. T. Pickering1,2
1Animal Breeding Research Organisation, Dryden Laboratory, Roslin, Midlothian, EH25 9PS, U.K.
2Department ofAnatomy, The Medical School, Bristol BS8 1TD, and