1. Medawar, P. B. Some immunological and endocrinological problems raised by the evolution of viviparity in vertebrates. Symp. Soc. Exp. Biol. 7, 320–338 (1953). The seminal paper that introduced the concept of the fetus as an allograft.
2. Trowsdale, J. & Betz, A. G. Mother's little helpers: mechanisms of maternal-fetal tolerance. Nature Immunol. 7, 241–246 (2006).
3. Steven, D. H. (ed) Comparative Placentation (Academic Press, New York, 1975).
4. Mossman, H. W. Vertebrate fetal membranes. (Rutgers University Press, New Brunswick, 1987).
5. Wooding, F. B. P. & Flint, A. P. F. in Marshall's Physiology of Reproduction Vol. 3 Ch. 4 (ed. Lamming, G. E.) 230–466 (Chapman and Hall, London, 1994).