1. Waddington, C. H. The Epigenetics of Birds (Cambridge Univ. Press, Cambridge, 1952).
2. Hornbruch, A. & Wolpert, L. J. Embryol. exp. Morph. 94, 257–265 (1986).
3. Wagner, M., Thaller, C. Jessell, T. & Eichele, G. Nature 345, 819–822 (1990).
4. Tickle, C. et al. Nature 296, 564–566 (1982).
5. Summerbell, D. J. Embryol. exp. Morph. 78, 269–289 (1983).