1. Wolpert, L. Positional information and the spatial pattern of cellular differentiation. J. Theor. Biol. 25, 1–47 (1969).
2. Wolpert, L. Positional information revisited. Development 107, (Suppl.) 3–12 (1989).
3. Saunders, J. W. & Gasseling, M. T. in Epithelial–mesenchymal interactions (eds Fleischmeyer, R. & Billingham, R. E.) 78–97 (Williams & Wilkins, Baltimore, USA, 1968).
4. Tickle, C., Summerbell, D. & Wolpert, L. Positional signalling and specification of digits in chick limb morphogenesis. Nature 254, 199–202 (1975).
5. Saunders, J. W. in Limb and somite morphogenesis (eds Ede, D. A., Hinchliffe, J. R. & Balls, M.) 1–24 (Cambridge Univ. Press, Cambridge, UK, 1977).