Lewis Wolpert is Emeritus Professor of Biology as Applied to Medicine, in the Department of Anatomy and Developmental Biology, University College London, London, UK. He is the author of The Triumph of the Embryo, A Passion for Science, The Unnatural Nature of Science, and Six Impossible Things Before BreakfastCheryll Tickle is Emeritus Professor in the Department of Biology and Biochemistry, University of Bath, Bath, UKAlfonso Martinez Arias is Professor of Developmental Mechanics at the University of Cambridge, UK