a | This is a painting by John Piper that was used as the frontispiece for Waddington’s book Organisers and Genes. In the picture, which is intended to represent the epigenetic landscape, the developmental pathways that could be taken by each cell of the embryo are metaphorically represented by the path taken by water as it flows down the valleys. The water is supposed to be flowing away from the viewer, towards the sea in the distance. But the bifurcations of the valleys look so unnatural that the flow of water actually appears to be towards the viewer. b | A later depiction of the epigenetic landscape. The ball represents a cell, and the bifurcating system of valleys represents the ‘chreodes’ or bundles of trajectories in state space. c | A rare view behind the scenes of Waddington’s landscape. Each valley in the landscape is formed by tension on guy ropes that are attached to complexes of ‘genes’, represented as pegs stuck in the ground. Panel a reproduced with permission from the frontispiece of Ref. 12 © (1940) Cambridge University Press; panels b,c reproduced with permission from Ref. 13© (1957) Geo Allen & Unwin.”

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