The importance of cell position in relation to boundaries in development goes back to Hans Driesch and his experiments over 100 years ago in the sea urchin embryo. These experiments convinced him that the way a cell developed depended on its position in the embryo. Driesch believed that the embryo had a system of coordinates, like x and y axes, that specified the position of cells within the embryo in order to determine their behaviour. He was, however, equally convinced that it was not physically possible to have such a system and invoked a mystical concept, which he called 'entelechy', to explain the positioning of cells during development. The first evidence that cells do indeed receive position-specific information came from E. N. Browne,who showed in 1909 that the hypostome, the mouth region of the adult hydra,could induce a whole new axis when grafted into the body of another hydra. But...

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