 At one end of a line of cells one postulates a source—a cell that produces a chemical (which I shall call a morphogen) and maintains it at a constant level. At the other end the extreme cell acts as a sink: that is, it destroys the molecule, holding the concentration at that point to a fixed low level. The morphogen can diffuse from one cell to another along a line of cells. (Crick 1970: 420)