Many systems having a linear pattern of spatial differentiation are capable of regulating this pattern during development or regeneration. This is very evident in the early development of the sea-urchin embryo, the development of cellular slime moulds, and in the regeneration of hydroids, planarians and vertebrate limbs. Hydra is a classic example of such a system in that removal or isolation of almost any region leads to the reconstitution of the original pattern. As a result of earlier studies on such systems attempts were made to provide a general explanation in terms of axial gradients, apical dominance, and polarity (Huxley & de Beer, 1934; Child, 1941). Work since this time has not invalidated in any striking manner these concepts, but has rather tended to extend them (e.g. Rose, 1952, 1957). In some ways the concepts have tended to remain descriptive rather than explanatory (Webster & Wolpert, 1966).

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