There is evidence which suggests that the polarity of regeneration in hydra is determined by axial gradients of some sort. The mechanisms which may be involved in the establishment and maintenance of the gradients have been investigated by studying the reversal of polarity in graft combinations.

Complete polarity reversal can be effected by a grafted hypostome or by a grafted hypostome and peduncle. Partial polarity reversal can be effected by a graft of a peduncle only. Changes in regional properties associated with polarity changes have been investigated using isolation and transplantation techniques.

The experimental results suggest that the axial gradient behaves as a gradient of a substance. Such a gradient could be produced by either (a) simple diffusion of a substance from a source or (b) unidirectional transport of a substance plus back-diffusion. Some of the experimental results are incompatible with mechanism (a). All the experimental results are compatible with mechanism (b).

Some of the problems raised by the interpretation of the axial gradient in terms of a polarized transport model are briefly discussed.

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