ABSTRACT
The regenerative ability of hydra has been frequently demonstrated since Trembley (1744) first witnessed the regeneration of a hydranth from the proximal half of an excised animal. Under normal conditions, the hypostome and tentacles always regenerate from the distal cut end, and proximal structures, the peduncle and basal disc, from the proximal cut end. This pattern of regeneration reflects the rigid polarity of hydra. Normal polarity and regeneration can be altered, however, by several techniques. Various graft combinations induce distal structures from regions destined to produce peduncle (Wetzel, 1898; King, 1903; Browne, 1909; Koelitz, 1911; Mutz, 1930; Burnett, 1961). An interspecific graft between the peduncle region of Hydra oligactis and the distal end of Hydra pirardi can result in the production of tentacles at the proximal end of the peduncle, which normally would form a base, and the inversion of polarity (Burnett, 1961). An extract prepared from an homogenate of hydra can induce multiple head and tentacle formation and inversion of polarity in gastric annuli (Lesh & Burnett, 1964; Lentz, 1965a; Lesh & Burnett, 1966). It appears therefore that regions proximal to the hypostome and tentacles are not irreversibly determined, but susceptible to redirection under appropriate stimuli. Although distal structures have been produced from proximal regions, no report exists of peduncle and basal disc formation at the distal end of an excised hydra.
