All animals do not display the same capacities for repairing injuries, i.e. they possess what may be termed different ‘regeneration potentials’. One parameter influencing the regeneration potential manifested by an organism is its developmental history, or ontogeny. Early in an animal's ontogeny many of its cells are still undergoing both structural and biochemical modifications. As the animal matures, its cells become increasingly differentiated, ultimately attaining a high degree of specialization (Hay, 1966; Balinsky, 1970).
Numerous studies support the contention that the younger an animal ontogenically, the greater is its regeneration potential. Investigations using amphibians have presented several cogent examples of this generalization (Balinsky, 1970). Analogous situations are also evident in the Cnidaria. In the hydrozoan, Hydra, the tentacles contain what are among the ontogenically ‘oldest’ cells in the organism (Tripp, 1928; Brien & Reniers-Decoen, 1949; Burnett, 1966; Campbell, 1967b). Unlike the remainder of the organism, these structures also reportedly possess no regeneration potential (Peebles, 1897; Kanaev, 1952).
Three explanations have been proposed to account for the absence of regeneration potential in isolated tentacles. Nussbaum (cited in Kanaev, 1952) suggested that the absence of interstitial cells in the tentacles precluded regeneration. However, other regions of the animal, when experimentally deprived of this cell type, retained their regenerative capacities (Brien, 1953; Diehl & Burnett, 1964).
A second explanation invoked the size of an isolated tentacle as a factor limiting regeneration. In disputing this suggestion, Peebles (1897) measured tentacle volume. Isolated Hydra grisea (Pallas) tentacles ranged from 0.01 to to 0.03 mm3, while the volume of the smallest body fragment capable of total regeneration was considerably smaller, 0.006 to 0.01 mm3. From these measurements she concluded that the size of isolated tentacles could not account for the inability to regenerate.