Regeneration and growth control in Nereis. II. An axial gradient in growth potentiality.

Caudal regeneration in Nereis diversicolor is that rapid proliferation of segments which occurs after loss of part of the posterior of the body and the reformation of the pygidium. The rate of segment proliferation is highest during the production of the first new segments. Thereafter, the rate declines rapidly with each passing week, though if the regenerated segments are amputated, rapid proliferation is again resumed (Golding, 1967). Two explanations of these results are possible. First, the declining rate of segment proliferation during regeneration may be the result of a declining concentration of hormone in the body fluids. There may be a feed-back from the regenerating tail to the ganglion. As segments are proliferated, the secretory activity of the ganglion is suppressed. Secondly, the level of hormone in the body fluids may remain constant, the proliferating region of the pygidium becoming progressively more insensitive as regeneration proceeds. It has also been shown that the rate of proliferation is correlated, not only with the stage reached in the regenerative process, but also with the number of segments amputated. The greater the number of segments lost, the faster the rate of segment proliferation which ensues. This is so with animals with ganglia in situ, and with those whose ganglia have been replaced by ganglia removed from intact donors. The same two explanations proposed for the initially high but rapidly declining rate of segment proliferation during regeneration are possible with respect to the correlation between the number of segments lost and the amount of ensuing regeneration. The first alternative is that removal of a large number of segments results in a high concentration of hormone in the body fluids, whilst the second is that the proliferating region so formed may be more sensitive to a given level of hormone than that formed after the loss of fewer segments.