6o6 REGENERATION AND GROWTH 7 



replacement-rate, i.e. the rate per arm, increased with the number removed. 

 As he pointed out, here the extent of injury as well as the amount of tissue removed 

 increased in that order, whereas it increases little with increasing amount removed 

 within one appendage. There is a mutual stimulation by the various wounds 

 and blastemata. Between arthropod limb-regenerates this effect is smaller (Zeleny, 

 I.e.; von Ubisch, 1915; Needham, 1946b) though probably significant (Przibram, 

 191 7; Huxley, 1932). The curve of rate/number of organs removed again is 

 sigmoid, i.e. sensitivity is maximal for an average degree of loss. 



The regeneration-rate of different organs varies considerably, in a pattern 

 which probably has biological significance (Needham, 1952). Even within one 

 organ (Needham, 1947a; Liischer, 1948) the detailed pattern, superimposed on 

 the main gradient considered above, may be complex. Sharp discontinuities are 

 evident in the gradient in the body of some Planarians (Brondsted, 1955) and 

 other animals. Such sharply defined changes must depend on local intrinsic differ- 

 ences. 



Age is one of the most important of natural factors affecting regeneration-rate, 

 though there are few detailed quantitative studies of this (Zeleny, 1909; Przibram, 

 1909; Abeloos, 1930; Curtis and Schultze, 1934; Du Noiiy, 1936; Needham, 

 1949a). Zeleny 's conclusions generally have been confirmed; most animals regener- 

 ate absolutely faster as they grow older and larger, but relatively more slowly, 

 per unit of body-size, and per unit of tissue removed, just as for an increasing 

 amount of loss in an individual of fixed age. It follows, therefore, that a larger 

 animal replaces a loss of a particular size more rapidly than a smaller individual, 

 and Anderson (1933) found this also for a two-dimensional wound in the carapace 

 oi Daphnia. Zeleny decided that even the relative rate of regeneration of the chela 

 of the crab, Portiimis, increases with age, but this was based on his conclusion that 

 the duration of the stadium did not change with age, whereas in fact his records, 

 when plotted graphically, or averaged, indicate a progressive lengthening of the 

 stadium from eleven to twenty days; the rate per day, per unit of body-size, 

 decreases with age as in other animals. 



In the limbs oi Asellus (Needham, 1949a) the absolute regeneration-rate per 

 day at first increases with body-size (age) but it is then constant over a considerable 

 range of bodysize, — probably incidental evidence that regenerative growth is 

 in arithmetic rather than in geometric progression (p. 603). The specific regener- 

 ation-rate, per unit of body size, therefore declines progressively with age (Fig. 3), 

 following a curve similar to that of Du Noiiy's index of cicatrisation, as a function 

 of amount lost. Like the specific rate of normal growth, the rate of decline itself 

 decreases progressively (Medawar, 1945). Both the initial rate of decline, and the 

 rate of slowing of this decline, are relatively greater for body-growth (Fig. 3) than 

 for regeneration. 



It follows that older individuals take longer than younger ones to regenerate 

 the same percentage-loss. In the tail of frog-tadpoles (Ellis, 1909) usually less than 

 100% of the loss is ultimately regenerated, and this percentage, again, declines 

 with age. The deficiency may be peculiar to larval Anura which cease to regener- 

 ate appendages at metamorphosis, but other indices of regenerative power also 

 decline with age. In Planarians the frequency of successful head-regeneration so 



