VI EFFECT OF EXTRINSIC FACTORS 623 



A triggered response may give no direct information on the nature of the process. 

 The direct effect of an increase in temperature is to speed regeneration, as it 

 does most physiological processes, yet a rise in temperature causes the degeneration 

 and casting of hydranths in Tubularia, and a fall in temperature induces regenera- 

 tion (Moore, 1939; Berrill, 1948). 



A second generalisation with a paradoxical quality is that low dosage of an 

 inhibitory agent often accelerates regeneration, e.g. alcohol (Morgulis, 1909; 

 Bilski, 1926), colchicine (Liischer, 1946; Havas, 1949), nitrogen-mustards (Sup- 

 niewski, 1949) and radiations (Schaper, 1904; Bardeen and Baetjer, 1904; Loof- 

 bourow, 1948). In most cases the acceleration is an indirect response to the slight 

 damage caused, though both effects of alcohol are direct (p. 621); similarly a 

 progressive temperature-increase may directly speed the process at first and directly 

 inhibit ultimately through heat-denaturation of enzymes. 



[a) Temperature 



Acceleration of regeneration by warmth was noticed by Trembley and other 

 early workers {cf. Morgan, 1901, p. 26). The regeneration of Hydra required 

 96-168 h. at 12 °C and only 48 h. at 26 ''C (Peebles, 1898; see also Moore, 1910) ; 

 that of planarians 35 days at 5 ""C and 4 days at 26 "C (Wolff and Dubois, 1948b), 

 14 days at 10 "C and 8 days at 16 °C (Vandel, 1921). In a crocodile a certain area 

 of skin healed in 29 days at 23 °C and in 1 1 days at 38 °C (Du Notiy, 1936, p. 99). 

 Moore (19 10) found a sigmoid relationship between regeneration-rate and tem- 

 perature, but Spiegelman and Moog (1944) obtained a virtually linear curve 

 over the physiological range, and a constant temperature-coefficient. 



Tadpoles cease to regenerate the tail at a lower limiting temperature of 14 °C 

 (Ellis, 1909) considerably above the lower lethal temperature, but planarians 

 show some regeneration down to 3 ^C (Morgan, 1901). The upper limiting temper- 

 ature for regeneration in poikilotherms is between 30° and 35 ^C (Korschelt, 

 1927; p. 695) and the maximal rate is reached 4-5' below this. For mammals the 

 normal body temperature is optimal, and further increase depresses regeneration 

 (Hutchinson and Burdeaux, 1951). For some components of regeneration in 

 mammals a subnormal temperature may be beneficial (Green and Stoner, 1954; 

 Moss and Weiss, 1955), while during the defence-phase fever is beneficial (Men- 

 kin, 1950) through differential action on the metabolisms of host and invading 

 microorganisms. Pyrogenic polysaccharides accelerate regeneration in mammals 

 but possibly not through their pyrogenic action. 



Temperature-change may affect all components of regeneration proportion- 

 ately to their normal rate (Nardi, 1937; Weisz, 1948), but sometimes it acts differen- 

 tially (Gross and Huxley, 1935; Berrill and Mees, 1936), and the quality of the 

 regenerate is then affected. Cold increases the incidence of heteromorphic regener- 

 ates, for instance legs in place of antennae, in arthropods (Przibram, 1934; 

 Kenk, 1935; Villee, 1942), while heat increases the frequency of replacement of 

 halteres by wings in the normal ontogenesis of Diptera. Temperature-change may 

 also affect the response to other agents, for instance (p. 627) it reverses that to 

 lithium salts (Trampusch, 1951; Mifume, 1953). 



Literature p. 64g 



