628 REGENERATION AND GROWTH 7 



and in other highly differentiated animals, e.g. in Anura (Bohn, 1904; Romeis^ 

 1915; Schmalhausen, 1925; Ltischer, 1946) and in spiders (Friedrich, 1906). 

 Starved annelids regenerate slowly (Morgulis, 1909; Liebmann, 1943). The less 

 highly differentiated Metazoa, Coelenterates and Planarians, which are able to 

 live on their own tissues to a remarkable degree, when fasting, can use the same 

 material for regeneration, which persists to an advanced stage of inanition. Some 

 cells and tissues are sacrificed to maintain and proliferate the more essential ones. 

 Well-fed a Planarian regenerates mainly by epimorphosis, but when starved, 

 mainly by morphallaxis (Chranova, 1938), — a fact which also throws light on 

 the nature and significance of morphallaxis itself Even animals as highly differ- 

 entiated as fishes are capable of living extensively on their muscle-tissues (Nardi, 

 1937) which, again, they can use for regeneration. Many of the lower Metazoa 

 regenerate from mouthless and even gutless fragments. 



Even in Planarians, however, prolonged fasting causes a progressive slowing 

 of regeneration (Child, 191 1; Sivickis, 1933-4; Wolsky, 1935; Brondsted, 1953, 

 1955) which, in Protozoa also, ceases before the stage of complete inanition 

 (Weisz, 1954). Some Protozoa, including Paramecium, are very dependent on food 

 (Tartar, 1954), possibly because their normal metabolic activity is so high. 



As opposed to starvation, brief fasting seems harmless or even beneficial to 

 regeneration, even in mammals. Previously fasted rats regenerate the liver better 

 than previously well-fed animals and in these rats continued fasting also is bene- 

 ficial (Rosenthal et al., 1951). Salamanders regenerate more rapidly after moder- 

 ate fasting (Morgan, 1906) and similarly the Oligochaete, Criodrilus (Janda, 1926), 

 Planarians (Morgan, 1901; Bardeen, 1901; Lillie, 1900; Sivickis, 1933) Coelenter- 

 ates (Beutler, 1926) and some Protozoa (Sokoloff, 1924; Chajfec, 1932). 



Animals generally have a poor appetite during the early stages of regeneration 

 (Bier, 191 7; Cuthbertson, 1946; Harkness, 1952; Needham, 1955, p. 210), at the 

 time when body-proteins are being mobilised and largely squandered as "nitro- 

 gen-flow". It is difficult to avoid the implication that excess protein is a sheer 

 embarrassment at this time. The magnitude of nitrogen-flow is proportional to the 

 protein-intake (Munro and Chalmers, 1945; Cuthbertson, 1946) and the negative 

 balance (nitrogen output — intake), even, may be greater (Grossman et al., 1954). 

 It seems very probable therefore that "flow"' and reduced intake of nitrogen, are 

 both beneficial. Anterior pituitary hormones will reduce N-flow but this does not 

 accelerate healing (Cuthbertson, 1954). In fact healing is retarded, whereas a low 

 level of anterior pituitary hormone increases flow, but improves healing (William- 

 son and Newman, 1954). 



The virtue of jettisoning excess protein may be to produce a more "youthful" condition 

 of metabolism (p. 596), essential for regenerative growth. Mildly starved Planaria do 

 become more youthful, metabolically as well as morphologically (Hyman, 19 19, 1920; 

 Abeloos, 1930) and it is probably significant that in old Planarians regeneration is improved 

 by fasting but not in young animals (Sivickis, 1933 Brondsted, 1955). In mammals, also, 

 fasting is more deleterious to young than to old individuals (Cuthbertson, 1944). Young 

 Clavellina (p. 589) regenerate by morphallaxis (Millot, 193 1, p. 135) and this contrast holds 

 also between fasted and fed Planarians. 



There is some indication (Crofts and Peters, 1945; Williamson and Fromm, 1952) that 

 nitrogen-flow reflects the destruction of most of the amino acids of proteins in order to pro- 



