VII 



INTERNAL FACTORS 629 



vide necessary quantities of a few essential amino acids, in particular of methionine, since 

 an external supply of methionine reduces and even reverses the negative nitrogen-balance 

 of the flow-period. However this mobilisation of methionine or other amino acids can 

 scarcely be a critical requirement, since regeneration may be quite normal in fasting ani- 

 mals. The action of administered methionine may be more indirect, — for instance it tends 

 to reverse the negative nitrogen-balance due to thyroid hormone (Gaunt, 1954; p. 205) 

 {cf. also p. 616). 



In the R-phase food also may impose a strain on metabolism. The gut of starved 

 Planariansis less sensitive to KCN than that of well-fed animals (see Child, 1941, 

 p. 114) and its oxygen-requirement is lower. This must be an advantage in the 

 trend tow^ards more reducing, anaerobic conditions. Fasting is accompanied by 

 a decrease in oxygen-consumption (Hyman, 191 9, 1920). This tends to reduce 

 pH also and facilitates the breakdown of body-proteins, and the release of SH- 

 groups. 



Fasting is beneficial also for the assertion of new individuations in regenerating 

 Planarians (Child, 1941, p. 350) i.e. it has qualitative effects. 



In the P-phase, food-protein is beneficial (Clark, 191 9; Harvey and Howes, 

 1930; Anon., 1950; Calloway et al., 1955), and fasting retards cell-proliferation 

 and growth (Schmalhausen, 1925; Nardi, 1937). Protein is then used with great 

 economy (Needham, 1952), as in the metamorphosis of insects (J. Needham, 1942), 

 and prior fasting may improve this economy, for animals previously well-fed 

 show a more adverse nitrogen-balance during the R-phase if intake is then reduced 

 (Grossman et al., 1954); although they regenerate less well than fasting animals 

 under any conditions, this inferiority is further increased if then they are made to 

 fast (Rosenthal ei a/., 1951). 



The strange squandering of protein in the R-phase is probably not more puz- 

 zling than the high rate of excretion of indispensable vitamins and other substances 

 which occurs even in the normal mammal. 



Vitamins. The specific roles of dietary constituents other than the vitamins 

 have been considered incidentally, in Section V. Little attention has yet been 

 given to the action on regeneration of vitamins other than vitamin C, ascorbic 

 acid (Arey, 1936; Holmes, 1942), but it may be pointed out that the "main- 

 tenance" function for which they all seem essential, is a low-grade regeneration- 

 process. Acute regeneration, also, is promoted by all those water-soluble vitamins 

 which have been tested. 



Scurvy is the result of faulty maintenance-repair, through a deficiency of vita- 

 min C. Lind, himself, noticed that men with scurvy also healed wounds slowly 

 (Hunt, 1 941) and it has been demonstrated experimentally that ascorbic acid 

 is necessary for maintenance (Pirani and Levenson, 1953) as well as for the initial 

 regeneration (Bourne, 1942; Murray and Kodicek, 1949) of the connective tissues 

 in general. It improves regeneration of the mammalian cornea (Boyd, 1955), 

 which is mesenchymal in origin. Its main function is in the synthesis of the mu- 

 copolysaccharides of the ground-substance (Bradfield and Kodicek, 1951; Kent 

 and Whitehouse, 1955) and of collagen (Murray and Kodicek, 1949). Bone, a 

 derivative connective tissue, similarly requires this vitamin (Bourne, 1942; Mur- 

 ray and Kodicek, 1949). It is thought also to control the number of polymorphs 



Literature p. 643 



