590 REGENERATION AND GROWTH 7 



(1927), Millot (1931), Abeloos (1932), A.E. Needham (1952). The subject is considered in 

 relation to morphogenesis in general by Huxley and De Beer (1934), Weiss (1939), Child 

 (1941), J. Needham (1942), Thompson (1942), Brachet (1950), Raven (1954) and Willier 

 et al. (1955)- Regeneration in specific groups of animals has been considered in the books of 

 Summers and Calkins (1941), Stephenson (1930), Wigglesworth (1950), Roeder (1953), 

 Hyman (1940, 195 1), Kiikenthal and Krumbach (1926 onwards), and Graisse (1948 on- 

 wards) . Recent symposia on the subject include those in the American Naturalist ( 1 940, 7^) , 

 Biological Symposia (J. Cattell). See also Bespoloe Razmnojenie i Regeneratsiya (M. A. Voronzova 

 and L. D. Liosner, 1957, Moscow, Gosudarstvennoe Izdatelstvo). 



II. THE GENERAL NATURE OF REGENERATION 



(a) The distribution of powers of regeneration among animals 



Some of the philosophical problems arising from this distribution (Needham, 

 1952) are not directly relevant here but it is important that powers of regeneration 

 are very widespread and are generally related to powers of normal growth. Among 

 Arthropods many Crustacea effectively grow throughout life (Needham, 1950b) 

 and regenerate well their limbs, whereas the Copepoda have "determinate" 

 growth and no power of regeneration in the adult. This is also true of insects, 

 which often regenerate well as growing larvae. The Rotifera and Nematoda have 

 determinate growth and poor powers of regeneration. At the other extreme are 

 many Protozoa, Porifera, Coelenterata, Platyhelmia, Nemertinea and Annelida, 

 which grow virtually indefinitely as individuals or through repeated fissions or 

 other processes of asexual reproduction. These are able to regenerate a complete 

 animal from almost any fragment larger than a critical size. A quantitative meas- 

 ure of their powers of regeneration is given by this minimal size of fragment 

 (Table i). Many Echinodermata and Tunicata also have extensive powers of 

 regenerating the whole body. These are all animals of simple organisation, and 

 in general increasing differentiation of tissues, whether in phylogenesis or ontogen- 

 esis, is correlated with a decreasing power of growth and of regeneration. Mere 

 increase in body-size is probably one aspect of differentiation and large oligo- 

 chaetes and urodeles regenerate less well than species having smaller individuals. 

 The Mollusca and the Vertebrata Craniata in general are large animals, highly 

 differentiated also in other ways, and their powers of regenerating external organs 

 are more limited. Their internal organs, however, retain remarkable powers. The 

 rat can regenerate 2/3 of its liver in a few days and although the adult Anuran 

 Amphibia can no longer regenerate limbs, a transected gut rejoins and heals 

 rapidly, even if the cut ends are divaricated. Even if one end is put out of reach in 

 an external fistula, the other end opens into the side of the viscus, and reestabhshes 

 continuity in twelve days or so (Goodchild, 1955). 



There are apparent exceptions to the general rule that powers of regeneration have de- 

 creased progressively during evolution. For instance a number of Urodele Amphibia have far 

 better powers of regeneration than those fishes which have been investigated. In this respect, 

 as in their return to a more fully aquatic mode of life, the Urodela generally may show 

 regressive evolution. 



These generalisations are still based on observations form relatively few species, 

 and they may hide considerable variation at lower taxonomic levels. Leeches, in 

 contrast to most Annelida have virtually no power of regeneration. Some plana- 



