644 REGENERATION AND GROWTH 7 



and Liischer and Von Muralt (1947), is a "homotropic" neural factor with a small, water- 

 soluble molecule whereas the "neurocletin" of Hoffman (1950) and the factor of Edds 

 (1953) are fat-soluble, probably originating from the lysis of myelin. Nucleoprotein, also, 

 stimulates the sprouting of nerves (Cohen et al., 1954), and these investigations clearly 

 are still at an early stage. 



(e) The vascular system and regeneration 



Much of the early work on regeneration was by embryologists not accustomed 

 to considering this system, but it is evident that the uses of the system have been 

 fully exploited in adult morphogenetic processes. Completely devascularised 

 tissues die, and partial devascularisation is compensated by new collateral vessels 

 (Engley et al., 1955) and probably also by other means, so that specific experimen- 

 tal work in general has indicated a minor role for the vascular supply to a surviv- 

 ing regenerate (Korschelt, 1927, p. 652; Janda, 1931 ; Wyburn- Mason, 1950; Goss, 

 1954) though experimental restriction of agents known to be distributed by the 

 blood stream shows clearly that it is important. Restriction of the blood-supply in 

 fact has been shown to retard regeneration in a number of cases: the oligochaetes 

 Rhvnchelmis {Janda., 1931) and Criodriliis (Zhinkin, 1936), some Amphibia (Stone, 

 1944) and some mammals (Bowes, 1943). Reciprocally haemorrhage retards in 

 Rhynchelmis (Janda, 193 1), and probably in the isopod, Asellus (Needham, 1953). 

 The membranes which form across the base of regenerates (p. 595) allow the 

 passage of vascular and nerve-supplies. 



The vascular system is very important for the control of many processes of 

 the defence- and demolition-period (Menkin, 1950), though inflammatory re- 

 actions probably do occur in avascular animals, and continue in isolated tissues, 

 in vitro (Cooke et al., 1953). It brings nutrients and removes unwanted products. 

 Cells for blastema-formation may travel this way, and injected cells are selectively 

 taken up by the appropriate tissue (Andres and Weiss, 1952), though the neoblasts 

 of oligochaetes are said to migrate alongside the nerve cord. It serves a mechanical 

 function in the growth of the regenerate of Arthropods. 



Many of these functions, the distribution of nutrients (Hammett and Chapman, 

 1938) and of cells, and the mechanical function (Hauschka, 1946; Berrill, 1948) 

 are performed by the "gastrovascular" system in Coelenterates. This system also 

 distributes the locally produced inhibitor-factor. 



In the vascular metazoa the locally produced controlling factors travel in the 

 blood stream and various active hormones throughout regeneration must be 

 carried this way. Compensatory hypertrophy of some organs is induced by cir- 

 culating, simpler metabolites, which accumulate and increase the "load" on the 

 organs (Hartmann, 1933; Bollmann and Mann, 1935). These metabolites are usu- 

 ally substances used by the organ, and the accumulation is due to loss of part of 

 the organ, so that the response is self-regulatory. For kidney-hypertrophy urea is 

 the most potent factor, and for other organs correspondingly critical "raw materi- 

 als". Size-adjustment of internally secreting organs may be controlled by the prod- 

 ucts of their activity rather than by the raw materials; those of many of the 

 endocrine organs of vertebrates act on the pituitary gland, affecting its output 

 of hormones, which act back on the "peripheral" organs (Gaunt, 1954), a two- 

 stage control, therefore. 



