iy8 GROWTH PRINCIPLES AND THEORY 2 



To arrive at a rational theory of growth, we can start with the following 

 considerations. 



1. Every living organism and living system in general is an open system, main- 

 taining itself in continuous import and export, building up and breaking down 

 of component materials. This is evidenced by the fact that metabolism is a basic 

 characteristic of living systems and, more particularly, by isotope experiments 

 which dramatically show that the turnover of the building materials of the 

 animal body takes place at a rate hardly expected in classical physiology (p. i47f ). 



2. Growth, that is, the increase of body mass in time, is not unlimited. As a 

 general rule, there is first a rapid increase which gradually slows down until the 

 organism reaches a steady state or is "adult". The limitation of growth is not, 

 in principle, due to a decline of the growth potency (whatever this may be) of the 

 component cells and tissues; rather it is regulated by factors lying in the organism as a 

 whole. Witness the fact that cells which have stopped growth and multiplication 

 in the adult organism may resume it if the restrictions imposed are removed, as 

 in regeneration, compensatory hyperplasia, tissue culture, etc. 



J. What is measured as "growth" is the outcome of processes of immeasurable 

 complexity, whether envisaged from the biochemical, cytological, physiological, 

 or morphological viewpoints. Not only the mechanisms of synthesis of proteins, 

 nucleoproteins, and other components, as well as the physicochemical basis of 

 mitosis are unknown; even if we knew, the individual chemical and physico- 

 chemical events would not give a clue as to how myriads of processes are interacting 

 in the organism in a way surpassing possible analysis. Furthermore, the growing 

 organism undergoes changes in many respects, such as chemical composition, 

 content of water, proteins and other compounds, the ratio between protein and 

 fat, changes of the shape of the body, different relative growth of organs, pro- 

 gressive differentiation of tissues, influence of hormones and consequent physio- 

 logical changes, and many others. 



4. Notwithstanding this complexity, processes in the organism as a whole can 

 be expressed by relatively simple quantitative expressions. As a rule, physio- 

 logical processes obey the allometric equation (p. 232f ) , i.e. their rate can be expressed 

 as a power function of body weight. The allometric equation was found to apply 

 to all physiological processes hitherto investigated, at least as a first approxi- 

 mation, and it is unlikely that others follow a radically different type of equation; 

 for if this would be the case, they would be incompatible with those where the 

 allometric equation holds true (Adolph, 1949). This applies even when the 

 phenomenon under consideration is the gross result of many unknown component 

 processes. For example, the rate of metabolism follows the allometric equation, 

 often in the simple form of the surface rule (p. 181), although it is the outcome of 

 innumerable and largely unknown processes of intermediary metabolism, and 

 the growing organism to which such relation applies undergoes many changes at 

 the biochemical, physiological, cellular, and morphological levels. The same 

 applies to any number of physiological processes. 



5. Animal growth can be considered the result of a counteraction of processes of 

 anabolism and catabolism, degradation and regeneration of the building materials 

 of the body. There will be growth so long as building up prevails over breaking 



