I INTRODUCTION I39 



reproductive structures, intracellular as well as intercellular (fibers, membranes, 

 shells, etc.) that grow. A more restricted definition was proposed by Linzbach 

 (1955), according to whom growth is any change-in-size of an organism "which 

 goes along with an identical self-reproduction of structured nuclef)proteins". 

 Although there is little factual criticism of this definition, it will hardly be of help 

 for better delimitation in dubious cases such as those mentioned, and it introduces 

 a hypothetical element, in so far as changes in nucleoprotein content would have to 

 be determined in any special case. 



It is obvious that growth cannot be equated with cell multiplication although, 

 as a rule, it is effectuated by it. There is cell multiplication without growth, as for 

 example, in cleavage and gastrulation ; and growth without cell multiplication, 

 as in cell-constant animals and tissues (p. i66). 



Furthermore, if growth and differentiation are contrasted, this is possible in the 

 abstract but not in the concrete case. In a crude approximation, it may be said 

 that during the embryonic period of an animal the development of its organiza- 

 tion, the histological, physiological, and biochemical differentiation of organs, 

 tissues and cells take place; in post-embryonic life the organization thus established 

 increases in size. However, there is no sharp borderline. Embryonic development 

 also is a period of rapid increase in size, and processes of differentiation continue 

 over the post-embryonic life cycle. 



Thus, a definition of growth is meaningful only if it is clearly understood that 

 in general growth is not an isolable phenomenon but rather a certain aspect of the 

 process of life. A definition of growth can only be of an "operational" nature, and 

 its value is exactly equal to what it may offer for a specific problem of research. 

 Growth is an abstraction; only in this sense can one discuss foundations, theory 

 and laws of growth. 



{b) Various aspects of the problem of growth 



Every living organism represents a hierarchy of organized systems. Hence, the 

 problem of animal growth must be envisaged from different viewpoints and at 

 various levels. Consequently, what is termed "growth" from a certain viewpoint 

 need not be termed so from another viewpoint. 



For example, if the growth of a tumor is investigated, this naturally is different 

 from normal growth of the organism as a whole. The adipose tissue of an ageing 

 human "grows" but not the individual. In a starving larva or fish, growth in 

 length may continue while the body loses weight. A tissue standing in constant 

 cell renewal like the epidermis "grows" during the full lifespan of the organism, 

 but the adult organism in which this process takes place is in a steady state. 



Growth in the biological sense is intimately connected with metabolism and 

 with the hierarchic organization of living systems in self-reproducing units 

 (Linzbach, 1955). If the molecule of a chemical compound increases in size, it 

 changes into another chemical compound. An organism, however, represents a 

 system consisting of units (proteins, nucleoproteins, cells, etc.) which may increase 

 in number but remain of the same kind within their respective species. Only this 

 fact allows for growth as increase in size of a system which remains specifically 

 identical. 



Literature p. 233 



