I INTRODUCTION 141 



To the first category belong genetic factors, hormones, processes of ageing and 

 differentiation, etc.; to the second, nutrition, vitamins, temperature, life space, 

 density of population, etc. A detailed discussion of the factors influencing growth 

 would widely surpass this presentation. Some of them, however, will be considered 

 at the adequate place. 



(c) Growth and differentiation 



There is an apparent antithesis between growth and difTerentiation of the 

 organism. Embryonic and undifferentiated cells, as e.g. mesenchymal elements, 

 as a rule manifest the largest growth capacity, while highly differentiated elements 

 such as nerve and muscle cells are incapable of division in post-embryonic life. 

 Similarly, it is a general rule, although with many exceptions, that regenerative 

 potencies decrease with progressive differentiation and evolutionary progress. 

 According to an often-stated theory (Minot and others), progressive differentia- 

 tion sets a limit to cell division and hence to growth: Undifferentiated protoplasm 

 grows at a constant rate, but difTerentiation continually diminishes its amount, 

 and so leads to a progressive decrease of growth rate (Schmalhausen, 1931). 



Notwithstanding the general antagonism which, as a rule, exists between growth 

 and differentiation, this viewpoint is evidently incorrect (Bertalanffy, 1951a; 

 Linzbach, 1955). Many undifTerentiated cells in the organism do not multiply 

 although they have the ability to reproduce. Such is the case, e.g. in a hydra or 

 planarian whose undifferentiated cells do not divide but immediately start to do 

 so in regeneration after an injury. The same is true of many vertebrate tissus, 

 cells of which do not divide in the adult organism bvit do divide in tissue culture 

 after explantation. Thus the growth of a tissue or organ does not simply vary with 

 the ratio between reproductive and non-reproductive cells. Conversely, the degree 

 of difTerentiation of a cell is no measure of its ability to grow and divide. Cells of 

 cell-constant animals or organs can show a vast increase of size without division 

 {cf. p. 166 and Fig. 4, p. 160). The cells of the liver are rather highly specialized 

 and do not divide in normal life. However, after extirpation of the larger part of 

 the rat liver compensatory hyperplasia takes place with a growth rate surpassing 

 that of malignant tumors. The same is shown by quantitative considerations. For 

 example, the heart is a highly differentiated and probably cell-constant muscle, 

 but it retains a high allometry constant (a = 0.95, cf. Fig. 4) from early develop- 

 mental stages to maturity. The highly differentiated skeletal musculature of man 

 grows faster than the rest of the body. Heart and liver are post-mitotic tissues 

 [cf. p. 163), but they show higher relative growth than the spleen which remains 

 mitotic during the whole life. Examples of this kind which can easily be multiplied, 

 demonstrate that notwithstanding the general antagonism between growth and 

 differentiation, the latter by itself is not a causative factor entailing the dis- 

 appearance of the ability to grow and reproduce in the cells, or the cessation of 

 growth in the organism as a whole. 



{d) Limitations of growth 



The size limit of an organism is determined genetically and, at least in verte- 

 brates and insects, by way of gene-dependent hormones. 



Literature p. 253 



