138 GROWTH PRINCIPLES AND THEORY 2 



of ''normality*' so introduced a scientifically inadmissible value judgment; for we 

 may define as "normal" that coordination of parts and processes in the organism 

 which guarantees its maintenance (BertalanfFy, 1946). This is not a subjective 

 point of view, but an objective criterion. However, in no way is this criterion easily 

 applicable. 



Protoplasrn is a notion whose extent cannot easily be delimited. For example, 

 a high content of water (about 80 percent) belongs to the living protoplasm. As 

 a rule, water content decreases during embryonic and post-embryonic develop- 

 ment. There are, however, instances to the contrary. Thus the increase in weight 

 of a toad's larva is essentially based upon water intake. At the time of hatching, 

 dry substance represents 42 percent of total weight; up to the 32nd day of larval 

 development, it decreases to less than 4 percent ; and only in metamorphosis does 

 it increase again to about 14 percent, with a concurrent loss of total weight 

 (Wurmbach, after Harms, 1955). The relation of these changes to hormonal 

 factors in amphibian metamorphosis (thyroxin) is obvious. Changes in water 

 content are considered to be generally characteristic for the development of 

 vertebrates (Linzbach, 1955; Harms, 1955). In the example mentioned, the 

 considerable intake of water is part of the "normar' life cycle of the toad larva, 

 and "biologically complete biomass" is formed. It depends solely on the viewpoint 

 taken whether total weight increase is considered as growth and hence water is 

 included ; or whether we are interested in growth as a process chemically defined, 

 say, as protein synthesis, in which case we have to exclude change in water 

 content and will choose protein content as standard. 



Similar considerations apply to the accumulation o{ depot material. It is plausible 

 not to consider the deposit of fat in a fattened swine as legitimate growth. But a 

 certain amount of depot materials, such as fat, glycogen, etc., and its proportion- 

 ate increase during growth is a prerequisite for normal function of the organism, 

 as it is indispensable for continuous activity with intermittent import of nutrition. 

 There is no possibility, however, to delimitate precisely the "normal"' amount of 

 such materials [cf. p. 222). Similarly, gross-weight increase passes from protein to 

 fat in the older rat, and in the ageing human fat also is deposited. These phenomena 

 are within the "normal" life cycle of the species concerned, and the biomass so 

 formed is as "functionally complete" as it can be in the age concerned. Again it 

 depends on our viewpoint whether we include or exclude such changes when we 

 speak of growth, depending upon whether the organism as a whole, certain 

 chemical mechanisms, or adipose tissue is the object of study. 



The difficulties of an exact delimitation of "biomass" are made particularly 

 clear by the results of isotope experiments. These show that depot materials such 

 as fat and seemingly inactive components such as bone are far from being stable 

 substances. Rather they participate in the continuous breaking down and regener- 

 ation within the organism, showing an amazingly high rate of turnover. 



Considerations of this kind make it difficult to give the definition of growth a 

 precise chemical meaning. There is no doubt that the growth of living systems is 

 essentially based upon units capable of self-reproduction which, on the basis of 

 present knowledge, can be defined as nucleoproteins . But it is hardly possible to 

 identify growth with the self-reproduction of these units since there are non- 



