208 GROWTH PRINCIPLES AND THEORY 2 



In cases available at present, the prediction from theory was verified to a surprising extent. 

 Even discrepancies previously assumed, disappeared with improvement of experimental 

 technique. Thus it was noted (Bertalanffy, 1942a, p. i87f. ; 1951a, p. 239) that the value of 

 '''-day ~ 0-045 calculated froin the growth curve of the rat, disagreed with protein loss 

 calculated from minimum N excretion after Terroine (0.0028) ; but isotope experiments 

 (Table 11, p. 200) showed precise agreement. This obviously goes far beyond accidental 

 correspondence. 



However, protein turnover does not cover the total of degradative processes in 

 the organism. Not only are components like fat, bone, dentine, etc. in continuotis 

 renewal as shown by isotope experiments; but especially the factor of differenti- 

 ation has to be taken into account. 



Gross living mass may be subdivided into "metabolically active" and "inert 

 body mass" (Benedict, 1938), the latter represented by components which, 

 although they do not leave the body, more or less cease to be involved in turnover 

 and growth^. Ftirthermore, during development a transition from "active" to 

 "differentiated body mass" takes place which is concealed in the "constant" x and 

 will presumably be significant in animals with rapid development and growth. 



By its very structure, the growth model and equations derived therefrom refer 

 to the "metabolically active" (Benedict, 1938), "essetitial" (Dunn, Murphy and 

 Rockland, 1947), or "generative mass" (Weiss and Kavanau, 1957). However, 

 gross body weight also includes "inert" or "reducible" mass. This refers, first, to 

 the shift of weight increase from protein to fat as in ageing mammals (Mayer, 

 1949). Secondly, the transitioii from "generative" to "differentiated" body mass 

 is not reflected in the model. 



Unfortunately, there is at present no quantitative measure for progressive 

 differentiation. Possibly investigation of turnover (p. i47ff.) and mitotic rates 

 (p. 165) of tissues could offer a way to distinguish between "active" and "differ- 

 entiated mass" and to determine the transition from the first to the latter; but 

 no study in this line appears to have been made. 



(m) Growth model of Weiss and Kavanau 



A much more elaborate model of growth has recently been proposed by Weiss 

 and Kavanau (1957). The basic assumptions of this model are: 



1. "Living mass" has the ability to reproduce and, in the absence of restraining 

 factors, reproduces exponentially. 



2. A living system consists of two components, "generative mass" which 

 remains in reproductive activity, and "differentiated mass", derived from the 

 former and consisting of terminal products and other secondary derivatives that 

 do not possess the ability to reproduce. In development there is a gradual tran- 



1 Weiss (1955a) has stated this very lucidly: "We certainly would not collect secretions, 

 such as slime, urine, sweat and sebum, over a measured period and add them to the growth 

 record. Yet, we do customarily include the bulk of cartilage and bone and other connective 

 tissues, which consists of residues of cellular secretions, just like those other ones, but 

 incidentally deposited, instead of extruded, hence accruing to the measured mass. Thus 

 what we measure is related not so much to the process of production as to the accident of 

 the disposal of the products. If they persist, we count them; if they drop out, we miss them." 



