VI GROWTH OF MAMMALS QIQ 



TABLE 14 



ABSOLUTE AND RELATIVE GROWTH RATES IN WEIGHT GROWTH 

 OF MALE WHITE MICE 



After Bertalanffy, 1938 



Bibliography cf. original. Second peak of growth rates is indicated by italics. 



Growth curves of mammals show segments or cycles. These are not arbitrary sub- 

 divisions as assumed by many authors to fit growth curves with a preconceived 

 formula. Rather these breaks in the growth curve are real and manifest in a 

 decrease and subsequent new increase of growth rates (Table 14). This is in 

 contrast to the monotonic decrease of specific growth rates in simpler growth 

 curves, e.g. offish (analysis of growth cycles in the mouse: Sailer, 1927, 1932; 

 mouse and rat: Bertalanffy, 1938). 



The break in the growth curve coincides with sexual maturation which further is ex- 

 pressed in many physiological changes. In the rat, all these changes, such as transition 

 from the first to the second growth cycle, the change in the dependence of basal and 

 resting metabolism on body size, the involution of the thymus, the changes in relative 

 growth and in Qoj of certain tissues, etc., occur at approximately 100 g body weight, 

 thus showing deep-reaching changes in metabolism connected with a shift of hormonal 

 balance. Naturally the sharp break assumed in these calculations is a mathematical 

 simplification of a process which, in reality, is uninterrupted. 



{b) Growth and hormones 



The characteristic course of growth in mammals is regulated by hormones 

 {cf Gaunt, 1954; Wolff' in the present volume). Only a few remarks relevant to 

 the present discussion will be made. 



The growth hormone of the hypophysis (somatotropin) is the most important 

 stimulator of growth in mammals. Thyroxin, with its effect of increasing metabolic 

 rate, is essential for normal growth and development. The androgenic steroid 

 hormones are, to an extent, growth stimulants in male but also female mammals, 

 even though their effect may be lacking or reversed in certain doses and conditions. 

 The glucocorticosteroids (cortisone), controlled by ACTH, have a growth- 

 inhibiting effect. 



l.ileralure p. 253 



