276 SENESCENCE AND REJUVENESCENCE 



a fixed quantity. Among the fishes, amphibia, and reptiles there 

 are, however, some forms in which growth apparently continues 

 during at least most of the life of the animal, although it is very 

 slow in later stages. Growth is apparently periodic rather than 

 continuous in all these cases, and its continuance throughout life 

 or up to a late stage is probably due to the fact that these animals 

 Per cent undergo partial rejuvenescence from time to 



time during periods of quiescence or star- 

 vation, a point which is discussed below 

 (pp. 299-300) . That the fundamental laws of 

 growth are essentially the same throughout 

 the organic world there is every reason to 

 believe. Everywhere apparently the rate of 

 growth is high in the young organism, or in 

 the young cells and tissues of the organism, 

 and decreases as development proceeds and 

 the rate of metabolism falls. With adequate 

 nutrition and under external conditions which 

 permit growth, the rate of growth appears to 

 be in a general way dependent upon the 



Days 38182838 55 77! io6 180 270 



FIG. in. Curve showing the decrease in rate of growth in male rabbits from 3 

 to 270 days after birth: each vertical interval indicated on the axis of ordinates repre- 

 sents i per cent increment in weight and each horizontal interval on the axis of abscissae 

 the length of time between successive weighings; during the first 38 days after birth 

 weighings were made every 5 days, after that at increasingly longer intervals. From 

 Minot's tables (Minot, '08). 



rate of metabolism. Discussion of the conception of growth as an 

 autocatalytic reaction which undergoes acceleration in rate to a 

 maximum is postponed to chap. xvi. 



NUTRITION, GROWTH, AND SENESCENCE 



The advance during the last ten years in our knowledge of the 

 chemical constitution of the proteid molecule, in which the work of 



