THE VARIATIONS DURING LIFE 



the pupal development. The result means, therefore, that no such 

 optimum exists. According to unpublished experiments by the writer 

 this appears to hold also for the embryonic development of the eggs 

 of an insect (A dims sulcatus). 



The review of the field of embryonic and pupal development shows 

 that so far the data for distinguishing between the energy required for 

 tissue building (or tissue disintegration) and the simple basal meta- 

 bolism of the tissues existing at any given moment are insufficient. 

 The available evidence points rather to the conclusion that the ex- 

 penditure of energy required for tissue formation is quite small. 



The Respiratory Exchange During Growth and Senile Decay. 



Comparative experiments on the standard metabolism at different 

 ages have been made on man by Magnus-Levy and Falk [ 1 899]. The 

 older experiments of Tigerstedt and Sondn [1895] were also made 

 on man, but standard conditions were not rigorously observed. The 

 general results of Tigerstedt-Sonden's experiments were very similar 

 to those of Magnus-Levy and Falk. 



Calculated per unit weight (kilogram) the metabolism of growing 

 individuals is much larger than of adults, and in old age there is a 

 distinct decrease. As will be shown in detail in the next chapter 

 adult individuals belonging to the same species, but of different size, 

 have a larger gas exchange per kilogram the smaller their size, while 

 it has been shown that when the metabolism is calculated per unit 

 surface (square metre) about the same figure is obtained for all sizes. 

 A comparison between young and adult human beings must, therefore, 

 be instituted on the basis of surface instead of weight, as there are 

 large differences in size. When this is done Magnus-Levy and Falk 

 find that the differences, though of course diminished, are not abolished 

 as the following table (XXV) shows : 



TABLE XXV. STANDARD METABOLISM OF MALES OF DIFFERENT AGE. 



Magnus-Levy and Falk have also made experiments on three sub- 



8 



