CONCLUSIONS FROM EXPERIMENTS 185 



constitute metabolism with a colloid substratum produced by the 

 reactions. 



The development of metabolic mechanisms, such as the striated 

 muscles, which are capable when stimulated of a very high rate of 

 metaboKsm, is in no sense an exception to or a contradiction of the 

 general law that a decrease in rate of metabolism is associated with 

 development. In the early stages of development correlative 

 functional stimulation of the cells of the organism certainly occurs 

 only to a very shght degree, so far as it occurs at all, and cannot be 

 compared to the degree of functional stimulation which occurs 

 in later stages after development of the stimulating mechanism — 

 in the case of striated muscle, the nervous system. This being the 

 case, we must compare the rate of metabolism in the unstimulated 

 or very sUghtly stimulated differentiated cell — not the rate of the 

 cell under strong stimulation — -with the rate of the embryonic cell, 

 if we are to attain a correct conception of the difference. Bearing 

 this point in mind, it is easy to see how great the difference in 

 rate is. In the case of striated muscle, for example, the rate of 

 metaboHsm in the earUer stages of development is siifiiciently 

 high to bring about the morphogenesis of the muscle without the 

 accelerating influence of nerve impulses, but later the muscle 

 atrophies unless its rate is frequently accelerated by nervous 

 stimulation. 



From this point of view senescence in its dynamic aspect con- 

 sists in a decrease in the rate of metabolism determined by the 

 changes in the substratum during development, and, in its morpho- 

 logical aspect, in the changes themselves. The idea that senescence 

 is in one way or another simply an aspect or result of development 

 itself has been more or less clearly expressed by various authors, 

 and various features of the developmental process have been re- 

 garded as the essential factors,' but discussion of the different 

 theories is postponed to a later chapter. 



Attention has already been called to the fact that growth may 

 give place to reduction and progressive development to regressive. 



' Among more recent writers who have advanced this view in one form or another 

 are the following: Cholodkowsky, '82; Enriques, '07, '09; Jickeli, '02; Kassowitz, 

 '99; Minot, '08, '13, and several papers of earlier date; Miihlmann, '00, '10. 



