60 EDWIN G. CONKLIN 
according to Popoff is so weakened that it is unable to assimilate 
nutriment, and consequently can only store up food instead of 
making protoplasm, does as a matter of fact form protoplasm 
throughout the whole of the growth period. 
So far as they go, therefore, these results do not support the 
view that senescence is due to either an increase or to a decrease 
of nuclear volume as compared with that of the protoplasm. 
But I think that this conflict between my results and those of 
Minot and Hertwig is, after all, confined to details, and that in the 
fundamental conception of the causes of senescence and rejuven- 
escence they may be brought into harmony. With the general 
thesis that senescence is associated with the accumulation in the 
cell of the products of metabolism and differentiation, and that 
rejuvenation consists in a return to a condition in which these 
products are largely eliminated, as Minot and Hertwig have 
urged, I am in hearty agreement; their assumption that changes 
in the nucleus-plasma ratio are the causes of these phenomena 
seems to me to be merely an error of detail. 
In a very suggestive paper, Child (’11) has recently maintained 
that senescence and rejuvenescence are caused by a decrease or 
an increase in the fundamental metabolic reactions. Anything 
which decreases the rate of metabolism, such as ‘‘decrease in 
permeability, increase in density, accumulation of relatively 
inactive substances, etc.,’’ will lead to senescence. ‘‘Rejuven- 
escence consists physiologically in an increase in the rate of metabo- 
lism and is brought about in nature by the removal in one way 
or another of the structural obstacles to metabolism” (p. 609). 
This hypothesis finds much support in the phenomena con- 
nected with the early development of the egg. It is well known 
that construct ve metabolism takes place only in the presence 
of nuclear material, and it has long been known that the nuclei 
of various kinds of gland cells give off substances which play 
an important part in the metabolism of the cell. Loeb (99) 
has shown that the nucleus is the oxidative center of the cell; 
Mathews identifies oxidase with chromatin; R. Lillie (’02) finds 
that oxidation takes place most rapidly in the immediate vicinity 
of the nucleus. If the rate of metabolism is associated with sen- 
