SOME CURRENT THEORIES 445 



the substratum. Such a change will doubtless decrease nucleo- 

 plasmic interchange, but this decrease will be secondary rather than 

 primary in the senescence process. Nucleoplasmic interchange 

 depends upon the metabolic conditions in the cytoplasm and in the 

 nucleus and may be altered' by changes in either or both. The 

 primary metabolic changes of age must occur throughout the proto- 

 plasm. On the other hand, to say, as Conklin does, that anything 

 which decreases nucleoplasmic interchange leads to senility and 

 anything which increases it renews youth is manifestly not true, 

 for low temperature may decrease and high temperature increase 

 the interchange, but such metabolic changes do not, properly 

 speaking, constitute senescence and rejuvenescence, although they 

 may in some cases result sooner or later in one or the other. 



The advances during recent years in our knowledge of the 

 colloids and the very natural and entirely justifiable desire to apply 

 the principles and conclusions of colloid chemistry to the living 

 organism have led various authors to suggest that senescence in 

 organisms is fundamentally a colloid change. In chaps, i, ii, and 

 viii I have called attention to these colloid changes and their impor- 

 tance for the problems of senescence and rejuvenescence. It can 

 scarcely be doubted that the colloid substratum of the organism 

 does undergo changes which are not essentially different from those 

 in non-living colloids and that such changes play an important 

 role in the process of senescence. They are perhaps, as I suggested 

 (pp. 49-50), the primary changes in embryonic protoplasm which 

 lead to decrease in metabolic rate and so initiate the processes of 

 differentiation and senescence. But something more than these 

 changes is involved in at least most cases of senescence, for differ- 

 entiation occurs, new structural substances are produced and 

 accumulate in the cell, and its metabolic activity often becomes 

 very different in character from that of the embryonic cell. While 

 these changes may depend in large measure upon colloid changes, 

 it is probable that changes in the chemical constitution of the 

 substratum may also contribute to its increasing stability and so 

 play a part in senescence. 



The occurrence of rejuvenescence has not, so far as I know, 

 been considered in connection with the suggestions that senescence 



