286 SENESCENCE AND REJUVENESCENCE 



increase in the amount of cytoplasm does occur in many cases, but 

 it is an incidental rather than a fundamental feature of senescence. 

 The important change is not the change in amount, but the change 

 in the proportion of chemically active to inactive, or more active 

 to less active substance. 



In the higher animals and man morphological differentiation 

 of the cells is much more conspicuous and varied than in the lower 

 forms, but the essential nature of the process is evidently the same 

 in all forms. Differentiation consists primarily, not in increase in 

 amount of cytoplasm, but in the accumulation of substances differ- 

 ent in some way from the embryonic cytoplasm and giving the cell 

 its characteristic structure. And it is unquestionably the increase 

 in these substances, not the increase in the amount of cytoplasm, 

 which determines the decrease in rate of metabolism and rate of 

 growth. The structural substances produced by different cells 

 differ in character in one way or another because in the course 

 of development different metabolic conditions arise in different 

 regions, and in the higher animals these conditions must be more 

 definite and fixed in character than in the lower organisms, because 

 the degree of individuation is higher, i.e., the correlation between 

 parts is more intimate and definite. These factors, together with 

 the limited regressibility in many parts, must also determine that 

 differentiation shall proceed farther than in the lower forms. The 

 structural differences in different cells are more permanent and more 

 conspicuous and in general involve the cell to a greater extent. 



So far as they have turned their attention to the phenomena of 

 senescence the anatomists, histologists, and pathologists have 

 often failed to recognize what the study of the lower organisms 

 forces us to admit as a fact, viz., that senescence is merely one aspect 

 of development, and have confined their attention to, and based 

 their theories upon, the morphological changes which occur in later 

 life, and particularly in what we are accustomed to call old age. 

 One reason for this attitude among those investigators who have 

 been chiefly concerned with man lies in the fact that old age in 

 man and the higher vertebrates is associated with certain morpho- 

 logical changes in the cells which seem to be different in character 

 and direction from the developmental changes. These changes 



