442 SENESCENCE AND REJUVENESCENCE 



nucleus. It would be of interest to know how Minot regarded 

 this cell. 



Delage ('03) believes that age and death are the result of differ- 

 entiation. In the course of differentiation the cells lose the capacity 

 for reproduction and finally for growth, and no cell is able to live 

 indefinitely without either growing or dividing. The idea that 

 cell reproduction prevents or retards senescence seems to be involved 

 in this view, but Delage does not attempt to develop it. 



Jennings has recently advanced a view very similar to that held 

 by Delage. Age and death, according to Jennings ('12, '13), are 

 the result of the increased differentiation of the higher organisms. 

 The infusoria do not necessarily die or undergo progressive race 

 senescence, as Maupas believed. In the more complex and highly 

 organized body of the higher animals the greater degree of differ- 

 entiation brings about loss of capacity to carry on the fundamental 

 vital processes, and so death finally results. Jennings fails to 

 note that the higher organisms differ from the protozoa, not merely 

 in the degree of structural differentiation, but in the absence or 

 limitation of agamic reproduction. As I have endeavored to show 

 (pp. 136-45), it is the repeated process of reproduction rather than 

 their low degree of differentiation which prevents progressive race 

 senescence and death in the protozoa. Each division brings about 

 some degree of rejuvenescence, which may balance the senescence 

 during the interval between divisions. Doubtless the capacity of 

 the protozoa to reproduce agamically and their low degree of differ- 

 entiation are associated with each other as results of a common 

 cause, but it is the repeated interruption of progressive develop- 

 ment by regression that prevents or retards old age and death. 



It remains to consider certain hypotheses which concern them- 

 selves more directly with the metabolic aspects of the age changes. 

 In his Allgemeine Biologic (1899), Kassowitz has attempted a 

 general consideration of biological phenomena on the basis of a 

 theory of metabolism which assumes that all metabolism consists 

 in the synthesis and destruction of the protoplasm molecule. All 

 non-protoplasmic (metaplasmic) substances, such for example as 

 fat, glycogen, starch, etc., which appear in the cell, must first have 

 formed part of the protoplasm molecules, and their formation is the 



