SOME CURRENT THEORIES 443 



result of chemical decomposition of these molecules. When the 

 cells are strongly stimulated, as they are during active function, 

 the protoplasmic molecules break down into substances which are 

 eliminated from the cell, such as carbon dioxide and the nitrogenous 

 excretion products. This Kassowitz terms active breakdown. But 

 even when the cells are not stimulated and functionally active to 

 any marked degree, protoplasmic breakdown still occurs, although 

 slowly and incompletely, and this inactive breakdown gives rise in 

 large part to the metaplasmic substances which accumulate in the 

 cell. The metaplasmic substances are, according to Kassowitz, 

 either quite incapable of further change in the cell after they are 

 once formed, or must be slowly transformed by the action of 

 enzymes before they can again take part in the synthesis of new 

 protoplasmic molecules. The presence of these metaplasmic 

 substances in the cell interferes with the passage of oxygen to the 

 labile molecules and with the transmission of stimuli and so favors 

 further inactive, as opposed to active, breakdown of protoplasmic 

 molecules. Consequently, when metaplasmic substances appear 

 in the cell, the inactive breakdown increases and this in turn leads 

 to further accumulation of metaplasm and so on. The result is a 

 decrease in functional activity and, sooner or later, death. From 

 , this point of view senescence and death are the result of a progres- 

 sive increase in the inactive breakdown and the metaplasmic sub- 

 stances formed by it. Death from old age finds its determining 

 factors in the chemical and physical constitution of protoplasm. 



In this theory the ideas of the accumulation of substance in the 

 cell and its effect upon metabolism as a basis for senescence is very 

 clearly and fully developed. And while there are various reasons 

 for dissenting from Kassowitz' theory of metabolism based on the 

 labile protoplasmic molecule (see pp. 13-18) and from the sharp 

 distinctions made between active and inactive breakdown and 

 between protoplasm and metaplasm, we can agree with him that 

 senescence and death are fundamental features of life and are 

 associated with an increase in stability of substratum of the cell. 



As regards rejuvenescence, Kassowitz is much less clear, 

 although he has in his ideas a satisfactory foundation for a theory 

 of rejuvenescence. In referring to Weismann's ideas concerning 



