ABSTRACT
There appear to be two main conceptions of the processes of senile decay. According to the first of these, the characters of old age are regarded as the result of inherent decay of the living machine. The theory suggests that the essential organisation of a cell, like that of an atom of radium, can only remain stable for a limited time, after which it undergoes irreversible and unavoidable decay. According to the second theory, senescence is not due to an inherent property of protoplasm itself, but is caused by the adverse effects exerted on the protoplasm by substances which are formed in the cell during its natural life. Thus, by a partial failure of the excretory mechanism an organism is eventually destroyed by its own activities. The analysis of the whole phenomenon is usually complicated by the fact that it is only towards the end of a long and active life that the symptoms of decline are sufficiently obvious to be of use for experimental purposes. Further, in most cases an organism may, during most of its life, throw off old or inefficient cells and replace them by others which are manufactured from the raw materials of its food. It, therefore, seems reasonable to select for experimental investigation a type of cell whose life is short, and which is incapable of absorbing food.
