434 SENESCENCE AND REJUVENESCENCE 



is the fact that reproduction in many of the simpler organisms 

 involves a disintegration of the original individual and the origin 

 of new individuals from its parts or certain of them. According 

 to von Hansemann ('93, '09), it is the atrophy of the sexual organs, 

 the final elimination of the germ plasm, which brings about the 

 changes of old age ending in death. These hypotheses are little 

 more than guesses based on observation of the life histories of 

 various organisms. 



Various authors have suggested that conjugation and fertiliza- 

 tion bring about rejuvenescence in some way. Maupas ('88, '89) 

 believed that the infusoria grow old and may finally die of old age 

 in the course of repeated agamic reproductions and that conjuga- 

 tion renews their capacity for growth and division, but later 

 investigators do not confirm these conclusions (see pp. 136-45). 

 Bernstein ('98) suggests that certain internal conditions whose 

 nature is unknown act as inhibitors of the growth impulse, and 

 that their effect increases during life and finally brings about death. 

 Fertilization, however, weakens or inhibits the inhibitors, and 

 growth proceeds anew until again gradually inhibited. According 

 to Buhler ('04) the molecular constitution of the organic substance 

 undergoes gradual change during life and becomes less and less 

 capable of metabolism, and fertilization re-establishes the original 

 constitution. Rubner ('89) has advanced a very similar view. 

 These hypotheses are merely statements of a supposed fact and do 

 not throw any light upon the problem of the nature of the processes 

 concerned in either senescence or rejuvenescence. 



The idea that age and death are the results of an intoxication, 

 a poisoning of the organism in one way or another, has been ad- 

 vanced by various authors, among whom Metchnikoff ('03, '10) 

 has received most attention. According to Metchnikoff man is 

 slowly poisoned by resorption of the products of bacterial activity 

 in the large intestine. One result of this intoxication is arterio- 

 sclerosis; another is that some of the phagocytes, the white blood 

 corpuscles, under the influence of the poisons depart from their 

 proper function as scavengers and protectors of the tissues and 

 begin to devour the cells of the highest organs of the body, even 

 those of the nervous system. While Metchnikoff 's ideas have 



