68 



ORGANISMS OF ONE CELL 



phenomena to weaken, give out and lead to old age. The 

 secreting cells for example ultimately cease functioning one 

 by one and their places in the tissue are taken by non-function- 

 ing connective- tissue cells; when enough of these are thus worn 

 out and replaced activity of the organ is impaired and the 

 general vitality of the entire organism is correspondingly 

 weakened. In the human organism this process results in 

 hardening of the tissues leading for example to cirrhosis of the 



liver or kidney, sclerosis of arteries, 

 etc., leading inevitably to death after 

 a longer or shorter time. 



The problem of old age therefore 

 resolves itself into the question why 

 do the individual cells give out? It 

 would seem that these differentiated 

 cells of the body are endowed with 

 a limited possibility of action or with 

 a " potential or vitality" which is 

 gradually exhausted by continued 

 use. Yet some of these epithelial 

 cells have the capacity to live far 

 beyond the limits of the natural life 

 of the organism to which they belong 

 under circumstances abnormal to the 

 organism. Thus epithelial cells of 

 the mouse under abnormal condi- 

 tions which we call cancer, may be 

 transplanted from mouse to mouse 

 for years after the normal length of 



life of the original animal a fact demonstrating that .epi- 

 thelial cells at least under these abnormal conditions have 

 a far greater potential of vitality than is represented by 

 the ordinary length of life of the individual. Weakness and 

 death of the individual cells therefore must be due to some de- 

 fect in the inter-relations of the many differentiated cells and 

 organs of the body of the metazoa and a defect which is cumu- 

 lative until the organisms are unable to carry on the necessary 



FIG. 29. Paramecium 

 caudatum in conjugation. 

 The micronuclei can be 

 seen in the process of di- 

 viding. The organisms are 

 united in the peristome 

 region. From a photo- 

 graph of a preparation. 



