194 THE POPULAR SCIENCE MONTHLY 



The possible causes of senescence and rejuvenescence may be classi- 

 fied as structural and functional, though these two should not be re- 

 garded as mutually exclusive. Indeed it is practically certain that both 

 structure and function are involved in these processes as in most other 

 vital phenomena. However, different students of this subject have 

 placed emphasis more or less exclusively upon either the structural or 

 the functional causes of senescence and rejuvenescence. 



Under the structural causes may be cited Minot's hypothesis that 

 senescence is caused by an increase in the amount of protoplasm as 

 compared with the nucleus. In 1890 he^* summarized his views on this 

 subject in the following words : 



We have then to state, as the general result of the studies which we har© 

 just made, that the most characteristic peculiarity of advancing age, of in- 

 creasing development, is the growth of protoplasm; the possession of a large 

 relative quantity of protoplasm is a sign of age. . . . We see that there is a 

 certain antithesis, we might almost say a struggle for supremacy, between the 

 nucleus and protoplasm. 



In several subsequent papers and books,^° Minot has developed this 

 idea at length. In his book on "Age, Growth and Death,"-^ he con- 

 cludes that 



Rejuvenescence depends on the increase of the nuclei, senescence depends 

 on the increase of the protoplasm and on the differentiation of the cells. 



E. Hertwig's^^ views are apparently diametrically opposed to those 

 of Minot, He finds that senescence or rather "depression" and 

 "physiological degeneration," in Adinospherium and Infusoria are 

 accompanied by an enormous growth of the nucleus. He regards the 

 immature egg cell with its great nucleus as in a condition of depression 

 similar to that found in the protozoa named. By the processes of matu- 

 ration and fertilization this nuclear material is greatly reduced and 

 thus the cells are brought back to a normal condition. 



As opposed to the hypotheses of Minot and Hertwig, it may be 

 pointed out that the larger part of a resting nucleus is composed of 

 achromatin which has been absorbed from the cell body, and that the 

 size of a nucleus depends chiefly upon the quantity of general proto- 

 plasm in a cell and upon the length of the resting period during which 

 the nucleus is absorbing this protoplasm. So far from there being an 

 antithesis between nucleus and general protoplasm, we find that the 

 general protoplasm is common to both ; small nuclei occur only in cells 



•* Minot, "On Certain Phenomena of Growing Old," Froc. Am. Ass'n Adv. 

 Sci., 29, 1S90. 



"Minot, "Ueber Vererbung und Verjtingung," Biol. Centralh., 15, 1S95. 



"Minot, "Age, Growth and Death," Putnams, New York, 190S. 



*" Hertwig, R., "Ueber die Kernkonjugation der Infusorien," Abh. Bayer. 

 Akad. Wiss., II. Kl., 17, 1889. 



