196 THE POPULAR SCIENCE MONTHLY 



differentiation products continues long enough it necessarily leads to 

 the death of the cell, since the continued life of the cell depends upon 

 the interaction between the general protoplasm and the nucleus. In 

 cells laden with the products of differentiation, the power of regulation 

 is first lost, then the power of division, and finally the power of assimi- 

 lation; and this is normally followed by the senescence and death of 

 the cells. 



In some cases the progressive transformation of protoplasm into 

 metaplasm may be reversed; in some manner the formed material is 

 dissolved and converted into general protoplasm, the protoplasm and 

 nuclei increase in size, the cells begin to divide and may become capable 

 of regulation. In short, this reversal of the differentiation process leads 

 to the rejuvenescence of senile cells. Minot^^ holds that differentiated 

 cells do not become undifferentiated — but at least it must be admitted 

 they may lose their products of differentiation and metabolism; gland 

 cells lose their secretion granules, egg cells their yolk, spermatozoa 

 within the egg their flagella, injured muscle cells their fibrillge, etc. In 

 such cases differentiation products are either eliminated from the cell 

 or are transformed into a more labile and more general form of proto- 

 plasm, though the latter is probably not undifferentiated. I have used 

 the term dediff erentiation for this process. 



Among functional causes of senescence may be mentioned the well- 

 known opinion of Metschnikoff, that the organism is slowly poisoned by 

 its own waste products. Metschnikoff especially emphasizes the effects 

 of intestinal fermentation and putrefaction in producing old age. 

 Zoologists are familiar with the fact that, in certain Polyzoa and Tuni- 

 cata which lack kidneys or efficient means of eliminating urea, or other 

 nitrogenous waste, the tissues gradually become laden with such waste 

 substances and the animal becomes senile and finally dies, but before 

 this happens it may give off one or more buds which are relatively free 

 from these waste products and which continue the life of the colony. 

 It is a general phenomenon both in plants and animals that buds are 

 composed of protoplasm which is not laden with products of differentia- 

 tion or metabolism, and hence they exhibit youthful characteristics 

 although the body from which they come may be senile. 



Another functional cause of senescence is to be found in a decrease 

 in the power of constructive metabolism. This factor has been recently 

 advocated by Child^' in a very valuable paper, in which he concludes 

 that anything which decreases the rate of metabolism, such as *' decrease 

 in permeability, increase in density, accumulation of relatively inactive 

 substances, etc.," will lead to senescence. On the other hand, " Re- 

 juvenescence consists physiologically in an increase in the rate of 



"Child, "A Study of Senescence and Rejuvenescence, etc.," Arch. Eniw. 

 Mech., 31, 1911. 



