44 HUMAN PHYSIOLOGY 



production of heat, for example, is something we can 

 scarcely hope to measure when only one cell is concerned, 

 but we cannot doubt that it is going on because we know 

 that it is evident when sufficiently large numbers of 

 active cells are massed. The phenomena we can most 

 easily make out in a study of solitary cells are the taking 

 of food, motion, and reproduction. A word may be 

 said about the last-named manifestation of life. 



Cell Reproduction. A cell which is well nourished and 

 otherwise in a favorable condition soon attains to a size 

 which marks the limit for its growth. Instead of in- 

 creasing further in bulk it cleaves into two parts which 

 are complete and living cells. The cells of the new 

 generation are at first undersized but these in their turn 

 grow to the standard of the species. It is curious to 

 reflect that there is a kind of physical immortality be- 

 longing to the one-celled organisms. Cell-division is 

 obviously not death. Many members of the family are 

 killed by accidental means, but they do not seem fated 

 to grow old and perish from any intrinsic property of 

 their own. The cell which is living to-day has descended 

 from a line of ancestral cells no one of which has ever 

 died. 



This assertion may be made with reference to the 

 cells of higher plants and animals as well as to those which 

 live alone. No cell that is living to-day has ever lost a 

 direct ancestor by death. Let us see in what sense this 

 is true. Take, for example, a cell in the human skin. 

 Its neighbors at the surface are rapidly dying, shrivel- 

 ling, and being cast off. This will be its own probable 

 fate. But, if we look backward instead of forward, 

 we recognize that this selected cell was formed a short 

 time ago by the cleavage of a preexisting cell. The 

 companion formed at the same time may have perished 

 but the fact remains that the ancestor did not die. The 

 same may be said of the parent cell in the previous 

 generation. 



As we trace the succession backward our attention 



