CELLS AND THEIR ASSOCIATION 45 



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 

 becomes fixed at last upon some cell in the embryo and 

 eventually upon the single cell, the fertilized egg or 

 ovum, from which all the countless host of cells in 

 the mature body are descended. This lived before the 

 individual whom we have chosen to picture. "Omnis 

 cellula e cellula," said Virchow in the last century. 

 "Omne vivum ex ovo," Harvey had written two hundred 

 years before. Either of these famous sayings will serve 

 to bring home to us the fascinating thought of the 

 uninterrupted living bonds which unite all forms of 

 the present with the most distant past. In retrospect 

 these threads of life seem endless, yet any one of them 

 may terminate at any moment. 





