CELLS AND THEIR ASSOCIATION 45 



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. 



When we think of our ancestors who have died 

 perhaps fourteen of them in a hundred years we must 

 be impressed by the disproportion between the mass of 

 their tissues which perished and the infinitesimal survival 

 in ourselves of what they transmitted to us. Someone 

 has said that the body is like a great lantern, serving 

 primarily to save the tiny, trembling flame of the germi- 

 nal life from being extinguished. It is a lantern which 

 wears out and only the flame can be saved to burn for a 

 time in a fresh protective shell. One contrast between 

 single-celled and many-celled forms of life will now be 

 clear: in the former the whole substance of one genera- 

 tion may live to constitute the next, in the latter the 

 accumulated cells of the body die and under the most 

 favorable conditions leave but the minutest part to 

 represent the stock. 



Another difference between the unicellular and the 

 more highly developed organisms is associated with sex. 

 A single bacterium may give rise to two, and these to 

 four descendants, and so on. A solitary member of 

 any of the more advanced types must mate with another 

 of opposite sex if the species is to be reproduced. The 

 fertilized ovum, before mentioned, which develops into 

 the embryo and so into the mature individual may be 

 called one cell, but it is more truly a composite of two 



