THE CELL 243 



new ones. In this case, it is only by a stretch of 

 language that we can speak of parent- and daughter- 

 cells, for the individuality of the pre-existing cell is 

 completely lost, and two fresh individualities have now 

 taken its place. 



Since all the cells of the animal or plant body arise 

 by the bipartition of pre-existing cells, it is clear that if 

 we follow these processes far enough back, in the case 

 of any individual organism, we may arrive at a period 

 at which only one cell was present. And under 

 ordinary circumstances this is actually the case. 

 Every individual among the higher animals and plants, 

 arising by the ordinary sexual method, existed at the 

 earliest stage of its embryonic history in the form of a 

 single cell, the fertilized ovum. And the first obvious 

 process in the development or embryology of the young 

 organism consisted in the division of this primitive 

 cell into two new cells. Each of these new cells then 

 divided again in like manner, and the multiplication of 

 cells continued until all the innumerable cells which 

 build up the organs of the adult body had finally 

 come into existence. When growth is completed cell- 

 divisions continue more slowly, producing new cells to 

 make good the wear and tear of the bodily tissues. 



As the number of cells increased, their relation to 

 one another in space was constantly changing. Dif- 

 ferent cells, too, became modified in different ways ; 

 for instance, the cells on the outside of the young 

 embryo took on a different form from those within, in 

 accordance with the different conditions to which 



they were exposed, and a host of other changes took 



16 a 



