THE CELL 243 
new ones. In this case, it is only by a stretch of 
Janguage 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—2 . 
