44 THE DOCTRINE OF IMMORTALITY 



rupted continuation of the living substance is assured in 

 various organisms. The simplest cases occur in the lower 

 organisms, in bacteria, etc., in unicellular plants and ani- 

 mals. In these the single individual, or the single cell, 

 grows up to a certain size and then divides. In this man- 

 ner the two daughter cells come to have part of the same 

 substance as the mother cell, and so it goes on. This 

 substance, so far as we can observe, does not change 

 essentially with time. In the higher plants and animals 

 we have in each case to do with many cells and we observe 

 that the functions are unequally distributed among these 

 cells. For the execution of various functions the cells become 

 unlike among themselves. This is the phenomenon of dif- 

 ferentiation of which we have already spoken. The majority 

 of the cells are destined for the care of the whole, and perform 

 their special functions. Some of the cells, however, are not 

 utilized in this manner, but serve for propagation. When 

 a flower unfolds in our garden we find in it certain special cells 

 which have to do with the propagation. These do not show 

 such differentiation as we may find in other cells of the plant, 

 but remain at first relatively simple in their structure. The 

 propagating cells mentioned separate themselves from the 

 mother plant and form the seed. As essential in this case 

 it appears that two cells are necessary for the process, one of 

 which we designate as the egg cell and the other as the seminal 

 cell. Two such cells unite and form a new cell, with which 

 the further development begins. The mother plant may 

 then die. We note in this case that the fate of the cells is 

 extremely unlike, in that some of them are given over to death, 

 while others remain permanently alive and serve for the 

 propagation of the species. In the next lecture, in which we 

 shall investigate the development of death, we shall occupy 



