PURITY OF THE GERM-CELLS 67 



eludes that there is no reason for supposing that these 

 have undergone any modification at all. 



If we consider the cells which build up an adult organ, 

 and for the moment regard each separate cell as an 

 individual, we see that each of these individuals 

 possesses an ancestry of cells stretching right back to 

 the fertilized ovum the single cell in which the whole 

 organism originated. So far as the later cell divisions 

 are concerned, the cell-lineage of a particular organ is 

 separate and distinct from that of the cells of any 

 other organ. At a certain distance back in the history 

 of the organism we shall come across a common cell- 

 ancestor for the cells belonging to a pair of neighbour- 

 ing organs, and the more widely separate the parts to 

 which the cells we are considering belong the further 

 back must we go before we find their ancestry merging 

 in a single cell. In a similar way as with other organs, 

 so it is found that the sexual cells or germ-cells of an 

 adult organism have a history quite distinct from that 

 of the cells of any other part of the body ; and these 

 cells are the only ones which are concerned in the 

 formation of the offspring. Thus we see that the 

 particular cell-lineage leading up to the germ-cells is 

 the only one which is continued into another genera- 

 tion ; all the others terminate with the death of the 

 individual creature of which they form a part. From 

 this point of view we may consider the nature of a 

 given series of animals as being determined only by 

 the particular series of cells which constitute the 

 direct ancestry of the germ-cells in each individual ; 

 the cells which make up the bodily structure are the 



52 



