THE CELL 227 



place too numerous for us to follow in detail. Thus 

 the complicated structure of the adult organism was 

 gradually arrived at by a process of development in 

 which cell-multiplication played a most prominent and 

 essential part. 



We have next to inquire what is the method of origin 

 of the original embryonic cell the fertilized ovum 

 from which the new animal or plant develops. 



As is indeed implied by the expression ' fertilized 

 ovum,' this cell arises by the fusion together of two 

 independent cells, such fusion constituting the process 

 of fertilization or impregnation. One of the cells 

 which took part in the fusion was derived from one 

 parent organism, and bore the distinguishing character- 

 istics of the cells which composed that parent or at 

 least some part of those characteristics whilst the 

 other was in like manner derived from the second 

 parent. 



It is to be observed that this fusion together of a 

 pair of cells, derived (in the case we are considering 

 namely, that of ordinary biparental reproduction) 

 from two separate individuals, results in the formation 

 of a complete new individuality, which arises definitely 

 at that point of time at which the fusion of the two 

 conjugating cells takes place. In this way the cells 

 of the offspring are seen to be of double origin, and it 

 is found that traits and characters derived from both 

 the father and the mother can co-exist in them side by 

 side. 



The cells which take part in the above-mentioned 

 fusion are known as gametes, or germ-cells male and 



152 



