244 RECENT CYTOLOGY 
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 
Dvum,” 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 
