SEXUAL REPRODUCTION 213 
which we may call the gametes, are concerned 
in the process, and they invariably coalesce 
to form one—the zygote. 
From the zygote, which is always a single 
cell, there springs a new generation which 
may multiply in various ways, but sooner 
or later a process supervenes which leads 
once more to the formation of new gametes. 
These in their turn may coalesce in appropriate 
pairs and so form new zygotes. 
In the more primitive unicellular plants the 
sexual cells or gametes are often apparently 
precisely similar to each other. They may 
also be externally indistinguishable from the 
ordinary vegetative organism itself, or at 
any rate from the newly formed individuals 
which have just arisen by vegetative propaga- 
tion. Nevertheless the sexual individuals are 
physiologically very distinct. If it were not 
so, they would scarcely be definitely impelled 
to unite, and to unite only in pairs. 
Closer examination reveals the fact that 
in sexual union the coalescence of the gametes 
is a very intimate one. Not only do the 
extra-nuclear protoplasms flow together, but 
the two nuclei also unite and mingle their 
contents in common. A study of the higher 
types, both of animals and plants, leads to 
the further conclusion that it is in the nuclear 
fusion, more than in anything else, that the 
significance of the sexual act is to be sought. 
We shall return to this point later, but it will 
be convenient and profitable in the first 
