SEXUAL REPRODUCTION 213 



which we may call the gametes, are concerned 

 in the process, and they invariably coalesce 

 to form one the zyg&te. 



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 



