6 THE INVERTEBRATA 



of fission, and often goes on in both, is carried out with cells (or, in 

 protozoa, with organized protoplasm) which existed as such in the 

 parent. In more highly organized animals the only protoplasm which 

 retains the required plasticity is that of the germ cells, and conse- 

 quently such animals have only the sexual reproduction which these 

 cells perform. The germ cells (gametes), before they reconstitute the 

 adult body, normally undergo the process known as conjugation or 

 syngamy, which is not an essential part of the reproductive process 

 but a provision for heritable variation whereby the race becomes 

 adaptable to its surroundings. Conjugation can only be performed by 

 uninucleate individuals and therefore, while in protozoa it sometimes 

 takes place between adults (hologamy, p. 31), in metazoa it always 

 requires the production of uninucleate young (the ova and sperma- 

 tozoa). The lower metazoa reproduce both by means of these gametes 

 and also asexually . In the higher animals, as we have seen, reproduction 

 is solely by gametes, though conjugation may be suspended for one 

 or more generations by the development of unfertilized ova (partheno- 

 genesis). 



