286 DEATH 



modes of reproduction. The fusion of male and 

 female pronuclei in the act of fertilisation of the egg 

 is the sexual process, and is equivalent to the 

 similar fusion of male and female pronuclei of 

 unrelated cells, seen in the conjugation of Infusoria. 

 On the other hand the successive acts of cell 

 division, by which the fertilised egg gives rise to 

 the embryo and the embryo becomes converted into 

 the adult, are asexual processes, equivalent to the 

 repeated acts of cell division by which the successive 

 generations of the Infusorian are produced. In the 

 Infusorian the number of such asexually produced 

 generations that can succeed one another is limited ; 

 so also is it in the Metazoon ; and the gradual 

 failure of the power to divide further leads in both 

 cases alike first to senescence or old age, and 

 ultimately to death. 



This comparison between Protozoa and Metazoa 

 in regard to the modes in which reproduction is 

 effected appears to be a just one. The striking 

 difference, that in the Protozoon the products of the 

 asexual process of cell division become independent 

 and similar unicellular animals, while in the Meta- 

 zoon they are component and differentiated units in 

 the body of a multicellular animal, does not affect the 

 comparison so far as concerns the essential point 

 i.*., the mode in which successive cell generations 

 come into existence in the two cases alike. A 

 further point of difference is- found in the consider- 

 ation that in the Infusoria all the asexually produced 

 cells retain, at any rate for a number of generations, 

 the power of conjugating with other cells ; while in 



