THE CELL 245 



female respectively, according to the sex of the parent 

 from which each is derived. In animals the female 

 gamete is known as the ovum, and the male as the 

 spermatozoon, and the product of their fusion, as already 

 said, is called the fertilized ovum. Germ-cells of a 

 similar kind arise in a slightly different way in plants. 

 The germ-cells are produced in special parts of the 

 organism known as the generative organs, which in 

 flowering plants are represented by the pistils and 

 stamens. 



A more convenient expression for the fertilized ovum 

 is that of zygote, a term which we have previously 

 encountered in the shape of the homo- and hetero- 

 zygotes of the Mendelian. By an expansion of meaning 

 the term zygote is also used to express the whole 

 organism which ultimately arises from the product of 

 fusion of a pair of gametes, and by this use the impor- 

 tance of the gamete, as opposed to the zygotic organism 

 as a whole, is brought into due prominence. 



We find, then, that the succession of generations in 

 the higher animals and plants, according to the common 

 use of this expression, depends upon the succession 

 of a much larger number of cell -generations. By 

 repeated divisions, each giving rise to a new generation 

 of cells, the fertilized ovum gradually develops into 

 the adult organism. By the division of certain 

 members of the later generations of cells which compose 

 this organism the gametes are produced. By the 

 conjugation of a pair of gametes a zygote of the 

 second generation arises, and the same processes are 

 continually repeated. 



