28 EVOLUTION OF LIVING ORGANISMS. 



ovum, is quiescent, stored with food-material to provide 

 for the nutrition of the developing embryo, and is gener- 

 ally of large size. The other, the spermatozoon of 

 animals or spermatozoid of plants, is on the contrary 

 small, active, and usually furnished with a vibratile whip- 

 like " tail," with the help of which it bores its way into 

 the ovum. The individual bearing ova is of the female 

 sex ; that bearing the spermatozoa, of the male sex. 

 But the names male and female are often conveniently 

 extended to the gametes themselves. Hermaphrodites 

 give rise to both kinds of germ-cells. 



In fertilisation one male gamete fuses with one female ; 

 and not only do the cell-bodies unite, but the nuclei of the 

 cells also combine into one nucleus. Thus the nucleus of 

 the resulting zygote contains chromatin from two in- 

 dividuals, since the cells usually come from different 

 parents. For it is only exceptionally that hermaphrodites 

 are self-fertilising. Unsuccessful male gametes, which do 

 not reach an ovum, perish sooner or later ; and likewise, 

 unfertilised ova die, except in those rare cases where 

 parthenogenesis occurs. Instances of parthenogenetic re- 

 production are the fresh-water stonewort, Chara crinita, 

 of which only females occur in northern Europe, and tha 

 plant lice (Aphidse) and certain other insects, which 

 propagate in this manner in the summer. 



The differentiation of the germ-cells and of sex has had 

 a profound influence on the evolution of organisms. To 

 secure the nourishment, fertilisation, distribution, and 

 survival of the reproductive cells is their chief function ; 

 to this end have been developed that wonderful diversity 

 and elaboration of structure, both bodily and mental, 

 found in living nature. 



Among the unicellular forms sexual reproduction may 

 be of a much simpler kind than that described above. 

 Often one individual may divide up into a number of 

 small "male" gametes, each capable of fertilising 

 another individual playing the part of a " female " ovum. 



