i8 CELL GENETICS 



1. Sexual Reproduction, i.e., propagation co-ordinating meiosis and 

 fertilisation in the life cycle. It has originally the function of 

 recombining genetic differences. Its antithesis is asexual repro- 

 duction, where the co-ordination does not occur either through the 

 absence of the system or, as in Apomixis, through its breakdown, 

 partial or complete. 



2. Sexual Differentiation, i.e., the production by a sexually repro- 

 ducing group of organisms of larger (female, ?) germ cells, which are 

 fertilised by smaller [male, c^) germ cells. Two degrees of sexual 

 differentiation occur ; first, differentiation between the cells them- 

 selves, or heterogamy ; and secondly, differentiation between the 

 individual organisms that bear them, or dioecism. Heterogamy 

 has originally the function of economy in allowing fertilisation by 

 motile germ cells at a distance to be combined with the provision 

 of food materials for the product by non-motile cells. It thus 

 increases the scope of cross-fertilisation and enlarges the inter- 

 breeding group. Its antithesis is isogamy. Dioecism has originally 

 the function of ensuring cross-fertilisation and of differentiating the 

 work of reproduction. Its antithesis is monoecism or herma- 

 phroditism. Incompatibility and heterothallism are restrictions of 

 sexual reproduction, analogous in effect to sexual differentiation, 

 but distinct from it inasmuch as they do not depend on a contrast 

 in size of the germ-cells and may be found with it or without it. 



