MALE AND FEMALE 15 



down from cell to cell and from generation to generation in all 

 organisms. In describing them it will be necessary to show their 

 bearing on the genotype and vice versa. For this purpose a point 

 of view is necessary rather different from the usual one, for while 

 the chromosomes contain the " something " which we identify with 

 the genotype they themselves cannot be directly identified with it. 

 Their form and behaviour have special properties of continuity and 

 autonomy, but, as we have already seen in regard to meiosis, they 

 are nevertheless controlled within certain limits by this "something." 

 This also will have to be constantly borne in mind. 



(ii) Conditions of Sexual Differentiation. Sexual differentiation 

 consists in the production of two different kinds of germ cells by the 

 sexually reproducing species, and this, as already shown, can be 

 attributed to differences in one of three conditions : environment, 

 position in development, genotype. The first two arise without 

 genetic differences, the third by the segregation of such differences 

 at meiosis. A few examples will provide an illustration of the 

 characteristic way in which these three methods operate. 



(a) Environment. When a fertilised egg of Bonellia viridis 

 (Echiuroidea) falls free it develops into a female ; when it falls on 

 the proboscis of a female it develops into a male which is parasitic 

 on the female (Baltzer) . Differences of genetic constitution and of 

 position in development therefore have no influence on the kind of 

 gametes produced by the organism. The external environment 

 alone is decisive. In the worm Di^iophiliis , a parental difference is 

 decisive, for large eggs develop into females and small eggs into 

 males (cf. Goldschmidt, 1920; Shen, 1936). 



(h) Development, {a) Differentiation in space : in hermaphrodite 

 plants and animals the kind of gamete to be produced by a given 

 mother-cell is determined by its position in the body. In Actinophrys 

 sol (Belar, 1926 b) the two products of one cell-division remain 

 attached and undergo meiosis side by side, each expelling two 

 " polar bodies." They then fuse, the one which completes its 

 reduction first taking the initiative. This is the minimum of 

 differentiation and is probably conditioned by difference of position 

 as in a multicellular hermaphrodite, {b) Differentiation in time : in 

 some plants or animals {e.g., protandrous MoUusca, Rana sometimes 



