THE DETERMINATION OF SEX 93 



only one or the other sex, as long as the primary germ-cells do 

 not all come to maturity. This is the case at least in the females 

 of the higher vertebrates ; and it is from them that the greatest 

 amount of evidence in this direction has been obtained. Russo 1 

 has recently maintained that treatment with lecithin causes an 

 increase in the number of female ova matured in the rabbit and 

 believes that he can distinguish the male from the female eggs 

 in the ovary. He admits however that the families in his tables 

 are selected and it seems that the differences he observed be- 

 tween the two kinds of eggs were probably due to degenerative 

 changes in some. 



One of the first writers to maintain that the ova bear either 

 maleness or femaleness was Beard, 2 who suggested that originally 

 there had also been two kinds of spermatozoa but that one 

 has disappeared in most animals, remaining in a functionless 

 condition in such cases as Paludina and Pygcera, which give 

 rise to the two kinds. His paper is somewhat speculative; 

 but there is a steadily accumulating body of proof that the 

 sex is irrevocably decided at least from the moment of fertili- 

 sation. This belief is supported by a number of different 

 facts. It has long been known that in man " identical twins " 

 are always of the same sex, i.e. that when twins are born 

 so like one another that they are distinguished with difficulty, 

 they are never of different sexes ; in these cases the twins 

 are produced by the division of one fertilised ovum and 

 during fcetal life are enveloped in the same membrane. 3 Twins 

 produced by the simultaneous development of two ova are 

 not more like each other than other brothers and sisters, and 

 are frequently of different sexes. A similar but perhaps even 

 more conclusive case is provided by the parasitic Hymeno- 

 pterous insects in which there is embryonic fission. Silvestri 4 

 has investigated two such insects, Litomastix and Ageniaspis. In 

 each the flies lay their eggs in the eggs of other insects, and at 

 the close of segmentation the embryonic cells become clustered 

 into groups, each of which produces a separate embryo. In 

 Litomastix the number of larvae so produced may be about a 



1 Atti Acad. Lincei, xvi. 1907, p. 362. 



2 Zool. Jahrbiicher, Anat. xvi. pp. 615 and 703, and other papers. 



3 See Galton, Human Faculty, 2nd ed. (J. M. Dent), p. 156. 



4 Annali R. Scuola Agric. Portici, vi. 1906, and Bollettino R. Scuola Ag. iii. 

 1908. 



