476 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1910. 



he observed between 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 male- 

 ness or femaleness Avas Beard/ who suggested that originally there 

 had also been two kinds of spermatozoa, but that one has disap- 

 peared in most animals, remaining in a functionless condition in 

 such cases as Paludina and Pygara^ which give rise to the two kinds. 

 His paper is somewhat speculative ; but there is a steadily accumulat- 

 ing body of proof that the sex is irrevocably decided at least from the 

 moment of fertilization. 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 wdien twins are born 

 so like one another that they are distinguished with difficulty, they 

 are never of different se^es ; in these cases the twins are produced by 

 the division of one fertilized ovum and during foetal life are en- 

 veloped in the same membrane.- Twins produced by the simultane- 

 ous 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 

 Hymenopterous insects in which there is embryonic fission. Sil- 

 vestri ^ 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 1,000; in Ageniaspis^ 

 10 to 20 ; but if only one egg be laid by the parasite in the egg of the 

 host, all the flies which hatch are of the same sex. Similar cases of 

 embryonic fission in parasitic Hymenoptera have been described by 

 Marchal,^ with the same results in respect of sex. 



Another line of argument tending in the same direction is drawn 

 from animals which have more than one kind of egg^ in which eggs 

 of one kind produce males, those of the other females. Some such 

 cases occur among parthenogenetic species, e. g., the rotifer Hydatina, 

 and Phylloxera among insects; but in other animals both kinds of 

 eggs require fertilization, and the larger always yield females, the 

 smaller males. This has been shown to be the case in Dinophilus 

 aiKitris by von Malsen (loc. cit.), in the mite Pediciilopsis by Renter ^ 

 and is suspected by Montgomery in a spider." In these cases there 

 can be no question of modifying the sex by external circumstances 

 after the egg is fully formed ; but it might perhaps be maintained that 



iZool. Jahrbiicher, Anat., vol. 16, pp. 615 and 703, and other papers. 



~ See Galton, Human Faculty, 2d ed. (J. M. Dent), p. 150. 



3Annali II. Scuola Agric. Portici, vol. 0, 190G, and Bollettino R. Scuola Ag., vol. ;?, 1908. 



^ Arch. Zoo. Exp. und Gen. (4), vol. 2, p. 257. 



B Festschrift fiir Palmen, 190.5-7, vol. 1. 



".Toiirn. Kxp. Zoo., vol. 5, p. 429. 



