Internal Factors of Sex Determination 411 



of the moth. Silvestri has seen the same egg pierced more than 

 once by the same fly, and we may readily suppose that two 

 individuals often deposit their eggs in the same egg of the 

 moth. 



A closely similar mode of development occurs in another 

 species, Polynotus minutus, which deposits its eggs in the larva 

 of Cecidomia destructor, the Hessian fly. Marchal found in 

 fourteen cases that the flies that emerge from the grub are all 

 of one or of the other sex, while in two cases there was a mixture 

 of both sexes. 



The Sex of Human Twins and Double Monsters 



There remains to be considered another class of facts, not dis- 

 similar from the last, that appear, if certain modern assumptions 

 are correct, also to- mean that the sex of the embryo is already 

 determined in the fertilized egg, and is not affected by subsequent 

 events. I refer to the case of human twins. It is said that two 

 kinds of twins occur : in one kind the individuals are not more 

 alike than any two children born at different times: these are 

 fraternal twins. They may be of the same sex or different sexes, 

 and not more often of the same sex than of different sexes. They 

 are supposed to arise from two eggs simultaneously set free from 

 the ovary. In the other kind of twins, the two individuals 

 closely resemble each other so closely, in fact, that they may 

 scarcely be distinguishable apart by their own parents. These 

 "identical" twins are said to be always of the same sex, and it 

 has been suggested that they arise from the same egg that has 

 become separated into two parts at some stage in its develop- 

 ment. This view seemed all the more plausible because in the 

 last decade it has been shown experimentally for some other eggs 

 that when the first two cells are separated each gives rise to an 

 entire embryo. This is true for the eggs of sea urchin and star- 

 fish, and in the vertebrates for the egg of amphioxus, the sala- 

 mander, and the fish. We do not know in these cases that the 

 isolated cells would produce individuals of the same sex, nor 

 do we know in man that the cells are really separated; but 



