Internal Factors of Sex Determination 4II 
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 bytheir 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 
