action of two factors, both unknown quantities. By the union of an egg and a sperm an organism is produced which has a certain sex. If we let X represent the unknown sex tendency of the ova, and Y the unknown sex tendency of the spermatozoon, then we may state the process of sex production thus: X + Y = sex of offspring. If we could discover the value of X or Y we could solve the equation and discover the secret of sex. Correns, as the result of experimentation with certain flowering plants since 1900, has found the value of X in these particular forms and has thus contributed invaluable facts for the reinterpretation of former observations and for the formulation of a wider generalization regarding sex phenomena.
Many facts are known which leave little doubt that in most animals sex is absolutely fixed in the fertilized ovum. A real exception is the case of the frog, where the embryo or even the larval tadpole is hermaphroditic, i. e., it contains both ovaries and testes. In a later stage of development one or the other of these pairs of organs degenerates when the frog becomes a definitive male or a definitive female.
In the human species twins are frequently of the same sex, either male or female. Such twins are known as "identical twins" when enveloped in a common chorion or fœtal membrane. They are the result of an independent development of accidentally separated cells at the two-cell stage of development. Double monsters likewise are always of the same sex. "Ordinary twins" are as frequently of opposite sex as of the same sex. In this case each fœtus is enveloped in its own chorionic membrane. Such twins are the result of the synchronous development of two ova simultaneously successfully fertilized. These facts show that sex is already determined in the fertilized egg and before the first segmentation. Similar evidence is contributed by Jehring, who studied poly-embryony in a species of armadillo (Tatusa hyhrida) found in Paraguay. Here as many as eight offspring appear at a single birth. Jehring reports that all eight fetuses are enclosed in a common fetal envelope. Hence the eight offspring must result from the development of the products of division as the eight-cell stage of segmentation. Since the offspring are all of the same sex, sex must have been already determined in the fertilized egg prior to the first cleavage. Again, Professor Silvestri, of Naples, has quite recently contributed new facts which lead to the same positive conclusion. He has discovered that Litomastix, a kind of bee (Chalcidæ), lays its eggs in the egg of a moth, Plusia. As the latter develops into a larval caterpillar, the egg of Litomastix segments into a chain of many eggs, each of which gives rise to an embryo bee. The caterpillar may contain a hundred such embryos and they are all of the same sex—female if the egg was fertilized; male if unfertilized. Sex must have been already fixed in the fertilized egg. But this is an