Page:Popular Science Monthly Volume 74.djvu/550

546 when the other combination occurs, 18 chromosomes from egg and 17 chromosomes from sperm, an organism results that has only 35 chromosomes, and this is a male. It seems, therefore, as if the accessory chromosome was a sex-determinant in some sense—probably only in the sense of a visible accompaniment of some hidden underlying physiological cause of sex—perhaps as the carrier of a specific enzyme or "hormone."

To bring these facts into line with Mendelian principles it becomes necessary to postulate (1) two kinds of eggs, just as there are two kinds of spermatozoa, (2) selective fertilization, and (3) dominance of femaleness, and the observed facts can be explained. If the accessory chromosome is a sex-determinant, then when an egg is fertilized by a sperm lacking this element, the egg itself must carry the factor that determines that the sex of the resulting organism of 35 chromosomes shall be a male. In the event of the other possible combination the egg selected must have been one carrying the female determinant and, since the accessory chromosome is a male-determinant and the union of the two produces a sex-hybrid, femaleness must dominate in order to yield a female organism of 36 chromosomes.

The most recent position is that of Dr. Correns, professor of botany in the University of Leipsig, to the effect that there is only one kind of egg (always female) and two kinds of spermatozoa. He has brought forth facts to amply support this view in flowering plants. He shows that the spermatozoa determine the sex and that the time of this determination is the instant of fertilization. Correns attempts to bring the second position above mentioned into harmony with his own, and the facts upon which Beard's position is based are given a reasonable interpretation. We shall consider these various positions from Correns's standpoint, and will note how he disposes of the obstacles above referred to.

There are now known about one hundred cases, mostly among the air-breathing arthropods, where there is a dimorphism of spermatozoa. But if fertilization determines the sex, what about such cases where eggs develop without fertilization, as in the drone honey-bee, ants and plant lice (aphids and phylloxerans)? These several examples until recently had been serious stumbling-blocks to the theory that sex is dependent on a dimorphism of spermatozoa. Nor is the matter yet as clear as we could wish it to be. All the fertilized eggs of the honeybee develop into females (workers or queens, depending upon the kind and amount of food they receive), the unfertilized eggs develop into males or drones. Meves has recently found that while the drone honey-bee produces two kinds of spermatozoa, one set, the male-producing, degenerates and becomes non-functional. Hence all fertilized eggs must develop into females. The facts become intelligible if we