According to the theory of cross-heredity of sex, female creatures should in the former instance be produced, and in the latter males, which same result is reached in accordance with the theory of Thury.
Attempts have been also made to apply Thury's theory to the human species. The menstruation of women has been compared with the rutting of the lower animals, and has been considered a protracted, oft-repeated rutting. Now, as an ovum is specially developed every month, it follows that this ovum requires a certain part of a month to attain a more or less advanced degree of ripeness. According to this, the ova which are fertilized a short time after menstruation will develop only female individuals, whilst those which have had a longer time in which to attain to ripeness would develop themselves into males.
The mucous membrane of the womb ought, about ten days before the beginning of menstruation, to thicken itself distinctly in consequence of a turgescence and dilatation of the vessels. In consequence it appears swollen and loosened, and it has reached the culminating point of swelling when the menstruation is at its highest. After menstruation the swelling does not immediately decrease, but lasts on for about nine days, until the mucous membrane returns to its normal condition. (Hensen.) Thus it seems that the swelling and hyperæmia in the womb appear at the same time as the conditions which lead to the ripening of the ovum. The fertilization of the human ovum would be, therefore, most efficacious at the time when the mucous membrane of the womb is also most appropriately prepared, and it is probable that the same moment is the one when all the other coincident factors are of a sort best calculated for the reception, the fixing, and the protection of the ovum. It is simultaneously with these processes in the mucous membrane of the womb, and in the other parts of the generative organs of the woman, that the ripening of the ovum is effected.
Now, it may appear not to be a matter of indifference (and may very likely even have some connection with the development of sex) whether the ovum is fertilized at a period during which the mucous membrane is passing through its changes in order to reach its highest point of swelling, or at the time, when, after menstruation, the mucous membrane is, during so considerable a period (nine days), passing through a retrograde metamorphosis in order to return to its normal condition. This protracted process seems to correspond to the protracted rutting in the form of a menstruation. If so, the human uterus, as Thury's theory would declare, would be prepared, to a certain extent, in different ways, for the reception of the ovum, according to the different sex-conditions of the future child.