“For this experiment a number of hens are taken which may be anticipated to be “setters,” unless the use of incubators is preferred. The hens are to be separated, and the eggs which each one lays in her own particular nest are to be marked with numbers corresponding to the days on which they were laid. “The eggs of the different hens are now to be rearranged, so that the eggs which each particular hen is given to hatch shall, as nearly as possible, have the same numbers. For example, if the number of the hens be six, and the period of laying up to the time when the hens begin to set be thirty days, one hen will have eggs to hatch with the numbers 1 to 5, the next 6 to 10, the third 11 to 15, the fourth 16 to 20, the fifth 21 to 25, the sixth 26 to 30. “In this way the doubt will be avoided, which necessarily arises, if I give the eggs of one hen, although marked, to be hatched by her alone. “In the latter case, it can very seldom be known with certainty from which egg-shell a cock or hen issued. “In this experiment, on the contrary, one can quietly wait until the cocks and hens in the growing broods of the different hens can be clearly distinguished and numbered, seeing that each brood has numbers (of the days of laying) of very nearly the same value. The experiment is easier and less subject to the possibility of error when the eggs belong to different varieties and are taken from known parents.” Fertilization in many cases alters the future sex as soon as it affects the germ in some corresponding manner. The germ develops itself, and, in the case of creatures whose ova develop without fertilization, evolves one, or the other, or both of the sexes. According to Pagenstecher, fertilization often alters the determination of the sex of the germs which attain to development in the ova. The point of time in the life of an ovum, at which it has reached that degree of ripeness which gives it such a character that the semen can no longer affect the determination of the sex, cannot be absolutely settled. A more powerful bull might beget female calves earlier in the late rutting time than an older one. After further explanations and critical expositions, which suggest themselves in the course of the examination of the theory, Pagenstecher lays stress upon the following important dicta of Thury’s teaching:-- 1. Sex depends upon the ripeness of the ovum at the time of fertilization. 2. The ovum which, at the moment of fertilization, has not yet reached a certain degree of ripeness, produces a female. If this degree of ripeness has been passed, the ovum, upon fertilization, produces a male.