It seems that in cases where a result was obtained it could be more easily explained by the principles which have been already described under the theory of cross-heredity of sex. The assumption of a greater or lesser degree of ripeness of the ovum which was to be developed was a very questionable one. The different processes with which we have become, in more recent times, acquainted as symptoms of the ripening of the ovum, are not here intended. Such symptoms can be observed both in the ova of the invertebrata and in those of the vertebrata. These symptoms, which are such as the attainment of the normal size of the ovum of the species in question, the protrusion of the orientation points (_Richtungskörperchen_), the steps towards the formation of a female anterior nucleus (_Vorkern_), etc., do not here apply, for the recognition of that higher degree of ripeness in the ovum which is necessary at the time of fructification for the development of a male individual. All the above symptoms occur alike in the ova destined for the male and for the female sex. That ripeness of the ovum upon which Thury’s theory insists, lies in the nature of the ovum apart from any anatomical signs. It is a condition of the ovum which we can only attempt to explain by laying down the principle that an ovum which has been for a longer time prepared in the female generative organs previously to its fertilization, must be riper than another which has had less time for this preliminary process. * * * * * We have mentioned above that on the occasion of Thury’s experiment, the desired result was effected in twenty-nine cases. Pagenstecher, Siebold, and Köll have dealt critically with Thury’s work. Coste was not in a position to confirm these experiments, nor to verify them. In order to test Thury’s results as applied to the human subject, Schröder obtained the assistance of young women, who were in a position to give him positive and accurate information respecting the time at which they became pregnant. The women could name the day on which they had had sexual intercourse, and knew the date of the last menses. From careful calculation of the interval between these dates, it was possible to ascertain approximately at what stage impregnation of the ova took place, the degree of ripeness of the impregnated ova could also be inferred from the space of time that had elapsed since the last menstruation, and the sex of the fœtus was noted. Schröder found that on an average of twenty-six cases in which boys were born, the conception had taken place 10.08 days after menstruation; on an average of twenty-nine cases in which girls were born, 9.76 days after. In consequence, he was not able to confirm Thury’s theory in the case of the human subject. * * * * *