Amongst cattle and sheep the first-born are more often females. (Girou.) Also in the human species a greater number of females are observed amongst the first-born. Here, on the one hand, regular intercourse with the male is to some extent unfamiliar; whilst, also, according to Thury, the choice of the date of marriage might be of importance. The constancy of the births may be explained by the regularity of the intercourse of the two parents in consequence of the reciprocal ties which surround family life. From illegitimate births more girls result than from marriage. The reason of this might be sought in the influence of the active excitement of the female at the time at which the conception took place, that is, shortly after menstruation, when the woman is most easily excitable. * * * * * The various theories respecting the explanation of the production of sex which are known to us, from the earliest accounts down to the modern predominance of natural philosophy, have been collected by W. His. (‘_Archiv für Anthropologie_,’ Band 4, 5.) Since then, it is true, during the course of successive studies, and more especially of those which have been made during the present century, many substantial alterations have appeared in the views held respecting the cause of the different development of sex. We have seen this above. Thus, discoveries have been made which have exercised a very wide influence, for example, that of the ovum in man and in the other mammalia (Baer, 1828), or the penetration of the spermatozoa into the protoplasm of the ovum as a necessary condition of fertilization. In later times it has been made certain that the head of the spermatozoon is a nucleus, and that only one spermatozoon penetrates into the interior of the ovum. Afterwards its head as a nucleus unites with a nucleus-part of the ovum, forming a new nucleus in the ovule, which, together with the surrounding protoplasm, serves as the point of departure of the further processes of development, and in this condition is described as an oosperm. After this follow other extensive details of the process of development, which will not be described here. It may be already seen, from the few principal factors of the process mentioned, that our theory respecting the development of the sex in the embryo will have to be substantially altered. We shall here adduce only the fundamental doctrines respecting the development of the generative organs laid down by Waldeyer in his masterly work on the ovary and the ovum. The teaching for which we are indebted to His, Kölliker, Schäfer-Korschelt, Heider, Duval, Kollmann, Minot, Bonnet, Bergh, Prénant, Balfour, Romiti, Kuppfer, and others, must also be well remembered, as well as the acquisitions of new information connected with the physiology of the embryo, and, of quite recent date, the mechanism of development (Roux), which can acquaint us more especially