Next after a number of theories which have been current on this subject, we reach some others, to which more or less value has been attributed. Morello attached weight to the concentration of the semen. Thin, liquid semen was to be favorable to the production of females; thick semen produced males. According to modern theories, based upon observation of invertebrate animals (O. Hertwig, Balfour, Landois, Minot, and others), it appears improbable that this position can be maintained. The supporters of this theory assert that a single spermatozoon suffices for the fertilization of an ovule, or to develop the ovum into a so-called “oosperm,” that is to say, to form a really fertilized ovum. (Perhaps the excess of semen serves for the earliest nutritive processes of the ovum; which would, however, be difficult to prove.) This fact has been also confirmed in the case of the higher animals, and it might be in a similar manner brought to an issue in the case of man. However, observations of this kind have not yet proved possible with man. But in the case when several spermatozoa penetrate the interior of a single ovum, anomalies in the process of development result from the formation of several nuclei. Such ova, also, perhaps in consequence of excess of semen, are very often aborted and perish. In order to throw light on the causes determining sex, Pflüger (in connection with results obtained by Born, which will be cited later) attempted to determine the relative numbers of the sexes, under normal conditions, in the case of the frog (_Rana fusca_). The numbers were taken by his pupils, A. von Griesheim and Dr. W. Kochs. The identification of the sex was made with a microscope under the supervision of Pflüger. According to Pflüger the Graafian follicles are easily identified with the aid of a microscope, if they are not in their earliest stage. They contain an ovum with a scanty yolk and large germinal vesicles with germinal markings. The whole is surrounded by connective tissue. According to Pflüger, the epithelium is wanting in such very young follicles. In these frogs after their metamorphosis the testicle consists of tubes, with multinuclear epithelia, and is easily distinguishable from the always large ovaries of the tadpoles. By different concentrations of semen, Pflüger attempted to influence the proportion between the male and female sexes.