The longer the ovum exists free from the ovary, the longer it remains without the surrounding of those elements (contained in the ovary) which are necessary to it. Apparently, in consequence of the absence of these elements, it is all the time drawing nearer extinction, or it may at least become gradually less capable of maintaining its own sex--which is feminine. At least, it appears, before it is overtaken by the total extinction which threatens it if it is not fertilized, to lose the energy necessary, when fertilization ensues, to maintain its sex, and so may become fitted to assume the opposite masculine sex. This much, however, seems to be quite certain, according to Mayerhofer, that the human ovary does not contain male and female ova already possessed of sex. Equally impossible is it to imagine male and female seminal filaments (spermatozoa) already existing in the organism, and provided with distinct capacities for generating definite sexes. We are unacquainted with any special anatomical signs indicating any such distinction, and do not, even after microscopic investigation, find ourselves prompted to assume the existence of such distinct forms as would allow us to conclude the existence of so fundamental a difference. It is true that in many of the lower animals different forms of spermatozoa are known. These are developed in one and the same testicle, and under the microscope whisk about confusedly with vivacious movements. We find this in the case of a kind of snail (_Murex brandaris_). If we observe a drop of the semen of this creature diluted with sea water, the greater number of the spermatozoa, possessing head, central portion, and tail, move about very energetically. Amongst them are other spermatozoa, distinctly larger and of different form, whose shape suggests spindle-like elements, ending in thin thread-like tails. All these objects exhibit a striking vivacity of movement. Whether these objects represent a particular kind of spermatozoa (as some have supposed), exhibiting definite sexual character, or whether they are not cells, striking on account of their movements, out of which spermatozoa are developed (the so-called spermatides or spermatogonia, transition forms out of which spermatozoa are developed), is at present an open question. H. A. Pagenstecher attempted an important readjustment of Thury’s theory, and tried to show that it might be made concordant with what had been elucidated by previous observations. He holds fast to the axiom (Joh. Muller, Home, Geoffrey St. Hilaire, etc.), that the embryo is at first sexless, and that the ovum after its fertilization still has this character, and must possess the potentiality of developing its sex in two different directions. The factors which determine sex must be external to the embryo.