In the case of a human embryo of less than five weeks, for instance, the sex to which it would afterwards belong cannot be recognised. In the fifth week of fœtal life processes begin which, by the end of the fifth month of pregnancy, have turned the genital rudiments, at first alike in the sexes, into one sex and have determined the sex of the whole organism. The details of these processes need not be described more fully here. It can be shown that however distinctly unisexual an adult plant, animal or human being may be, there is always a certain persistence of the bisexual character, never a complete disappearance of the characters of the undeveloped sex. Sexual differentiation, in fact, is never complete. All the peculiarities of the male sex may be present in the female in some form, however weakly developed; and so also the sexual characteristics of the woman persist in the man, although perhaps they are not so completely rudimentary. The characters of the other sex occur in the one sex in a vestigial form. Thus, in the case of human beings, in which our interest is greatest, to take an example, it will be found that the most womanly woman has a growth of colourless hair, known as “lanugo” in the position of the male beard; and in the most manly man there are developed under the skin of the breast masses of glandular tissue connected with the nipples. This condition of things has been minutely investigated in the true genital organs and ducts, the region called the “urino-genital tract,” and in each sex there has been found a complete but rudimentary set of parallels to the organs of the other sex. These embryological conclusions can be brought into relation with another set of facts. Haeckel has used the word “gonochorism” for the separation of the sexes, and in different classes and groups of creatures different degrees of gonochorism may be noted. Different kinds of animals and plants may be distinguished by the extent to which the characters of one sex are rudimentary in the other. The most extreme case of sexual differentiation, the sharpest gonochorism, occurs in sexual dimorphism, that is to say, in that condition of affairs in which (as for instance in some water-fleas) the males and females of the same species differ as much or even more from each other as the members of different species, or genera. There is not so sharply marked gonochorism amongst vertebrates as in the case of crustacea or insects. Amongst the former there does not exist a distinction between males and females so complete as to reach sexual dimorphism. A condition much more frequent amongst them is the occurrence of forms intermediate in regard to sex, what is called abnormal hermaphroditism; whilst in certain fishes hermaphroditism is the normal condition.