In the _uterus bicorporeus_, the union of the cornua is higher up, so that the lower portion is single, while the upper part alone is double, consisting of two strongly curved cornua. This conformation is peculiar to ruminating animals. If two ova be present they are separate from each other, each being contained in its own distinct body or cornu, but a portion of the membranes extends along the common cervix, from one body to the other. [Illustration: Uterus bifundalis.] A still higher grade is the _uterus bifundalis_, where the fundus alone is double, the cornu being formed only by this portion. This formation is observed in the horse, ass, &c.: the common cavity is here the receptacle of the ovum, so that in the unimpregnated state, the cornua appear only as appendices, into which a portion of the membranes extend. In the _uterus biangularis_, the double formation has nearly disappeared, except at the fundus, where the uterus imperceptibly passes into the tubes: this is the case among the edentata, and some of the monkey tribes. The highest grade is the _uterus simplex_: every trace here of the double form is lost; the fundus no longer forms an acute angle, where it bifurcates into two cornua; but is convex. We now for the first time see the divisions of the uterus into body and cervix distinctly marked. [Illustration: Uterus biangularis.] The human uterus presents a similar variety of forms, as it gradually rises in the scale of development during the different periods of utero-gestation. It is at first divided into two cornua, and usually continues so to the end of the third month, or even later; the younger the embryo the longer are the cornua, and the more acute the angle which they form; but even after this angle has disappeared, the cornua continue for some time longer. [Illustration: Uterus simplex.] The uterus is at first of an equal width throughout; it is perfectly smooth and not distinguished from the vagina either internally or externally by any prominence whatever. This change is first observed when the cornua disappear and leave the uterus with a simple cavity. The upper portion is proportionably smaller, the younger the embryo is. The body of the uterus gradually increases, until at the period of puberty it is no longer cylindrical, but pyriform: even in the full-grown foetus the length of the body is not more than a fourth part of the whole uterus; from the seventh even to the thirteenth year it has only a third, nor does it reach a half until puberty has been fully attained. The os tincæ or os uteri externum first appears as a scarcely perceptible prominence projecting into the vagina; it increases gradually, in size until the latter months of gestation, when the portio vaginalis is relatively much larger than afterwards.