_Placenta._ The placenta is formed essentially by the chorion and decidua; it is a flat, circular, or more or less oval mass, soft, but becoming firmer towards its edge. It is the most vascular part of the ovum, and by which it is connected most intimately with the uterus. Its longest diameter is generally about eight, its shortest about six inches; its greatest thickness is at that spot where the umbilical cord is inserted, which is usually about the middle of the placenta, although it occasionally varies considerably in this respect, the cord coming off sometimes at the edge. The placenta, as ordinarily seen after labour, is barely an inch in its thickest part, but when filled with blood or injection it swells very considerably, and is then little short of two inches. It is generally attached to the upper part of the uterus in the neighbourhood of one of the Fallopian tubes, and more frequently on the left side than on the right; its inner or foetal surface is smooth, being covered by the chorion, which at this part is much thicker. The placenta cannot be distinguished from the other parts of the ovum until the end of the second month, at which period it covers nearly half the surface of the ovum, gradually diminishing in relative size, but increasing in thickness and absolute bulk up to the full period of utero-gestation. It forms a spongy vascular mass, its uterine surface being divided unequally into irregular lobes called _cotyledons_. The uterine surface of a full-grown placenta is covered by a pulpy membrane, resembling in structure the decidua which covers the chorion, and of which it seems to be a continuation. This is always found present at the end of pregnancy: it covers the lobes of the uterine surface of the placenta, descending into the sulci which runs between them: in some parts it is thicker than in others, especially where it is connected with, or in fact becomes, the decidua of the chorion or decidua reflexa. This membrane, which has been called the _placenta decidua_, is pretty firmly attached to the vessels of the placenta, so as not to be separated without rupture; but by maceration, its texture is more or less destroyed, so that we may easily distinguish the extremities of these vessels. "This decidua, or uterine portion of the placenta," says Dr. Hunter, "is not a simple thin membrane expanded over the surface of the part: it produces a thousand irregular processes, which pervade the substance of the placenta as deep as the chorion or inner surface; and are every where so blended and entangled with the ramifications of the umbilical system, that no anatomist will perhaps be able to discover the nature of their union. While these two parts are combined, the placenta makes a pretty firm mass, no part of it is loose or floating; but when they are carefully separated, the umbilical system is evidently nothing but loose floating ramifications of the umbilical vessels, like that vascular portion of the chorion, which