The Placenta is an organ most important to the mammalian embryo. It conveys not only nourishment but also oxygen from the maternal blood to that of the foetus. It is, of course, necessary that the animals whose ova do not contain large stores of food should in some way provide the substances necessary for the life of their embryo, and it is by means of the placenta that this is brought about. The embryo of oviparous animals does not require a placenta for its nutrition, since there is inside the egg a large store of highly nutritious albuminous and fatty materials; the shell is pervious to air, and the chick's blood can in the allantois be oxidized by the air directly. A bird's egg contains in itself all the necessaries which the placenta supplies, and when impregnated only requires the heat of the mother's body to develop a chick.

While an ovum is descending the Fallopian tube, the mucous membrane of the uterus becomes turgid, and, as before mentioned, if the ovum be unimpregnated it is cast out of the body, part of the substance of the lining membrane of the uterus is desquamated and discharged with a fluid largely composed of blood. This takes place approximately every four weeks, and hence is called menstruation. If the ovum be impregnated, however, the mucous membrane of the uterus not only becomes turgid, but its cells proliferate, and considerable thickening of the tissue takes place. The mucous membrane is then called the decidua. When the ovum reaches the uterus it ordinarily becomes embedded in that part of the decidua which occupies the fundus of the uterus. The decidua here grows excessively, and becomes much thickened, and on either side of the ovum a projection is sent from the decidua which meets below the ovum, and completely encircles it.

The name decidua vera is given to the membrane lining the general cavity of the uterus, while that part lining the fundus, to which the ovum is attached, is called the decidua serotina, and its processes surrounding the ovum receive the name of the decidua reflexa.

The placenta is developed from two sources, one arising from the membranes of the foetus, and the other belonging to the mother.