The maternal part is formed from the decidua serotina, which becomes much thickened and very vascular where the placenta is attached. The fcetal placenta is derived from the chorion, which sends out a number of finger-like processes, which subdivide and into which the allantois, as it spreads over the chorion, sends prolongations. The mesoblastic layer of the allantois gives rise to the capillaries which are in these processes. The capillaries spring from the branches of the umbilical arteries which pass along the umbilical cord to reach the chorion. The vessels of the decidua serotina or maternal placenta end in large sinuses, lined by endothelial cells. The blood is carried to these sinuses by the uterine arteries, and from them by the uterine veins. The walls of the sinuses are provided with unstriped muscular tissue, which can close the inlets from the arteries, and thus shut out the blood. The villi of the foetal placenta, dipping into these uterine sinuses, are covered with a single layer of thin, scaly cells, so that the foetal blood is only separated from the maternal by the walls of the capillaries and these thin cells, and thus the interchange of nutrient materials and gas readily go on between them; it is very similar to the conditions of the lung alveoli, where the blood is separated from the air with which it interchanges gases by the cells of the capillary wall and of the lung alveolus.

Series of diagrams representing the relationship of the decidua to the ovum at different periods.

Fig. 275. Series of diagrams representing the relationship of the decidua to the ovum at different periods. The decidua are colored black, and the ovum is shaded transversely. In 4 and 5 the vascular processes of the chorion are figured (copiedfrom Dalton).

1 Ovum entering the congested mucous membrane of the fundus - decidua serotina. 5. Decidua reflexa growing round the ovum. 3. Completion of the decidua around the ovum. 4. General growth of villi of the chorion. 5. Special growth of villi at placental attachment, and atrophy of the rest.

Anteroposterior section through a gravid uterus and ovum of five weeks (semi diagrammatic).

Fig. 276. Anteroposterior section through a gravid uterus and ovum of five weeks (semi-diagrammatic). (Allen Thomson).

a. Anterior wall of uterus, p. Posterior wall of uterus, m. Muscle substance, g. Glandular layer, ss. Decidua serotina. r. Decidua reflexa. v. Decidura vera. ch. Chorion. uu. Uterine cavity, c. Cavity of the cervix.

Though the capillaries of the foetus are in such close relation to the blood of the mother, it must be distinctly understood that there is no direct communication between the vessels of the foetus and those of the mother, and therefore it is not possible to inject the vessels of the mother through those of the foetus, or vice versa.

The nutrient materials from the maternal blood together with oxygen diffuse through the walls of the foetal capillaries, the effete matter, on the other hand, passing from the capillaries to the blood in the veins which surrounds and bathes these vessels. The placenta increases with the growth of the foetus till shortly before birth, when it is said to undergo a certain amount of degeneration. It is cast out of the uterus after the expulsion of the foetus with the membranes attached to it. It is, however, only the superficial layer of the maternal placenta (which is intimately connected with the foetal placenta) that is cast off, the deeper layer remaining in the uterus, and undergoing various changes during the reduction of this organ to its normal size.

After ligature of the umbilical cord, the intimate relationships of the maternal and foetal circulation cease, and it is thought that this causes the inlets of the uterine sinuses to contract, so that when the placenta separates from the uterine walls, the arterioles leading to the sinuses are contracted and possibly occluded with clots. The uterine blood current is thus prevented from escaping into the uterine cavity after parturition, and causing profuse hemorrhage.

The uses of the placenta may be briefly summed up as:

(1) Alimentary, as it supplies the place of the organs of digestion by supplying the foetal blood with nutritive material.

(2) Respiratory, as it performs the function of the lungs, the foetal blood receiving oxygen from the oxyhaemoglobin of the mother, to which it gives up its C02.

(3) Excretory, as it does duty for the kidneys, removing the urea, etc., from the foetal blood.