called the umbilical arteries; these will, however, be further described when treating of the foetal placenta.

As the foetus becomes developed, the part of the allantois in connection with the body becomes gradually obliterated. A part of it remains as the urinary bladder, and the rest forms a fibrous cord, which runs from the apex of the bladder to the umbilicus, and is known as the urachus.

(4) The Chorion is the external covering of the ovum. At first it consists simply of the zona pellucida or vitelline membrane, and then it is called the primitive chorion. Later it is supplemented by that part of the somatopleure removed from the embryo in the process of forming the amnion. This blends with the primitive chorion and strengthens it, and while lying beneath the zona pellucida, receives the name of the subzonal membrane. The chorion at first is a smooth membrane, but villous processes early grow out from it. These villi are chiefly developed at its upper part, where they aid in the formation of the foetal placenta.

The allantois, when it has spread over the chorion, becomes blended with this membrane, and fills the villous processes with the blood vessels it contains.

THE PLACENTA.

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.

Relation of the Fatal to Maternal Placenta.-The maternal part is formed from the decidua serotina, which becomes much thickened and very vascular where the placenta is attached. The foetal 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

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 (copied from Dallon).

1. Ovum entering the congested mucous membrane of the fundus-decidua serotina. 2. 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.

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

Antero-posterior 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. Decidua vera. ch. Chorion. u.u. Uterine cavity. c. Cavity of the cervix.

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.

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 fœtus and those of the mother, and therefore it is not possible to inject the vessels of the mother through those of the fœtus, 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 fœtus 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 fœtal blood receiving oxygen from the oxyhæmoglobin of the mother, to which it gives up its CO,.

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