A. Embryo of four weeks. (Kollfker after Coste).

B. Anterior, and C. posterior views of the heart of an embryo of six weeks. (K'olliker after Ecker.) 1. Upper limit of buccal cavity, c. Buccal cavity, b. Lies between the ventral ends of the 2d and 3d branchial arches, d. Buds of upper limbs, e. Liver, f. Intestine. 1. Superior vena cava. i'. Left superior vena cava or connection between the left brachio-cepha-lic vein and the coronary vein. 1". Opening of inferior vena cava. 2. 2/ Right and left auricles. 3. 3' Right and left ventricles. 4. Aortic bulb.

The primitive heart is connected at its upper end with the two aortae, and at its lower end with the omphalo-mesenteric veins.

After a time the tube shows signs of division into three parts; the upper part becomes the aortic bulb, next to which is formed the cavity of the ventricle, continuous with which is the auricular space. The tube also, which at first lies in a straight line, now becomes twisted on itself, the auricular part becoming posterior and superior, while the ventricle, with the aortic bulb, remains anterior and somewhat below.

Diagram of the circulation of a chick at the end of the third day.

Fig. 304. Diagram of the circulation of a chick at the end of the third day. (Foster and Balfour).

H. Heart. A A. Aortic arches (2d, 3d and 4th). Ao Dorsal aorta. L. of A., R. of A., Right and left omphalomesenteric arteries..S. T. Sinus terminalis. R. of, and L. of. Right and 1 ft omphalo-mesenteric veins. S. V. Sinus venosus. D. C. Duct of Cuvier. S. Ca. and V. Ca. Superior and inferior cardinal veins.

Each primitive cavity of the heart is divided into two by the gradual growth of partitions, and thus the four permanent heart cavities are developed.

Externally a notch shows the division of the ventricle into right and left cavities, while from the inside of the right wall there grows a projection which subdivides the ventricle internally. This septum is, however, not at once complete at its upper part, a communication between the right and left sides of the heart remaining for some time above this partition. With the growth of the inter-ventricular septum, the external notch becomes less prominent, but is permanently recognizable as the inter-ventricular groove.

In the auricles a fold develops from the anterior wall, which ultimately unites with a process of later development from the posterior wall. This septum is not complete during foetal life, but is interrupted by an opening leading from one auricle to the other, called the foramen ovale.

Simultaneously with the appearance of the posterior process of the septum, another fold arises, which is placed at the mouth of the inferior vena cava, and forms the Eustachian valve.

The aortic bulb likewise, by a projection from the inner wall of the cavity, becomes divided into two canals, the anterior of which remains in continuity with the right ventricle, while the posterior is continuous with the left ventricle. The anterior thus becomes the pulmonary artery, and the posterior the permanent aorta.

The primitive circulations of a human embryo may be divided into two, which differ in their time of appearance and in the accessory organs to which they are distributed. Though they may, for the sake of clearness, be described as two independent circulations, they are not strictly so, as they exist for a short time coincidently, and arise in connection with one another from the same heart.

(a) The earlier or vitelline circulation is that which is directed to the yolk sac, the embryo obtaining nourishment from the vitellus or yolk; this is an organ of quite secondary importance in the mammalian embryo, and hence this circulation may be better studied in some such animal as the chick, which depends, throughout its embryonic life, on the vitellus for nourishment. In the human embryo the vitelline circulation is chiefly of importance for the few days immediately preceding the development of the placental circulation.

The aortic bulb is continuous with two vessels which run on either side of the primitive pharynx; these are the aortae, and from each of them a large branch is given off. These omphalomesenteric arteries pass to the yolk sac, and there become split up into a number of small vessels, the blood from them being returned partly by corresponding omphalo-mesenteric veins, partly by a large vein running round the periphery of the vas-60 cular area known as the sinus terminalis. The sinus terminalis opens partly into the right and partly into the left omphalomesenteric veins, which subsequently unite into a common venous trunk, called the sinus venosus, which is continuous with the . primitive auricle.

Diagram of the vascular system of a human foetus.

Fig. 305. Diagram of the vascular system of a human foetus. (Huxley).

H. Heart. T. A. Aortic trunk, c. Common carotid artery, c'. External carotid artery. c'. Internal carotid artery, s. Subclavian artery v. Vertebral artery. 1 2.3.4.5, Aortic arches. A'. Dorsal aorta. 1. Omphalo-mesenteric artery, dv. Vitelline duct, o'. Om-phalo-mesenteric vein. v'. Umbilical vesicle. vp. Portal vein L. Liver, uu Umbilical arteries. u"u". Their endings in the placenta u'. Umbilical vein. Dv. Ductus venosus. vh. Hepatic vein. Cv. Inferior vena cava. vil. Iliac veins, az. Vena azygos. vc. Posterior cardinal vein. DC. Duct of Cuvier. P. Lungs.

This vitelline circulation in the human embryo persists but a short time. After the fifth or sixth week of foetal life it becomes obliterated, the yolk then being atrophied, and the placental circulation well developed.

(b) The later or placentalcirculation is developed in the meso-blastic layer of the allantois, especially in that part which is in relation with the decidua serotina. The allantois, when fully developed, extends to the chorion, over which it spreads, sending in processes to occupy the villi. These chorionic villi are embedded in the decidua of the uterus, and are especially developed at the upper part, which is in connection with the decidua serotina or maternal placenta.

The primitive aortae, which were at first two separate tubes, become united in the dorsal region of the embryo, so that the two aortic arches end in a single vessel, which extends to the middle of the embryo, and there divides into two branches, each of which gives off a vessel called the vitelline or omphalomesenteric artery.

From the branches of the aortae arise two large vessels, which, running along the allantois, spread out over the chorion, being especially directed to the upper part of this membrane; these are the umbilical or hypogastric arteries, which carry the blood from the aortae to the foetal placenta.

Veins arise from the terminal networks of these arteries, and combine to form the two umbilical veins. ' The umbilical veins take a similar course to the arteries, and convey the blood to the venous trunk formed by the junction of the omphalo-mesenteric veins.

After a time the right umbilical and right omphalo-mesenteric veins disappear, while from the trunk formed by the junction of the left umbilical and left omphalo-mesenteric veins, branches are given off to the liver (yenee advehentes), and at a point nearer the heart, vessels are received from the liver (venae, revchentes).

To the part of the vessel intervening between the origin of the venae advehentes and the entrance of the venae revehentes is given the name of the ductus venosus.

Thus it may be seen that in the placental circulation the blood is conveyed from the aorta, by the umbilical arteries, to the foetal placenta, undergoes changes, owing to its close relation-ship to the maternal blood. From the placenta it is returned by the umbilical vein, which sends a part through the liver and a part direct to the heart. The more minute details of foetal circulation will be described later on.

Diagram of the heart and principal arteries of the chick.

Fig. 306. Diagram of the heart and principal arteries of the chick. (Allen Thomson.) B. and C are later than A.

1, 1. Omphalo-mesenteric veins. 2. Auricle. 3. Ventricle. 4. Aortic bulb. 5, 5. Primitive aortae. 6, 6. Omphalo-mesenteric arteries A. United Aorta.