The blood is returned from the head by the two primitive jugulars, which unite with the cardinal veins conveying the blood from the trunk and lower extremities to form a vessel on each side, called the duct of Cuvier.

From the lower extremity of the embryo the inferior vena cava commences by the union of the external iliac veins; this passes up and opens into the venous trunk common to the left vitelline and left umbilical veins.

The left vitelline becomes continuous with the vessels from the common trunk going to the right side of the liver (the right vena advehens), and forms the main trunk of the portal vein (Fig. 308, B. and D.).

At this stage of the formation of the veins there are three trunks opening into the auricle, the right and left ducts of Cuvier and the inferior vena cava.

As development proceeds, the lower parts of the cardinal veins join the external iliac veins, forming the common iliacs, and so return their blood into the inferior vena cava.

The upper parts of the cardinal veins become continuous with the posterior vertebral veins which convey the blood from the parietes of the embryo. Between the latter a communicating branch is established, which helps in the formation of the azygos vein.

The ducts of Cuvier, which at first were placed almost at right angles to the auricle, become more oblique in their direction as the heart descends.

Between the primitive jugular veins a cross branch is developed, which conveys the blood from the left side of the head and upper extremity of the duct of Cuvier of the opposite side.

The left duct of Cuvier, below the communicating branch, atrophies and forms part of the coronary veins of the heart.

The connection between this and the vein above the cross branch being, in the adult, represented by a small vein, or a band of fibrous tissue, called the vestigial fold of the pericardium.

The cross branch from the left to the right jugular becomes the left innominate vein. The right duct of Cuvier and the right jugular, below the entrance of this cross branch, form the superior vena cava; while the part of the right primitive jugular immediately above the entry of the left innominate vein forms the right innominate vein.

Diagram illustrating the circulation through the heart and the principal vessels of a foetus.

Fig. 309. Diagram illustrating the circulation through the heart and the principal vessels of a foetus. (Cleland).

a. Umbilical vein. b. Ductus venosus. f. Portal vein. e. Vessels to the viscera. d. Hypogastric arteries, c. Ductus arteriosus.

The posterior vertebral vein of the right side forms the vena azygos major; the corresponding branch of the opposite side, together with the part of the left primitive jugular below the cross branch, forms the left superior intercostal vein and the superior vena azygos minor. The lower part of the left posterior vertebral vein, together with the connecting branch to the right vein, remain as the inferior vena azygos minor.