Fig. 246.—Diagram of embryonic vascular system, without the portal circulation. (Parker, after Wiedersheim.) The dorsal aorta is formed by the junction of the right and left aortic roots arising from the confluence of the branchial arterial arches.   Fig. 247.—Diagram of the circulation of the yolk-sac at the end of the third day of incubation in the chick. (After Balfour.) The median portion of the first aortic arch has disappeared; but its proximal end forms the external, its distal the internal carotid arteries. The whole blastoderm has been removed from the egg and is viewed from below. Hence the left appears on the right, and vice versa. Arteries in black. Veins in outline.

1. Early Stage.—In the earlier developmental stages in mammalian embryos the primitive dorsal aorta extends caudad along the ventral aspect of the vertebral axis, giving off paired vitelline or omphalo-mesenteric arteries to the yolk-sac and allantoic arteries to the embryonic urinary bladder or allantois (Figs. 246 and 247).

The blood is returned from the vascular area of the yolk-sac by two vitelline or omphalo-mesenteric veins, which unite near the heart to form a common trunk, continued as the sinus venosus into the caudal or auricular extremity (venous end) of the primitive tubular heart (Figs. 246, 247 and 248).

2. Development of Allantois. Stage of Placental Circulation.—The placental circulation, replacing the temporary vitelline circulation of the earliest stages, is inaugurated by the appearance of two umbilical veins, which pass cephalad, imbedded in the tissue of the ventral mesogastrium, to empty into the sinus venosus near the vitelline veins (Fig. 249). The umbilical veins return the oxygenated blood from the placenta to the embryo. At first the right umbilical vein is the larger of the two.

Fig. 248.—Schema of vitelline veins.   Fig. 249.—Schema of umbilical veins, early stage.

The sinus venosus at this time also receives two large veins, transversely directed, called the ducts of Cuvier, which are formed near the heart by the union of the anterior cardinal (primitive jugular) and posterior cardinal veins, draining respectively the head end of the embryo, and the body walls and Wolffian bodies.

The vitelline veins are placed on each side of the primitive small intestine, and become connected with each other by a broad anastomotic branch (Fig. 249). When the hepatic outgrowth buds from the duodenum the vitelline veins send out branches which break up into a wide-meshed capillary network in the mesodermic tissue enveloping the hepatic cylinders. Hence at this period the circulation in the vitelline veins is made up of three districts:

(a) Distal segment of veins, coursing along duodenum, and joined by a transverse anastomosis, before reaching the liver bud (subintestinal veins).

(b) Middle segment, from which capillary vessels are derived, ramifying upon and between the developing hepatic cylinders.

(c) Proximal segment, formed by the continuation of the proximal part of the vitelline veins into the sinus venosus of the heart.