The development of the venous system, especially in connection with the liver, presents a somewhat complicated series of successive conditions. After having become familiar with the principal typical embryonal stages, as shown in the following diagrams, the student is strongly recommended to cement this knowledge by the comparative examination of the venous system. The permanent veins of the lower vertebrates, while in many cases not strictly homologous to those of the higher forms, yet are excellent objects for study, since they serve to illustrate temporary stages in the development of the mammalian venous system, and to that extent are of aid in comprehending one of the most difficult and important chapters in human anatomy. At the conclusion of the diagrammatic consideration of the mammalian development a number of comparative facts will be put together for this purpose. =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). [Illustration: 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.] 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). [Illustration: 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.] [Illustration: FIG. 248.--Schema of vitelline veins.] =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. [Illustration: FIG. 249.--Schema of umbilical veins, early stage.]