LATER DEVELOPMENT OF VASCULAR SYSTEM 363 
a longitudinal anastomosis communicating with the internal 
carotid in front. 
The two omphalomesenteric arteries are originally independent 
(Chap. V), but as the dorsal mesentery forms, they fuse in a 
common stem extending to the umbilicus. The anterior mesen- 
teric artery arises from this. The coeliac and posterior mesen- 
teric arteries arise independently from the dorsal aorta (Fig. 216). 
Mesonephric arteries arise from the ventro-lateral face of the 
dorsal aorta and originally supply the glomeruli; they are very 
numerous at ninety-six hours, but become much reduced in 
number as the renal portal circulation develops; some of them 
persist as the definitive renal and genital arteries. 
The umbilical arteries arise from the same pair of segmental 
arteries that furnishes the primitive artery of the leg. Thus 
on the fourth day the umbilical arteries appear as branches of 
the sciatic arteries; but later the umbilical arteries become much 
larger than the sciatic (Fig. 216). The right umbilical artery is, 
from the first, smaller than the left. On the eighth day its inter- 
mediate portion in the region of the neck of the allantois is much 
constricted, and it gradually disappears. The caudal artery is 
the narrow posterior extremity of the dorsal aorta behind the 
umbilical arteries. 
I do not find a stage in the chick when the umbilical arteries unite 
directly with the dorsal aorta by way of the intestine and dorsal mesen- 
tery, though no doubt indirect connections exist at an early stage. In 
mammals (Hochstetter) the primitive umbilical artery has such a 
splanchnic course, but a secondary connection in the somatopleure soon 
replaces the primary splanchnic path. 
Il]. THe Venous System. (See Chapter VI for origin of the 
first venous trunks) 
We shall take up the development of the venous system in 
the following order: (a) the system of the anterior vene cave 
(venz cave superiores); (b) the omphalomesenteric and um- 
bilical veins and the hepatic portal system; (c) the system of the 
inferior vena cava. 
The anterior vene cave are formed on each side by the 
union of the jugular, vertebral, and subclavian veins. The jugular 
is derived from the anterior cardinal veins, which extend down 
the neck in close proximity to the vagus nerves. The embryonic 
