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 c celiac 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. 



III. 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 venae cavse 

 (venae cavae superiores); (b) the omphalomesenteric and um- 

 bilical veins and the hepatic portal system; (c) the system of the 

 inferior vena cava. 



The anterior venae cavae 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 



