DEVELOPMENT OF THE CIRCULATORY SYSTEM 843 



one vein disappear, leaving one omphalo-mesenteric artery and one vein. 

 Soon after, as the circulation becomes established in the allantois, the 

 vessels of the umbilical vesicle and the omphalo-mesenteric vessels are 

 obliterated and the first circulation is superseded by the second. 



As the septum between the two ventricles makes its appearance, 

 that division of the right aortic arch which constitutes the vascular 

 portion of one of the branchial arches disappears and loses its connec- 

 tion with the abdominal aorta ; a branch, however, persists during the 

 whole of intra-uterine life and constitutes the ductus arteriosus, and 

 another branch is permanent, forming the pulmonary artery. 



TJie Second, or Placental Circulation. As the omphalo-mesenteric 

 vessels disappear, and as the allantois is developed to form the chorion, 

 two vessels the hypogastric arteries are given off, first from the 

 abdominal aorta ; but afterward, as the vessels going to the lower 

 extremities are developed, the branching of the abdominal aorta is such 

 that the vessels become connected with the internal iliac arteries. The 

 hypogastric arteries pass to the chorion through the umbilical cord and 

 constitute the two umbilical arteries. At first there are two umbilical 

 veins ; but one of them afterward disappears, and there is finally 

 but one vein in the umbilical cord. It is in this way the umbilical 

 arteries carrying the blood to the tufts of the foetal placenta, which is 

 returned by the umbilical vein that the placental circulation is 

 established. 



Corresponding to the four visceral arches, which have been 

 described in connection with the development of the face, are four 

 pairs of vascular arches. These, with the two aortae, constitute the 

 five branchial or arterial arches, not counting an unimportant rudimen- 

 tary branch connected with the fourth arch, described by Zimmermann. 

 These arches are numbered from above downward. 



In the development of the arteries distributed to the head and upper 

 extremities, the first change observed is the disappearance of the first 

 arch, its stem being continued forward as the temporal artery, giving 

 off the internal maxillary and perhaps the facial (His). The second 

 arch afterward disappears and is replaced with what is thought to form 

 the lingual artery. The third arch remains and, with the upper part of 

 the posterior aortic root, forms the internal carotid. The common 

 carotid and the external carotid are formed from the anterior aortic 

 root, the posterior and the anterior aortic roots coming from the same 

 primitive vessel. The fourth arch on the right side becomes the sub- 

 clavian artery. It joins also with the anterior aortic root to form the 

 innominate artery. The fourth arch on the left side remains as the 

 arch of the aorta. The fifth arch on the left side becomes the pulmo- 



