432 GENERATION. 



disappear, and we have then but one omphalo-mesenteric ar- 

 tery and one vein. Soon after, as the circulation becomes 

 established in the allantois, the vessels of the umbilical vesi- 

 cle 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 the first circulation is superseded by the second.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 ap- 

 pearance, that division of the right aortic arch which consti- 

 tutes 'the vascular portion of one of the branchial arches dis- 

 appears and loses its connection 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. 



% 



The 8econd, 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 be- 

 come connected with the internal iliac arteries. The hypo- 

 gastric 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 umbili- 

 cal vein, that the placental circulation is established. 



Corresponding to the four visceral arches, which we have 

 described in connection with the development of the face, 1 

 are four vascular arches. One of these disappears, and the 

 remaining three undergo certain changes, by which they are 

 converted into the vessels going to the head and the supe- 

 rior extremities. The anterior arches on the two sides are 

 converted into the carotids and subclavians ; the second, on 

 the left side, is converted into the permanent aorta, and the 



> See page 411. 



