590 EMBRYOLOGY. 



(d) The truncus arteriosus is divided into pulmonary artery and 



aorta by the development of a special partition, which 

 begins above, grows downward, and joins the ventricular 

 partition. 



(e) The complete separation of the atria first takes place after 



birth by the permanent closure of the foramen ovale. 



11. At the ostium atrioventriculare and at the ostium arteriosum 

 the first fundaments of the valves are formed as thickenings of the 

 endocardium (endocardial cushions) projecting inward. 



Development of the Chief Arterial Trunks of Man and Mammals. 



12. From the truncus arteriosus there arise five pairs of visceral- 

 arch vessels (aortic arches), which run along the visceral arches, 

 embrace the head-gut laterally, and unite dorsally to form the two 

 primitive aortse. 



13. The two vessels fuse at an early period to form the unpaired 

 aorta lying under the vertebral column. 



14. In Mammals, of the five pairs of visceral-arch vessels the first 

 and second degenerate ; the third furnishes the proximal part of the 

 carotis interna ; the fourth arch becomes on the left side the aortic 

 arch, on the right side the arteria anonyma brachiocephalica and 

 the proximal part of the subclavia ; [the fifth early disappears ;] the 

 fifth [sixth] arch gives off branches to the lungs, and becomes the 

 pulmonary artery, but on the left side remains until the time of 

 birth in open communication with the aortic arch through the 

 ductus Botalli, whereas the corresponding portion on the right side 

 atrophies. 



15. After birth the ductus BotalU is closed and converted into the 

 ligament of the same name. 



16. From the aorta two pairs of large arterial trunks go to the 

 foetal membranes — to the yolk-sac the vitelline arteries (arterise 

 omphalomesentericse), to the allantois and placenta the umbilical 

 arteries. 



17. The vitelline arteries subserve the vitelline circulation, and 

 afterwards, with the reduction of the umbilical vesicle, degenerate. 



18. The iimbUical arteries, which continually become larger with 

 the increasing development of the placenta, arise from the lumbar 

 portion of the aorta, pass forward [ventral] in the lateral wall of the 

 pelvis, then at the side of the bladder and along the inner surface of 

 the abdominal wall to the umbilicus and umbilical cord. 



