932 



GENERATION. 



development of the heart, which is formed of the twisted portion of the central vessel, 

 will be described farther on. Simple, undulatory movements take place in the heart of 

 the chick at about the middle of the second day ; but there is not, at that time, any 

 regular circulation. At the end of the second day or the beginning of the third, the cur- 

 rents of the circulation are established. The tune of the first appearance of the circula- 

 tion in the human embryon has not been accurately determined. 



FIG. 309. Area vasculosa. (Bischoff.) 

 a, a, Z>, sinus terminals ; c, omphalo-mesenteric vein ; d, heart ; ,/,/, posterior vertebral arteries. 



In the arrangement of the vessels for the first circulation of the embryon, the heart is 

 situated exactly in the median line and gives off two arches which' curve to either side 

 and unite into a single central trunk at the spinal column below. These are the two aorta3, 

 and the single trunk formed by their union becomes the abdominal aorta. The two aortic 

 arches, one of which only is permanent, are sometimes called the inferior vertebral arteries. 

 These vessels give off numerous branches, which pass into the area vasculosa. Two of 

 these branches, however, are larger than the others, pass to the umbilical vesicle, and are 

 called the omphalo-mesenteric arteries. In the embryon of mammals, there are, at first, 

 four omphalo-mesenteric veins, two superior, which are the larger, and two inferior ; 

 but, as development advances, the two inferior veins are closed, and we then have two 

 omphalo-mesenteric arteries and two omphalo-mesenteric veins. At about the fortieth 

 day, one artery and one vein disappear, and we have then but 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 and the first circulation is superseded by the second.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 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 ai'teriosus, and another 

 branch is permanent, forming the pulmonary artery. 



