DEVELOPMENT OF THE CIRCULATORY SYSTEM. 431 



the body of the embryon, it carries the plexus of vessels of 

 the area vasculosa with it, the vessels of communication with 

 the embryon being the omphalo-mesenteric arteries and veins. 

 As these processes are going on, the great central vessel of 

 the embryon .becomes enlarged and twisted upon itself, at a 

 point just below the cephalic enlargement of the embryon, 

 between the inferior extremity of the pharynx and the supe- 

 rior cul-de-sac of the intestinal canal. The excavation which 

 receives this vessel is called the fovea cardiaca. The differ- 

 ent stages of 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 

 currents of the circulation are established. 1 The time of the 

 first appearance of the circulation in the human embryon has 

 not been accurately determined. 



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 aortse, 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 nu- 

 merous 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-mesen- 

 teric 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 



1 BURDACH, Traite de physiologic, Paris, 1838, tome in., pp. 510, 511, 

 158 



