270 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



mesenteric artery. By the enlargement of capillaries connecting the ventral and 

 dorsal aortae a second pair of aortic arches is formed at this stage (Fig. 263). 



Development and Transformation of the Aortic Arches. In embryos 4 to 

 5 mm. in length five pairs of aortic arches are successively developed, the first, 

 second, third, fourth and sixth (Fig. 264) . An additional pair of transitory vessels 

 which extend from the ventral aorta to the sixth arch appear later in embryos of 



Aortic arch 3 



Aortic arch 

 z 



Aortic arch 

 l 



Dorsal 

 Aortic arch 4- 

 Aortic arch 6 



Esophagus 

 Trachea 



Ventral aorta. 



\Pulmonary artery 

 Bui bus cord is 



Aortic arch 5 . 



Int. carotid artery. 

 Aortic arch Z 



Aortic arch 6 

 Pulmonary artery 



Bui bus cord is 



Aortic arch A 



Aortic arch 5 



Dorsal aorta- 



FIG. 265. Aortic arches of human embryos: A , of 5 mm.; B, of 7 mm. (after Tandler). I, II, III, IV, 



pharyngeal pouches. 



7 mm. but soon degenerate (Fig. 265, B}. They are interpreted as being the fifth 

 pair in the series. From each dorsal or descending aorta there develop cranially 

 the internal carotid arteries. These extend toward the optic stalks where they 

 bend dorsad and caudad, connecting finally with the first intersegmental arteries 

 of each side. The descending aortae are now fused to their extreme caudal ends 

 and the umbilical arteries take their origin ventrally. Twenty-seven pairs of 

 dorsal intersegmental arteries are present. From the seventh cervical pair of these 



