262 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



the ventral aorta to the sixth arch appear later in embryos of 7 mm., but soon 

 degenerate (Fig. 272 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 

 (Fig. 271). The descending aortae are now fused to their extreme caudal ends and 

 the umbiHcal arteries take their origin ventrally. Twenty-seven pairs of dorsal 



First cervical artery 



Pulmonary artery 

 Ant. cardinal vein 

 Post, cardinal vein 



Subclavian artery 

 Caeliac artery 



Vertebral artery 



Olocyst 



Dorsal aorta' 



Vena capitis medialis 



Biilbns cordis 

 Opiitlmlmic artery 

 A nt. cerebral artery 

 Common cardinal vein 



Vitelline artery 

 (Superior mesenteric) 



Caudal artery 

 Umbilical artery 



Inf. mesenteric artery 



Fig. 271. — Arteries and cardinal veins of the right side in a 4.9 mm. human embryo (modified after 

 Ingalls). X 20. H, Heart; I-VI, aortic arches. 



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

 the subclavian arteries of the upper limbs arise. Of the ventral viteUine vessels 

 three are now prominent, the caeliac artery in the stomach-pancreas region, the 

 vitelline or superior mesenteric in the small intestine region, and the inferior mesen- 

 teric of the large intestine region. 



Of the aortic arches the third pair is largest at 5 mm. (Fig. 272, ^4). From 

 the sixth pair are given off the small pulmonary arteries to the lungs. At 7 mm. 

 the first and second aortic arches are obliterated (Figs. 272 B and 273), but the 



