DEVELOPMENT OF THE ARTERIES 



267 



2. The lateral (visceral) branches of the descending aortae are not segmentally 

 arranged. They supply structures arising from the nephrotome region (meso- 

 nephros, sexual glands, metanephros, and suprarenal glands). From them later 

 arise the renal, suprarenal, inferior phrenic, and internal spermatic or ovarian 

 arteries. 



3. The ventral {splanchnic) branches are at first rather definitely intersegmen- 

 tal. Primitively they form the paired vitelline arteries to the yolk sac (Figs. 

 268 to 270). Coincident with the degeneration of the yolk sac the prolongations 

 of the ventral vessels to its walls disappear, and the paired persisting arteries, 

 passing in the mesentery to the gut, fuse to form unpaired vessels from which 



Seventh segmental 

 artery 



Cceliac artery 

 V. pancreas 



Yolk stalk 

 Vitelline artery 



Mesonephric arteries 



Dorsal aorta 



R. umbilical artery 

 Cloaca 



Tenth dorsal segmental artery 

 Dorsal aorta 



Cceliac axis 



Vitelline artery 



R. umbilical 

 artery 



Inf. mesenteric 

 artery 



Common iliac artery 

 A B 



Fig. 278. — Reconstructions showing the development of the umbiUcal and iliac arteries (after Tandler) : 

 A, 5 mm. human embryo; B, 9 mm. human embryo. 



three large arteries are derived, the cceliac artery, the superior mesenteric, and the 

 inferior mesenteric (Fig. 271). 



The primitive coeliac axis arises opposite the seventh intersegmental artery. Together 

 with the mesenteric arteries, it migrates caudalward until eventually its origin is opposite 

 the twelfth thoracic segment (Mall). This migration, according to Evans, is due to the un- 

 equal growth of the dorsal and ventral walls of the aorta. Similarly, the superior mesenteric 

 artery is displaced caudad ten segments, the inferior mesenteric artery three segments. 



The Umbilical and Iliac Arteries. — As previously described, the umbilical 

 arteries arise in young human embryos of 2 to 2.5 mm. from the primitive aortae 

 opposite the fourth cervical segment. They take origin from a plexus of ventral 



