304 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



the dorsal aorta with the subintestin al vein in the neighborhood of 



the anus, and later give off vessels to the regTon of the rectum. 

 When, as in all classes, from the amphibia upward, a urinary bladder 

 is developed from the rectal (cloacal) region, the hypogastrics form 

 its blood supply, these vessels being the vesical arteries. In the 

 amniotes the distal end of the anlage of the bladder forms a fcetal 



w 1 s—ii — 



Fig. 323. Fig. 324. 



Fig. 323. — Diagram of early relations of vertebral arteries in an amniote. av, 

 vertebral artery; da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary 

 artery; ra, radix aortae; sa, subclavian; va, ventral aorta; 4-6, aortic arches. 



Fig. 324. — A, side view of developing anterior arteries of Laceria, after van Bem- 

 meln; the vertebral artery not developed behind; B, ventral view of the relations of the 

 arteries at the base of the vertebrate brain, av, vertebral artery; b, basilar artery; cw, 

 circle of Willis; da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary 

 artery; ra, radix aortae; sa, segmental arteries; sc, subclavian; 2-6, aortic arches. 



structure known as the allantois, described in another section (p. 382), 

 and parts of the vesical arteries are carried out as allantoic arteries 

 (figs. 327, 329), into the new formation. Since these pass through 

 the umbilicus, they are also known as the umbilical arteries. Later, 

 when the umbilicus disappears, the allantoic arteries are lost and 

 only the rectal and vesical arteries remain of the hypogastric trunks. 



