THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



243 



side, arise from the dorsal aorta in the lumbar region of the embryo and pass out 

 through the belly stalk (later the umbilical cord) to end in the chorionic villi. 

 These are known as the allantoic or umbilical arteries. The blood from the 

 chorionic villi is carried back to the embryo by the umbilical veins which, 

 although a single trunk in the umbilical cord, pass cranially through the body wall, 

 one on each side, and open into the ducts of Cuvier (Figs 216 and 217). The 

 course and distribution of the blood vessels in the placenta have been considered 

 in the chapter on foetal membranes (see p. 131): 



The Arteries. The simplest form of the arterial system is as follows: 

 The single dorsal aorta extends from the cervical region to the caudal end of 

 the embryo and is situated in the medial line ventral to the notochord. In the 

 cervical region a branch extends cranially on each side of the medial line, the 

 two forming the dorsal aortic roots. From the latter, other branches arise and 



Dors, aortic root 



Vent, aortic root 



Vent, aortic trunk 



Dors, aortic root 



A 





Oesophagus 



Trachea 

 Pulmonary artery 



FIG. 218. From reconstruction of aortic arches (i, 2, 3, 4, 6,) of left side and 



pharynx of a 5 mm. human embryo. Tandler. 



I-IV Inner branchial grooves. 



pass ventrally, one in each branchial arch, forming the aortic arches. These 

 unite ventrally on each side to form a single vessel, the ventral aortic root, 

 and the two roots unite to form the single ventraTSortic trunk which joins the 

 cranial end of the heart. Somewhat caudal to the middle of the embryo a 

 branch of the aorta passes ventrally through the mesentery as the omphalomes- 

 enteric artery which enters the umbilical cord. Still farther caudally the 

 paired umbilical (allantoic) arteries are given off from the aorta and pass out 

 into the umbilical cord (Figs. 215, 216, 217). 



The conditions which exist at this stage in the region of the aortic arches in 

 mammalian embryos are indicative of the conditions which persist as a whole 

 or in part throughout life in the lowest Vertebrates. The changes which occur 

 in Mammals, however, are profound and the adult condition bears no resem- 

 blance to the embryonic. Yet certain features in the adult are intelligible only 

 from a knowledge of their development. In the human embryo six aortic arches 



