204 THE DEVELOPMENT OF THE CHICK 



The first aortic arch loses its connection with the dorsal aorta 

 at about the stage of 30 somites, and the second arch similarly 

 during the fourth day; the ventral ends of these arches retain 

 their connection with the ventral aorta and constitute the begin- 

 ning of the external carotid. Thus the third, fourth, fifth and 

 sixth aortic arches remain. Their transformation belongs to the 

 subject-matter of Chapter XII. 



The pulmonary artery appears as a posterior prolongation of 

 the ventral aorta on each side at about the 35 s stage. It thus 

 appears successively in later stages as a branch from the base of 

 the fourth and sixth aortic arches. 



The Internal Carotids. The loop where the mandibular arch 

 joins the dorsal aorta may be called the carotid loop; it is situated 

 in front of the oral plate at the base of the fore-brain on each 

 side (Fig. 93). It enlarges to form a sac, and when the connec- 

 tion with the mandibular arch is lost, sends out branches into 

 the tissue surrounding the brain. These are of course a direct 

 continuation of the dorsal aorta on each side. 



The segmental arteries are paired branches of the dorsal aorta 

 in each intersomitic septum. They pass dorsally to about the 

 center of the neural tube and arch over laterally to enter the 

 segmental veins, and thus unite with the cardinal veins. 



The Development of the Venous System. The main outlines 

 of the development of the venous system have been already 

 considered. 



The somatic veins, i.e., the anterior and posterior cardinal 

 veins and their branches, enter the sinus venosus through the 

 ducts of Cuvier. The original position of this duct as we have 

 seen is about the level of the second somite. The formation of 

 the cervical flexure, however, carries a number of somites forward 

 above the heart, so that at about the stage of 32 s it comes to 

 lie in the region of the eighth and ninth somites. The relation 

 between the somatopleure and the heart in this region has been 

 already described. 



The anterior cardinal veins are the great blood-vessels of the 

 head, and become the internal jugulars in the course of develop- 

 ment. Owing to the order of development of the body, the 

 anterior cardinals are formed before the posterior cardinals. At 

 the 15-16 s stage they lie at the base of the brain, dorsal and 

 lateral to the dorsal aortse, and extend forward to the region of 



