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 aortie 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 35s 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, 7.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 32s 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. Owine to the order of development of the body, the 
anterior cardinals are formed before the posterior cardinals. At 
the 15-16s stage they lie at the base of the brain, dorsal and 
lateral to the dorsal aorte, and extend forward to the region of 
