244 Vertebrate Embryology 



about the fourth or fifth day, the middle 

 parts of the first and second, the mandibular 

 and hyoid, aortic arches disappear. The lower 

 ends of these two arches persist as the small 

 mandibular or lingual arteries (Fig. 76, A L): 

 while their upper or dorsal ends persist as the 

 carotid arteries, each of which immediately 

 divides into an internal and an external 

 branch (Fig. 76, A C), the former going to 

 the brain and the latter to the face. 



By the sixth day, the ventricular septum is 

 complete, and its front edge fuses with the 

 hinder edge of the septum that divides the 

 truncus arteriosus into right and left sides. 

 The front edge of this latter septum arises 

 between the fourth and fifth aortic arches in 

 such a way that all of the blood that comes 

 from the left side of the truncus arteriosus 

 (and consequently from the left ventricle) 

 passes into the third and fourth aortic arches ; 

 while the blood from the right ventricle passes 

 into the fifth aortic arch. 



About the seventh day, the right and left 

 parts of the truncus arteriosus separate com- 

 pletely from each other, the right branch 

 remaining in connection with the fifth aortic 

 arch, as the pulmonary trunk ; and the left 



