THE LATER DEVELOPMENT OF THE CHICK 341 



condition chiefly by a series of degenerations. During the 

 third and fourth days the first (mandibular) and second (hyoid) 

 arches disappear, leaving only their continuous ventral roots 

 as the root of the external carotid artery. The lateral dorsal 

 a,orta3 have meanwhile continued forward, as the internal ca- 

 rotid arteries, supplying the brain and other organs of the head. 

 These parts of the lateral dorsal aorta3 become separated, 'dur- 

 ing the sixth and seventh days from the remainder of the lateral 

 dorsal aorta3 by an interruption in these vessels just back of the 

 dorsal attachment of the third aortic arch; thus the internal 

 carotid arteries arise only from the third aortic arch, which is 

 consequently known as the carotid arch (Fig. 134, B). The 



A 



FIG. 134. Aortic arches of the chick. From Lillie (Development of the 

 Chick), A, after Locy. A. Of the left side of a chick of four and one-half days; 

 from an injection. B. Reconstruction from sagittal sections of an eight-day 

 embryo. Ao. A. Arch of the aorta (systemic arch) ; A.o.m., vitelline artery; Car., 

 carotid artery; Car. ext., external carotid artery; Car. int., internal carotid artery; 

 D.a., ductus arteriosus; d.Ao., dorsal aorta; p. A., pulmonary artery; S'cl., sub- 

 clavian artery; 3-6, third to sixth aortic arches (first to fourth branchial aortic 

 arches). 



ventral roots of the first and second arches (external carotids) 

 remain as branches from the bases of the carotid arch (roots of 

 the third arches), supplying the mandibular region. 



The fourth, or systemic arch, is at first symmetrically devel- 

 oped like the others, but during the fifth and sixth days it 

 becomes reduced on the left side and correspondingly enlarged 

 on the right. Finally the left systemic arch wholly disappears, 

 and the anterior part of the left division of the truncus remains 

 only as the stem of the left carotid artery. Meanwhile, it will 

 be recalled, the right side of the truncus has connected with the 



