AORTIC ARCHES OF BIRDS — GLENNY 531 



mesoderm of the hypomere, and is of the same nature as the blood 

 vessels, and that, as the hypomere closes below in the median ventral 

 line, the two cavities are brought together and fuse to form the heart. 

 With the disappearance of the ventral mesocardium the heart comes to 

 lie free in the coelom. 



Patten (1939) points out that the paired ventral aortic roots ex- 

 tend anteriorly from the bulbo-conus arteriosus (anterior heart 

 chamber), and that the ventral aortic roots and the omphalomesenteric 

 veins constitute direct continuations of the paired endocardial primor- 

 dia of the heart. This is not in contradiction to Jolly's view of vessel 

 formation within the embyro. 



At about the 44-hour stage of incubation the heart begins regular 

 contraction, thus establishing the circulation of the blood. 



In birds and mammals the ventricle is divided into left and right 

 compartments by the interventricular septum. The atrium is like- 

 wise divided by the interatrial septum, and the sinus venosus, still 

 recognizable, is incorporated into the wall of the right auricle ac- 

 cordmg to Quiring (1933). The systemic or aortic root and the pul- 

 monary root form by a splitting of the conus arteriosus into two main 

 trunks. The aortic or sj^stemic root passes from the left ventricle to 

 the body, while the pulmonary root passes from the right ventricle 

 to the lungs. Thus the bird heart is comprised of two embryonic 

 chambers, each of which is secondarily divided into two compart- 

 ments, while the other two heart chambers of the lower vertebrates 

 and the chick embryo are lost through incorporation and further 

 structural and functional modifications. The atrium and ventricle 

 of the early embryonic heart alone persists as the primary heart 

 chambers, and the valves of the conus arteriosus persist in the pul- 

 monary and systemic roots at the point of junction of these vessels 

 with the ventricles. 



As reported by Twining (1906), Lillie (1908, 1919), Patten (J929), 

 Hughes (1934), and others, the aortic arches make their appearance 

 (in the chick embryo) in order and at approximately the following 

 levels of development or incubation: (1) first aortic arch appears in 



Figure 108. — a, Amniote aortic arch arrangement, lateral view; b, same, ventral view; 

 c-i, ventral views of main cervical and thoracic arteries; c, in Bufo melostictus (modified 

 after Bhaduri); d, in Alligator mississippiensis (modified after Reese); e, in Sphsnodon 

 punctatus (USNM 19260);/, in Boa constrictor (after Hafferl); g, in birds (generalized); 

 h, in Emys (modified after Hafferl); i, in mammals (modified after Patten). Explanation 

 of symbols: 1-6, aortic arches; a., axillary artery; a.r., aortic root; b., basilar artery; 

 br., brachial artery; c, coracoid artery; c.c, common carotid artery; c.d., dorsal carotid 

 artery; c.v., ventral carotid artery; c.n.v., comes nervi vagi; d.a., dorsal (abdominal) 

 aorta; d.b., ductus botalli; d.c, ductus caroticus; e.c, external carotid artery; i., innomi- 

 nate artery; i.e., internal carotid artery; i.m., internal mammary artery; l.a., ligamentum 

 aortae; l.b., ligamentum botalli; I.e., ligamentum caroticum; p., pectoral arteries; r.a., 

 radix aortae; s., subclavian artery; s.c, subscapular artery; t., thoracic artery; v., vertebral 

 artery; v.a,, ventral aorta. 



