AORTIC ARCHES OF BIRDS — GLENNY 529 



in the other tetrapod vertebrates, and to attempt to show homologies 

 such as do exist. 



Early development of the avian vascular system 



In discussing the early development of the vascular system of the 

 chick, Patten (1929) states that the early vessels are formed from 

 mesodermal cells that lie in the path of their development and that 

 the walls of these early vessels are one cell-layer in thickness. As a 

 result, no clear structural differences between the precursors of both 

 arteries and veins arise until a much later period in development. 

 Balfour (1873) has stated that the blood vessels of the chick arise not 

 as spaces or channels between the mesoblast cells but as a network 

 formed by united processes of the mesoblast cells, and that it is 

 thi'ough these processes that the blood flows. He also stated that 

 first traces of blood vessels are to be found in the pellucid area at about 

 30 hours of incubation. 



Hyman (1942) states that the blood and blood vessels arise from 

 mesenchyme cells of the mesoderm by forming patches of cells and 

 that the central cells become modified into blood corpuscles, while 

 the peripheral cells become oriented so as to form tubes, the early 

 blood vessels. Essentially all these views represent the same concept, 

 but expressed in slightly different ways. 



The vitelline veins are the earliest vessels to form in the embryo 

 and are found to develop on the surface of the jolk sac in the splanchnic 

 hypomere and then pass to the embryo in the gut mesentery and 

 finally come to enter the heart at its caudal end. 



The ventral aorta is observed to arise at the cephalic end of the 

 heart, with which it then becomes connected. The ventral aorta 

 then extends anteriorly to the anterior end of the pharynx, at which 

 point it bifurcates to form the anlagen of the ventral radices aortae, 

 which then turn laterally and dorsally and pass around the pharynx, 

 on either side, and curve posteriorly, dorsal to the pharynx, as the 

 dorsal radices aortae which carry the blood backwards to the vitel- 

 line arteries which in turn pass to the yolk sac. Thus the first aortic 

 arch which now lies within the mass of the mandibular visceral arch 

 comes to be the first of the true aortic arches formed. Subsequently 

 five additional pairs of vessels communicating between the ventral 

 and dorsal radices aortae come into existence for a varying length of 

 time, depending upon their ultimate fate in the adult bird and the 

 function which they serve in the embryo or in the adult. Later, the 

 two dorsal radices aortae, posterior to the pharyngeal region, come to 

 fuse, thus forming the single median dorsal aorta (abdominal aorta) 

 of the adult vertebrate. 



Jolly (1940) has pointed out that the origin and mode of formation 

 of the large embryonic vessels is still a matter in question and cloaked 



