402 EMBEYOLOGY OF THE LOWER VEETEBEATES cm. 



be justified in believing that in the Amniota an aortic arch -has 

 disappeared in front of the last one. The persisting vestiges of this 

 fifth arch, which are now known to occur commonly in the embryos 

 of the Amniota, were detected first by van Bemmelen (1886) in 

 Eeptiles and Birds. A good example of such a vestigial fifth arch is 

 seen in the embryo of the Crocodile (Fig. 189). 



The cause of the reduction of this fifth aortic arch is probably to 

 be recognized in the fact that it receives its blood from the pulmonary 

 cavity of the conus and of the ventral aorta (Graham Kerr, 1907*). 

 As a consequence of this, during the evolution of the lungs as the 

 main organs of respiration a larger and larger proportion of the 

 blood in the cavity mentioned has become drawn off to the lungs, 

 leaving less and less for arch V, with the natural result that the 

 latter has become reduced to the verge of disappearance. 



Before leaving the subject of the aortic arches it is necessary to 

 point out that their diagrammatic arrangement as shown in Figs. 186 

 and 187 is commonly much obscured in the adult. For during 

 development there occur not merely the disappearance of large 

 portions of the original scheme of arches and the straightening out 

 of the unnecessary curves, but also other complications. The chief 

 of these are due to the longitudinal vessels — ventral aorta or aortic 

 roots — lagging behind in their growth in length. This leads, 

 according to the position in which it takes place, to the crowding 

 together of the ventral or the dorsal ends of consecutive aortic 

 arches, and their mere approximation may be succeeded by actual 

 fusion so that two or more arches may come to have a common root 

 emerging from the ventral aorta, or a common terminal portion 

 opening into the aortic root. 



Pulmonary Artery. — The pulmonary artery makes its first 

 appearance in Crossopterygian fishes (Polypterus) as a branch from 

 arch VI towards its ventral end which passes to the lung and 

 adjoining parts of the pharyngeal wall. Throughout the series of 

 lung-breathing Vertebrates it develops similarly as an outgrowth of 

 the sixth aortic arch. A result of the main blood-stream of this arch 

 passing off into the pulmonary branch is that the dorsal part of the arch 

 lying beyond the point of origin of the pulmonary artery becomes as a 

 rule reduced in size, forming the duct of Botallus. Except in the case 

 of certain Eeptiles (p. 397) the duct of Botallus becomes in normal 

 individuals of the Amniota completely obliterated soon after birth. 



In the Lung-fishes the point of origin of the pulmonary artery is 

 displaced up to the dorsal end of arch VI where it is fused with 

 arch V. Thus arch V is able to carry blood directly to the pulmonary 

 artery and correlated with this it does not undergo the reduction in 

 size which has taken place in the Amniota. 



In Zepidosiren and Frotopterus an important development of the 

 pulmonary artery takes place inasmuch as its area of distribution 

 extends on to the lung belonging morphologically to the other side of 



