138 EARLY EMBRYOLOGY OF THE CHICK 



final disappearance. In many embryos of this age the first 

 arches, and in a few the second also, have disappeared alto- 

 gether. This leaves only the third, fourth, and sixth pairs of 

 aortic arches. These arches persist intact for some time, and 

 parts of them remain permanently, being incorporated in the 

 formation of the aortic arch and the main vessels arising from 

 it, and in the roots of the pulmonary arteries. 



In reptiles, birds, and mammals the main adult vessels which 

 connect the heart with the dorsal aorta are derived from the 

 fourth pair of aortic arches of the embryo. The paired condi- 

 tion of these arches persists as the adult condition in reptiles, 

 but in birds and mammals one of the arches degenerates before 

 the end of embryonic life. In birds the left arch degenerates 

 leaving the right one as the adult aortic arch; in mammals the 

 right arch degenerates leaving the left as the aortic arch of the 

 adult. 



The dorsal aortae, at first paired, later become fused to form a 

 median vessel. The fusion begins at about the level of the 

 sinus venosus and progresses cephalad and caudad (Fig. 35). 

 Fusion extends cephalad but a short distance, never involving 

 the region of the aortic arches. Caudally the aortae eventually 

 become fused throughout their entire length. 



Early in development the aorta gives rise to a segmentally 

 arranged series of small vessels which extend into the dorsal 

 body wall. At the level of the anterior appendage buds a pair 

 of the segmental arteries become enlarged and extend into the 

 wing buds as the sub-clavian arteries. Coincident with the 

 development of the allantois, segmental vessels opposite the 

 allantoic stalk become enlarged and extend'into it as the allan- 

 toic arteries. The external iliac arteries to the posterior ap- 

 pendage buds arise as branches of the allantoic arteries close to 

 their origin from the aorta (Fig. 47) . 



The three main arteries which in the adult supply the ab- 

 dominal viscera are represented in four-day chicks only by the 

 omphalomesenteric arteries. The omphalomesenteric arteries 

 arise as paired vessels (Fig. 35), but in the closure of the ventral 

 body wall of the embryo they are brought together and fused to 

 form a single vessel which runs in the mesentery from the aorta 

 to the yolk-stalk (Fig. 47). With the atrophy of the yolk-sac 

 the proximal part of the omphalo-mesenteric artery persists as 



