528 EMBRYOLOGY OF THE CHICK 



parts of them do, it is from these three pairs that the main blood vessels 

 arise. 



In reptiles, birds and mammals, all the main vessels of the adult 

 connecting the heart with the dorsal aorta are derived from the fourth 

 pair of embryonic aortic arches. 



It is important to remember this, as our studies in comparative 

 anatomy will consist of the study of an amphibian, a dogfish, a turtle, 

 and a cat or rabbit, and the student will be required to show similarities 

 and differences of this nature in the different groups. 



In reptiles the aortic arches remain in pairs (Fig. 309), but in birds 

 the left arch degenerates, while in mammals it is the right arch which 

 degenerates. The dorsal aortae, which began as paired vessels, now 

 fuse close to the sinus venosus. The portion extending cephalad is 

 fused for a very short distance, though never involving the region of 

 the aortic arches. 



Quite early in development there are segmental vessels arising from 

 the aorta which extend into the dorsal body-wall. The pair at a level 

 with the anterior appendage-buds enlarge and extend into the wing- 

 buds as the subclavian arteries. 



We have already mentioned the pair opposite the allantoic stalk 

 which has enlarged to become the allantoic arteries. 



The external iliac arteries which supply the posterior appendage- 

 buds arise as branches from the allantoic arteries close to the origin of 

 the aorta. 



At four days, the chick embryo still has the omphalomesenteric 

 arteries as its main visceral supply. It will be remembered that these 

 arteries are paired originally, but as the embryo (which must be con- 

 sidered as having its ventral portion open and thus lying extended over 

 the yolk of the egg), comes to have its ventral walls meet and grow 

 together, the omphalomesenteric arteries, like the heart and other paired 

 structures which later become fused to form a single vessel or organ, 

 are brought together and fused, thus forming a single vessel which 

 comes to lie in the mesentery and runs from the aorta to the yolk-stalk. 



The proximal portion of the omphalomesenteric artery persists as 

 the superior mesenteric of the adult, after the atrophy of the yolk-sac. 



The inferior mesenteric artery and the coeliac artery arise from the 

 aorta independently at a later stage. 



The cardinal veins are the main afferent systems of the early em- 

 bryo. They form on the second day as paired vessels on each side of the 

 midline and extend both headward and tailward. The anterior and pos- 

 terior cardinal veins on the same side come together to form the duct 

 of Cuvier, which duct runs ventrally and enters the sinus venosus. On 

 the fourth day there is practically no change in the cardinal veins. 



Later, the proximal portions of the anterior cardinal veins become 

 connected by a new transverse vessel which forms, and enters into the 



