4:82 EMBRYOLOGY OF THE CHICK 



distance caudad, the single aorta again divides into two vessels from 

 which the large vitelline artery, already mentioned, is given off on each 

 side. The dorsal aortae, now greatly diminished in diameter, continue 

 into the tail. 



The first pair of aortic arches formed are called the mandibular 

 aortic arches (Fig. 284). 



A second pair now form behind the first, and before the close of 

 the day there may be still a third pair, all of which connect in a similar 

 manner to the first with the bulbus arteriosus and the dorsal aorta. 



The sinus terminalis is now also much better developed than before 

 and a true circulation has been established, which can carry the yolk- 

 food-granules (after these have been converted into usable food) to the 

 embryo. 



It is essential that a somewhat detailed knowledge of the circulation 

 be obtained. 



The blood is brought to the heart by the vitelline veins (Figs. 277, 

 284). The heart then contracts and forces it through the aortic arches 

 into the dorsal aorta. Here it passes tailward, a small portion going into 

 the tail itself, but the greater part is carried to the vascular area. There 

 are two ways in which the blood now gets back into the vitelline veins. 

 First, it may pass directly to the veins from the arteries through the 

 connecting capillaries, or, second, it may pass into the sinus terminalis 

 at a middle point on each side, and then pass forward and backward 

 through this large vessel. The greater portion, however, in this second 

 method passes forward toward the head from where it is returned to 

 the heart through two large parallel vessels. The part which passes 

 backward is again distributed to the vascular area as there are no con- 

 necting vessels with the tail of the embryo. The vitelline veins and 

 arteries run parallel to each other, though the veins lie a little forward 

 from the arteries. 



In the embryo itself, the cardinal veins are the main afferent ves- 

 sels. At thirty-eight hours the anterior cardinals can be seen. These 

 are a pair of vessels which return the blood from the head of the embryo 

 to the heart. The posterior cardinals are also paired, and return blood 

 from the caudal region. 



Both anterior and posterior cardinal veins unite on each side of the 

 body to form a short common vessel before entering the heart, the 

 right and left ducts of Cuvier, or common cardinal veins. These Cuvier- 

 ian ducts then turn ventrad on each side of the fore-gut and enter the 

 sinus venosus at the same point the omphalomesenteric veins enter. 



The omphalomesenteric veins [so called because they pass through 

 the umbilicus (navel) as umbilical vessels connecting the offspring with 

 the mother in the higher forms], are established in the chick from thirty- 

 three to thirty-six hours' incubation. They are postero-lateral exten- 

 sions of the self-same endocardial tubes which formed the heart. They 



