DEVELOPMENT OF THIRD DAY 501 



the conus end comes to lie caudal to the sinus or venous end, a position 

 that the higher vertebrates all retain in the adult stage. In fishes, the 

 atrial region of the heart remains caudal to the ventricular portion even 

 in the adults. 



The point where the two vitelline veins meet to empty into the 

 heart becomes pushed farther and farther caudad, so that the two veins 

 unite and form a common opening into the heart. All blood from the 

 vascular area to the heart passes through this single common tube, 

 though in a short time the right vein will dwindle away and disappear. 

 The tube is then an opening for the left vitelline vein only. This com- 



wr 



Fig. 297. 



Diagrammatic lateral view of the chief embryonic blood-vessels of the chick, 

 during the sixth day. a, Auricle ; al, allantoic stalk ; ao, dorsal aorta ; c, 

 cceliac artery ; ca, caudal artery ; cl, cloaca ; cv, caudal vein ; da, ductus arteriosus ; 

 dv, ductus venosus ; ec, external carotid artery ; ej, external jugular vein ; t, in- 

 testine ; ic, internal carotid artery ; ij, internal jugular vein : I, liver ; m, meso- 

 nephros ; ma, mesenteric artery ; mv, mesenteric vein ; p, pulmonary artery ; pc, 

 posterior cardinal vein ; pv, pulmonary vein ; 8, sciatic artery ; sc, subclavian 

 artery ; scv, subclavian vein ; st, yolk-stalk ; sv, subcardinal vein ; ul, left umbilical 

 artery ; ur, right umbilical artery ; uv, left umbilical vein ; v, ventricle ; va, vitelline 

 artery; vca, anterior vena cava (anterior cardinal vein) ; vp, posterior vena cava; 

 vv, vitelline vein ; y, yolk-sac ; 3, 4, 6, third, fourth, and sixth aortic arches. 

 (From Lillie's "Development of the Chick," by permission of Henry Holt & Co., 

 Publishers.) 



mon tube-like entry into the heart is called the meatus venosus ; the por- 

 tion nearest the heart is the sinus venosus, and the portion lying more 

 distal, the ductus venosus. 



The dorsal aorta gives off numerous branches supplying various 

 portions of the body of the embryo, the blood being returned by two 

 large veins on each side of the body. That from the anterior part of the 

 embryo being carried through the anterior cardinal veins and that from 

 the posterior part of the body being carried by the posterior cardinal 

 veins, the anterior and posterior cardinals then unite into a common vein 

 before emptying into the sinus venosus, and this common vein is called 

 the duct of Cuvier. 



