Development of the Third Day 199 



projecting loop, the ventricular portion, is be- 

 coming more pointed, as the apex of the 

 heart, and is separated by slight constrictions 

 from the auricular region, on the one hand, 

 and the bulbus, on the other. There is, at this 

 time, no separation of the heart into right and 

 left sides. 



The point at which the two vitelline veins 

 unite to empty into the heart now becomes 

 pushed farther towards the tail, so that, in- 

 stead of these veins emptying simultaneously 

 into the auricular portion of the heart, they 

 first unite to form a large single vessel which 

 then leads into the heart (Figs. 66 and 76, VE). 

 This single vein which brings all of the blood 

 from the vascular area back to the heart, first 

 through the two vitelline veins, and then, as the 

 right vein dwindles and disappears, through 

 the left vein only, is called the meatus venosus; 

 the portion nearest to the heart being some- 

 times called the sinus venosus ; and the part 

 farther from the heart the ductus venosus. 



The dorsal aorta now gives off numerous 

 branches (Figs. 65 and 76) to various parts of 

 the constantly enlarging embryo, and the 

 blood that is carried away from the heart by 

 these branches is brought back chiefly by two 



