DEVELOPMENT OF THE HEART. 975 



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arises, the bulbus arteriosus. This speedily divides into two branches, 

 which give off two anterior and two posterior vertebral arteries ; the 

 former supply the cephalic part of the embryo, and the latter soon 

 unite, to form the descending aorta. The primitive aorta is very early 

 connected with two small arteries, named the omphalo-mesenteric or 

 meseraic arteries, which pass laterally on to the yolk-sac, hence they 

 are also named vitelline arteries, and branch out into the vascular area ; 

 they convey the blood from the heart and body of the embryo, over 

 the yolk-sac. After a time, one of these arteries disappears, the left 

 usually remaining. From the vascular area, the blood is returned to 

 the embryo, by the anterior recurved ends of the terminal sinus, which 

 appears to have the function of a vein. But from the hinder part of 

 the vascular area, other veins proceed, and end in posterior branches, 

 which join the recurved ends of the terminal sinus to form the two 

 omphalomesenteric veins; these enter the embryo, and become con- 

 nected with the posterior extremity of the still straight, sac-like, heart. 

 After a time, the right omphalo-mesenteric vein shrinks, the left one 

 remaining pervious. 



In the Bird, the vessels of the vascular area become very large, and 

 form the vitelline vessels ; they send vascular processes into the sub- 

 stance of the yolk, and thus food is absorbed by, and supplied to, the 

 embryo. The yolk-sac is the temporary alimentary chamber of the 

 embryo, and, in certain lower animals, it remains so ; its vessels are 

 represented by the abdominal veins in the Non-vertebrate forms. The 

 vessels of the yolk persist in the Bird up to the period of hatching, 

 and for some time afterwards, until the contents of the yolk-sac are 

 completely absorbed. But in the Mammalian ovum, the omphalo- 

 mesenteric artery and vein continue very small, and soon become ob- 

 literated. The circulation through the vascular area to the terminal 

 sinus, in them, is probably chiefly respiratory. 



But now a change takes place in the circulation outside the embryo 

 both in the Bird and in the Mammal. The allantois grows, and two 

 arteries proceeding from the posterior vertebrals, named the hypogas- 

 tric or umbilical arteries, proceed out upon it ; whilst two veins return- 

 ing upon it, also named umbilical, join the common trunk of the om- 

 phalo-mesenteric veins. The right umbilical vein then disappears, and 

 the left one, enlarging, as the single left omphalo-mesenteric vein di- 

 minishes, soon appears as the chief median vein, carrying the blood 

 from the allantois, or its modification, the placenta, back to the heart 

 of the embryo. It now passes through or beneath the liver. When 

 the vitelline vessels, in the Mammalian embryo, have shrunk, the 

 proximal part of the left omphalo-mesenteric vein, forms the trunk of 

 the portal vein, and the part beyond this, after joining with the umbili- 

 cal vein, constitutes the ductus venosus, with which vessels the liver 

 and its veins are now seen to be connected. 



As the embryo develops, the heart itself undergoes a process of 

 bending or curvature, like an italic letter S. At the same time, it 

 enlarges, and acquires thicker walls ; the anterior or arterial part, which 

 gives off the bulbus arteriosus, is now placed nearer to the under side 

 of the embryo, whilst the posterior or venous part, which receives the 



