FORMATION OP HEART AND FIRST BLOODVESSELS. 



935 



tine, ultimately come to open into the right vena hepatica advehens of the um- 

 bilical vein, and thus constitute the origin of the portal vein. That portion 

 of the umbilical vein which lies between the two systems of hepatic branches 

 just mentioned, remains throughout the whole of fetal life, and is termed the 

 Ductus venosus Arantii. It was formerly believed that the nutrient matter 

 of the yolk passes directly through the vitelliue duct, into the (future) di- 

 gestive cavity of the embryo, and is from it absorbed into its structure; but 

 there can now be little doubt, that the vitelliue vessels are the real agents 

 of its absorption, and that they convey it through the general circulating 

 system, to the tissues in process of formation. They correspond, in fact, to 



S.T. 



Diagram of the circulation of the Yolk Sac at the end of the third day : H, Heart. AA, the 2d, 3d, 

 and 4th Aortic Arches; the first has become obliterated in its median portion, but is continued at its 

 proximal end as the external Carotid, and at its distal end as the internal Carotid. Ao, Dorsal Aorta. 

 R.O/.A, and L.Of.A, Right and left omphalo-mesaraic arteries. S.T., Sinus terminalis. R.OJ and L.O/, 

 Right and left omphalo-mesaraic veins. S. V, sinus venosus. D.C, Ductus Cuvieri. S.Ca.V and V.Ca, 

 Superior and inferior Cardinal Veins. 



the Mesenteric veins of Invertebrated animals, which are the sole agents in 

 the absorption of nutriment from their digestive cavity ; and the blastodermic 

 vesicle to be regarded as the temporary stomach of the embryo, remaining 

 as the permanent stomach in the Radiated tribes. 



778. The first rudiment of the Mammalian Heart, which is the earliest of 

 the permanent organs of the embryo that comes into functional activity, 

 consists of an aggregation of cells, forming a thickening of the mesoblast 

 beneath the anterior portion of the intestinal canal, and lying in the general 

 pleuroperitoueal cavity (Fig. 329, Ht), the innermost cells becoming de- 



