. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 278 COMPARATIVE MORPHOLOGY OF VERTEBRATES. smaller veins empty. Blood is brought to the yolk by the omphalomesenteric arteries, which are also distributed to the yolk sac, dividing up distally into a net- work of capillaries connecting distally with the vitelline veins. By these the blood is carried to the liver and through the portal circulation to the heart. In the mam- mals a similar vitelline circulation is developed, but as the yolk sac contains no yolk, it is of minor importance. In the amniotes an outgro
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 278 COMPARATIVE MORPHOLOGY OF VERTEBRATES. smaller veins empty. Blood is brought to the yolk by the omphalomesenteric arteries, which are also distributed to the yolk sac, dividing up distally into a net- work of capillaries connecting distally with the vitelline veins. By these the blood is carried to the liver and through the portal circulation to the heart. In the mam- mals a similar vitelline circulation is developed, but as the yolk sac contains no yolk, it is of minor importance. In the amniotes an outgrowth, the allantois (p. 318), arises as a diverticulum from the hinder end of the alimentary canal, increases in extent, growing downward and carrying the ventral body wall before it. Branches of the hypogastric arteries, known as the allantoic arteries, extend into it and are connected by capillaries. FIG. 284.—Diagram of embryonic circulation in a large-yolked vertebrate; compare with fig. 282. aa, aortic arches; al, allantois; an, anus; ca, cv, caudal artery and vein; da, dorsal aorta;, dc, Cuverian duct; h, heart; ha, hypogastric (allantoic) artery; i, jugular vein; /, liver; oma, omv, omphalomesenteric artery and vein; pc, postcardinal vein; si, subintes- tinal vein; st, sinus terminalis; va, ventral aorta; y, yolk; ys, yolk stalk. with umbilical veins which arise from the subintestinal vein behind the vitelline veins. There thus is formed an allantoic circulation which is both respiratory and nutritive in character. In the reptiles both of the umbilical veins persist through the foetal life (only one shown in fig. 273), but in birds and mammals one aborts, leaving the other as the efferent vessel of the allantois. With the end of foetal life (at hatching or at birth) both the vitelline and the allantoic circulations disappear, leaving only inconspicuous rudiments. The entrance of the Cuverian ducts into the heart was mentioned on page 271. These ducts are a pair of trans
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