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 transverse vessels which enter 

 the sinus venosus, one from either side, and, together with the hepatic 

 veins, mark the posterior limit of the heart. Each develops outside 

 of the somatic wall of the hypomere and extends dorsally until it reaches 

 the level of the top of the ccelom (fig. 282). In this course, in the 

 fishes, each receives an inferior jugular vein which comes from the 

 head, bringing back blood from the muscles of the lateral and ventral 

 branchial regions. At its dorsal end each Cuverian duct divides into 



