1642 A MANUAL OF ANATOMY 



tube to the sinus venosus, which is situated close to the caudal or 

 posterior end of the developing heart. 



The two vitelline arteries are branches of the two primitive dorsal 

 aortcB, and are therefore connected with the developing heart only 

 indirectly. 



The vitelline circulation is of little importance in Man, but it is 

 a considerable factor in the case of oviparous animals — e.g., Birds — • 

 the yolk in such animals being very abundant and specially nutri- 

 tive. 



The Allantoic Circulation. 



As retrogression of the yolk-sac and vitelline circulation takes 

 place, the nutritive and respiratory functions of the foetus are 

 assumed by the allantois and allantoic circulation. The left vitel- 

 line artery becomes entirely obliterated, but the proximal or 

 intra-emhryonic portion of the right vitelline artery persists, and 

 gives rise to the superior mesenteric artery. 



As previously stated, the allantois is a blind diverticulum from 

 the ventral wall of the hind-gut. This diverticulum consists of 

 splanchnopleure — ^that is to say, entoderm and splanchnic meso- 

 derm — ^which is continuous with the splanchnopleure of the wall of 

 the gut. This part of the allantois is known as the allantoic diver- 

 ticulum. At the umbilical orifice it enters the allantoic stalk, which 

 extends from the umbilicus to the placental area of the chorion, or 

 chorion frondosum. The mesodermic tissue of the allantoic stalk 

 contains the two allantoic arteries and the two allantoic veins. 



The allantoic arteries grow from the caudal ends of the two 

 primitive dorsal aortcB. In the region of the chorion frondosum 

 they divide into branches which enter the cotyledons or tufts of 

 arborescent chorionic villi, and render these important structures 

 vascular. In this manner the peripheral part of the allantoic stalk 

 and the chorion frondosum are brought into continuity. The blood 

 is conveyed from the primitive dorsal aortas to the chorion frondosum 

 by the allantoic arteries, which leave the body of the foetus through 

 the umbilical orifice; and, after having undergone the necessary 

 changes, it is conveyed back into the body of the foetus by the 

 allantoic veins. These veins enter the foetal body through the 

 umbilical orifice, and, after passing along the intestinal tube, they 

 open into the sinus venosus at the caudal or posterior end of the 

 developing heart. 



The foregoing constitutes the allantoic circulation, which is of 

 special importance in Birds, Reptiles, and Fishes. In Man, how- 

 ever, this circulation is of a transitory nature, and soon gives place 

 to the placental circulation. 



The Placental Circulation. 



The nature of the placental circulation has been described in 

 connection with the placenta. It is sufficient to state in this place 



