VITELLINE CIRCULATION. 701 



to the yolk sac, the embryo obtaining nourishment from the 

 vitellus or yolk ; this is, however, an organ of quite secondary 

 importance in the mammalian embryo, and hence this circulation 

 may be better studied in some such animal as the chick, which 

 depends, throughout its embryonic life, on the vitellus for nourish- 

 ment. In the human embryo the vitelline circulation is chiefly 

 of importance for the few days immediately preceding the devel- 

 opment of the placental circulation. 



The aortic bulb is continuous with two vessels which run on 

 either side of the primitive pharynx ; these are the aortse, and 

 from each of them a large branch is given off. These omphalo- 

 mesenteric arteries pass to the yolk sac, and there become split 

 up into a number of small vessels, the blood from them being re- 

 turned partly by corresponding omphalo-mesenteric veins, partly 

 by a large vein running round the periphery of the vascular area 

 known as the sinus terminalis. The sinus terminalis opens partly 

 into the right and partly into the left omphalo-mesenteric veins, 

 the omphalo-mesenteric veins themselves subsequently uniting into 

 a common venous trunk, called the sinus venosus, which is con- 

 tinuous with the primitive auricle. 



This vitelline circulation in the human embryo persists but a 

 short time. After the fifth or sixth week of foetal life it becomes 

 obliterated, the yolk then being atrophied, and the placental cir- 

 culation well developed. 



(6) The later, or placental circulation, is developed in the meso- 

 blastic layer of the allantois, especially in that part which is in 

 relation with the decidua serotina. The allantois, when fully de- 

 veloped, extends to the chorion, over which it spreads, sending in 

 processes to occupy the villi. These chorionic villi are imbedded 

 in the decidua of the uterus, and are especially developed at the 

 upper part, which is in connection with the decidua serotina or 

 maternal placenta. 



The primitive aortse, which were at first two separate tubes, be- 

 come united in the dorsal region of the embryo,, so that the two 

 aortic arches end in a single vessel, which extends to the middle 

 of the embryo, and there divides into two branches, each of which 

 gives off a vessel called the vitelline or omphalo-mesenteric artery. 



