PLACENTAL CIRCULATION. 699 



quently uniting into a common venous trunk, called the sinus 

 veuosus, which is continuous 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 yelk 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 

 developed, extends to the chorion, over which it spreads, sending 

 in processes to occupy the villi. These chorionic villi are im- 

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

 at the upper part, which is in connection with the decidua sero- 

 tina or maternal placenta. 



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

 become 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-inesen- 

 teric artery. 



From the branches of the aortse arise two large vessels, which, 

 running along the allantois, spread out over the chorion, being 

 especially directed to the upper part of this membrane ; these are 

 the umbilical or hypogastric arteries, which carry the blood from 

 the aortse to the foetal placenta. 



Veins arise from the terminal networks of these arteries, and 

 combine to form the two umbilical veins. The umbilical veins 

 take a similar course to the arteries, and convey the blood to the 

 venous trunk formed by the junction of the omphalo-mesenteric 

 veins. 



After a time the right umbilical and right omphalo-mesenteric 

 veins disappear, whilst from the trunk formed by the junction of 

 the left umbilical and left omphalo-mesenteric veins branches 

 are given off to the liver (the venae advehentes), and at a point 

 nearer the heart vessels are received from the liver (the vence 

 revehentes). 



To the part of the vessel intervening between the origin of the 



