THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 549 



(b) The First Developmental Conditions of the Large Vessels. Vitettine 

 Circulation, Allantoic and Placental Circulation. 



At both ends, in front and behind, the heart is continuous with 

 the trunks of blood-vessels, which have been established at the same 

 time with it. The anterior or arterial end of the cardiac tube is 

 elongated into an unpaired vessel, the truncus arteriosus, which con- 

 tinues the forward course under the head-gut, and is divided in the 

 region of the first visceral arch into two arms, which embrace the 

 head-gut on the right and left and ascend within the arch to the 

 dorsal surface of the embryo. Here they bend around and run back- 

 ward in the longitudinal axis of the body to the tail-end. These 

 two vessels are the primitive aortce (figs. 107, 116 ao); they take 

 their course on either side of the chorda dorsalis, above the entoderm 

 and below the primitive segments. They give off lateral branches, 

 among which the arterice omphalomesentericce are in the Amniota 

 distinguished by their great size. These betake themselves to the 

 yolk-sac and conduct the greatest portion of the blood from the two 

 primitive aortas into the area vasculosa, where it goes through the 

 vitelline circulation. 



In the Chick, the conditions of which form the basis of the following 

 account (fig. 303), the two vitelline arteries (R.Of.A, L.Of.A) quit 

 the aortse at some distance from their tail-ends, and pass out laterally 

 from the embryonic fundament between entoderm and visceral middle 

 layer into the area pellucida, traverse the latter, and distribute them- 

 selves in the vascular area. They are here resolved into a fine net- 

 work of vessels, which lie, as a cross section (fig. 116) shows, in the 

 mesenchyme between the entoderm and the visceral middle layer, 

 and which are sharply bounded at their outer edge (toward the 

 vitelline area) by a large marginal vessel (fig. 303/&7 7 ), the sinus ter- 

 minalis. The latter forms a ring which is everywhere closed, with 

 the exception of a small region which lies in front, at the place 

 where the anterior amniotic sheath has been developed. 



From the vascular area the blood is collected into several large 

 venous trunks, by means of which it is conducted back to the heart. 

 From the front part of the marginal sinus it returns in the two 

 vence vitellince anteriores, which run in a straight line from in 

 front backwards and also receive lateral branches from the vascular 

 network. From the hind part of the sinus terminalis the blood is 

 taken up by the venae vitellinse posteriores, of which the one of the 

 left side is larger than the one of the right ; the latter afterwards 



