218 EMBRYOLOGICAL TYPES 



time, and place the cavity of the fore gut in communication 

 with that of the amnion. 



In the embryo, paired dorsal aortae develop, beneath the 

 notochord. Anteriorly they connect with the aortic arches, 

 and posteriorly at an early stage they simply spread out over 

 the yolk-sac on each side forming the vitelline arteries. Later, 

 the single median dorsal aorta arises by the fusion of the paired 

 vessels (as far forwards as the pharynx), and it extends back 

 behind the vitelline arteries into the tail as the latter is formed. 



The aortic arch in the 4th visceral arch becomes the 

 systemic (that on the left disappears), that in the 6th arch 

 the pulmonary. The pulmonary arches also connect with 

 the dorsal aorta by the ductus arteriosus. 



The cardinal veins also arise as paired vessels, on each side 

 of the aorta, and they communicate with the heart across a 

 transverse septum by means of the ductus Cuvieri. Beneath 

 the hind portion of the posterior cardinal veins, the subcardinal 

 veins arise, in the region of the mesonephros. The sub- 

 cardinal veins acquire connexion with the developing inferior 

 vena cava. 



As the anterior intestinal portal moves farther back in the 

 embryo, the fusion of the two posterior vitelline, or omphale- 

 mesenteric veins, with one another becomes more extensive. 

 This combined vessel, which lies in the region of the developing 

 liver and behind the heart, is known as the ductus venosus. 

 At the hind end of the ductus venosus, the two posterior 

 vitelline veins are separate, but farther back still they fuse 

 together again twice : in one place dorsal to the gut, and in 

 another place behind again, ventral to the gut. The piece 

 on the left side between the dorsal point of fusion and the 

 ductus venosus disappears ; the piece on the right side between 

 the dorsal fusion and the ventral fusion behind it disappears 

 also. The net result of all these modifications is that the 

 vitelline veins run into the embryo on each side from the yolk- 

 sac and join beneath the gut. From this point a single vein 

 runs forwards and makes one complete twist round the gut 

 in the direction of the thread of a corkscrew, and runs into the 

 ductus venosus. Part of these posterior vitelline veins becomes 



