LATER DE\ ELOPMEXT OF VASCULAR SYSTEM 375 



tinuous with the dorsal aorta. The pulmonary trunk divides into 

 right and left arches from which the small pulmonary artery is 

 given off on each side, and the arch is continued without per- 

 ceptible diminution in size as the ductus Botalli (ductus arteri- 

 osus) to the dorsal aorta. Thus the greater quantity of blood 

 pumped by botli sides of the heart passes into the dorsal aorta 

 by way of the right aortic arch, and the right and left ductus 

 Botalli; but part of the blood from the left ventricle passes into 

 the carotids. The main branches of the dorsal aorta are (1) 

 coeliac, distributed to stomach and liver mainh% (2) omphalo- 

 mesenteric to the 3'Olk-sac and mesentery, (3) right and left 

 umbilical arteries (of which the left is much more important, the 

 right soon disappearing), to the allantois and leg, (4) segmental 

 arteries to the body-wall, (5) the caudal arteries. 



The anterior venae cavae (former ducts of Cuvier) return the 

 blood from the head, wing, and walls of the thorax to the right 

 auricle; but owing to the formation of the sinus septum, the left 

 vena cava opens directly into the right auricle to the left of the 

 sinus valves, and the right one, also independently, to the right of 

 the sinus valves. The proximal portion of the vena cava 

 inferior is the original meatus venosus, and it receives the 

 right and left hepatic veins, the last of w^hich receives all the 

 blood from the allantois through the umbilical vein (original 

 left). 



There is also an hepatic portal and a renal portal circulation. 

 The hepatic portal system is supplied with blood mainly from 

 the yolk-sac, but also from the veins of the alimentary canal by 

 the mesenteric vein; the latter is a relatively unimportant vessel 

 at eight da3^s, but groW'S in importance and becomes the entire 

 hepatic portal vein after absorption of the yolk-sac. The hepatic 

 portal vein branches wdthin the liver into a system of capillaries 

 which reunite to form the right and left hepatic veins. Thus 

 all the absorbed nutrient material passes through the capillaries 

 of the liver, where certain constituents are no doubt acted on 

 in some important, but little understood, way. 



The renal portal circulation persists through the period of 

 functional activity of the mesonephros. The afferent vein is 

 the posterior cardinal which is supplied by the segmental veins 

 and the veins of the leg and tail. The blood flows through the 

 capillaries of the mesonephros into the subcardinal veins, and 



