LATER DEVELOPMENT OF VASCULAR SYSTEM 379 
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 both 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) 
ceeliac, distributed to stomach and liver mainly, (2) omphalo- 
mesenteric to the yolk-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 vene cave (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 which 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 days, but grows in importance and becomes the entire 
hepatic portal vein after absorption of the yolk-sac. The hepatic 
portal vein branches within 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 subeardinal veins, and 
