THE CIRCULATORY SYSTEM. 531 



opment in mammalia, is the umbilical vesicle. In birds, the yolk-sac is 

 carried completely into the abdomen through the umbilical opening ; in 

 mammals it remains exterior. It does not appear that the contents of 

 the yolk are directly absorbed from the cavity of the sac, but they are 

 carried by the ramifying vessels to the liver. These vessels are there- 

 fore counterparts of the mesenteric. Eventually folds arise on the lin- 

 ing membrane of the yolk-sac over which these vessels pass, and which 

 facilitate absorption. In fish, at this stage, the yolk-bag hangs down, 

 and respiration takes place upon its surface. 



From the caudal extremity of the embryo the allantois emerges as a 

 mass of cells, of which the interior liquefy, and the exterior* 



T . * . <7 ' , .. , The allantois. 



then constitute a sac. In birds and in reptiles it reaches 

 considerable development ; in the former extending entirely over the 

 yolk-sac, but in mammals it is soon replaced and shrivels up. It dis- 

 charges the function of a urinary bladder, and, indeed, a portion of it 

 continues to do so in man. Its disappearance is the signal that the em- 

 bryo is now depending on the placenta. 



To return now to the development of the circulatory system. At 

 about the end of the eighth week, as we have seen, the ven- Development 

 tricle is divided by a septum, the division of the auricle not of the circuia- 

 occurring till a little after, and even then not being perfect, tory syst ' 

 an aperture, the foramen fcvale, existing. This is the state of things at 

 about the twelfth week: of the five branchial arches two disappear, 

 the aortic bulb then divides into two tubes, which .are to be the aorta 

 and pulmonary artery respectively. Next, one of the branchial arches 

 forms the subclavian and carotid arteries. Of the middle pair, the right 

 is obliterated, but the left remains to constitute the arch of the aorta. 

 Of the lowest pair, the right forms the right and left pulmonary arteries, 

 and the left constitutes the ductus arteriosus. 



The blood-system having reached its full development, the foetal circu- 

 lation may be described us follows : From the placenta ox- The f^i c j r . 

 idized blood is brought through the umbilical vein, a part cuiation. 

 passing into the ascending cava through the ductus venosus, and the rest 

 into the liver through the vena portas, from which, by the hepatic vein, 

 it also reaches the ascending cava. In its passage to the heart it be- 

 comes adulterated with blood derived from the trunk and lower extrem- 

 ities. It next gains into the right auricle, and, to some extent, is kept 

 from contamination with the venous blood coining through the descend- 

 ing cava by means of the Eustachian valve, which directs the arteri- 

 alized blood through the foramen ovale into the left auricle, from which 

 it gains the left ventricle, and also directs the venous blood of the de- 

 scending cava into the right ventricle. The blood which is in the left 

 ventricle is driven therefrom into the ascending aorta, and supplies the 



