THE VASCULAR SYSTEM 323 



Ultimately, in Sauropsida, animals with very large eggs en- 

 cased in a porous shell, the allantois comes to line the entire 

 shell and serves as the embryonal respiratory organ; in mam- 

 mals it forms the main part of the placenta and umbilical cord, 

 and functionally replaces the yolk sac, which is here a useless 

 rudiment, although equipped with its full complement of 

 blood-vessels. In both cases the allantois is cast away from 

 the embryo at birth, haemorrhage being prevented by an 

 atrophy of the blood-vessels at the point at which they leave 

 the body. 



Further important modifications of the circulatory system 

 are caused by the development of liver and kidneys and by 

 the increase in bulk of the intestine. Owing to an original 

 continuity between the yolk sac and the intestine, the veins 

 from this latter organ empty into the vitelline veins, forming 

 a compound vein, composed of intestinal and vitelline branches, 

 the omphalo-mesenteric. Of these the right one does not 

 develop beyond a certain point, and the main, and ultimately 

 the entire, duty falls upon the left. About this the develop- 

 ing liver grows, and in such a way as ultimately to include it 

 within its substance, and as a result of this that part of the 

 vein which runs through the liver becomes divided into a 

 system of capillaries. The result of this is that the blood 

 coming from both yolk and intestine has no longer any way 

 of getting directly into the heart through a large vessel, but 

 must first pass through the capillary system of the liver, and 

 be re-collected upon the other side. From this stage on the 

 single omphalo-mesenteric vein, that originally of the left side, 

 becomes known as the portal vein, and the collecting vein upon 

 the other side of the liver, which brings the blood from that 

 organ into the heart, forms the hepatic. Throughout this 

 portion both of the original vitelline veins are preserved, and 

 it thus happens that there are two hepatic veins, but only one 

 portal. 



A similar change is that inaugurated by the development of 

 the embryonic kidney. The blood comes back from the tail 

 in a median caudal vein, which, posterior to the cloaca, divides 



