524 EMBRYOLOGY OF THE CHICK 



least until the nephroi are formed). It is the allantoic circulation which 

 permits the escape of carbon dioxide and other waste matters. 



Therefore, the intra-embryonic circulation has nothing to do with 

 either manufacturing blood or throwing out waste matter (until the 

 nephroi are formed) ; it serves only as the carrier, distributor, and col- 

 lecting system of both food and waste materials. 



As all three systems, intra-embryonic, vitelline, and allantoic send 

 their vessels to and from the heart, the contents of all three systems 

 mingle in that organ, although of course the vitelline circulation is the 

 richer in food material, and the allantoic the richer in waste matter. 



It is at this point that the student must again remember that arteries 

 need not necessarily carry blood rich in food matter, but that an artery 

 is any blood-vessel carrying blood away from the heart under a high 

 pressure. This pressure probably accounts for the fact that arterial 

 walls are thicker and stronger than venous walls. 



Veins are the carriers of blood to the heart. 



THE VITELLINE CIRCULATION 



This has been described in detail at an earlier period. 



THE ALLANTOIC CIRCULATION 



We have already spoken of paired vessels extending through each 

 segment of the embryo which arise from the aorta at about the level 

 of the allantoic stalk. One pair of these segmental vessels increases in 

 size as the allantois grows, and is distributed over the allantois in a rich 

 plexus. As the allantois lies close under the shell, there is thus afforded 

 a large area where gases can easily be exchanged and oxygenation be 

 brought about. After such oxygenation and the extrusion of the carbon 

 dioxide, the allantoic blood is gathered by the allantoic veins, and car- 

 ried back to the heart. 



The excretory ducts later develop in the embryo and then empty 

 into the allantoic stalk close to its cloacal end. It is at this time that 

 the allantois begins to function as a receptacle for solid waste matters, 

 which, after the fluid parts have been evaporated, retains this waste- 

 matter until it is thrown off at the birth of the animal. 



The right and left allantoic veins run cephalad in the lateral body- 

 walls of the chick, and enter the sinus venosus, one on each side of the 

 omphalomesenteric vein. These two allantoic veins will shortly fuse 

 and form a single umbilical vein (Fig. 308). 



The yolk-sac is regarded as a diverticulum of the intestine, and the 

 allantois as a diverticulum of the urinary bladder, which itself is an out- 

 growth of the alimentary tract. 



These outgrowths carry their blood vessels with them. Therefore, 

 the omphalomesenteric artery and the vitelline veins (these latter are 



