274 COMPARATIVE ANATOMY. 



embryo undergoes a longer intra-uterine existence, the allantois 

 coming into close connection with the walls of the uterus by means 

 of its villi : the allantoic vessels extend into the wall of the uterus 

 and come into more or less close relations with the maternal 

 vessels, thus serving for the respiration and nutrition of the foetus. 



FIG. 218. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK DURING THK 

 LATER DAYS OF INCUBATION. (After Balfour.) 



//, heart; VS.R, right precaval ; VS.L, left precav.il; S. V, sinus venosus : the 

 two precavuls are the original "ductus Cuvieri " : they open into the sinus 

 venosus ; J, jugular vein ; SU. V, anterior vertebral vein ; W, subclavian ; 

 V.C.I, postcaval ; UP, hepatic veins ; DV, ductus veuosus ; PV, portal vein ; 

 M, mesenteric vein ; Of, vitelline vein ; U, allantoic vein. The three last- 

 mentioned veins unite together to form the portal vein. 



In this way arise a placenta and a placental circulation (coin]). 

 pp. 10-12). The embryos of all Mammals except Monotremes 

 and Marsupials reach this high stage of specialisation. 



The allantois then no longer has a simple sac-like form, but 

 becomes solid. The part outside the body of the fcetus disappears 



