274 



COMPARATIVE ANATOMY. 



embryo undergoes a longer intra-uteriue existence, the allantois 

 comingTnto 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 witli the maternal 

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



FIG. 218. DIAGRAM OF THE VKNOI-S ('nirri.ATioN IN TIIK CHICK DUIUN<; THU 

 LATER DAYS OK INCUBATION. (After Balfour.) 



//, In-art; r,V. /.', riht precaval ; VS.L, left precaval ; S.J', sinus veiiosus : tin- 

 two preeavals an- tin- original "tluctus (Juvieri " : tln-y opi-n into tin- sinus 

 venosus ; J, jugular vein ; SU.1", anterior vertebral vi-in ; //', subrlavian ; 

 V.C.I, postcaval ; HP, hepatic veins ; DV, ductus venosus ; JT, portal vein ; 

 .17, iiu-si'iiti-ric 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 i 



pp. 10-l^J. Tin- riiil>i-yos of all Mammals except Monotremes 



;md Marsu]iia Is rcacli this high stage of specialisation. 



The allantois then no longrr has a, simple sai'-like form, hut 

 becomes solid. The part outside the body of the f'n-tus disappears 



