256 Vertebrate Embryology 



The course of the circulation at the end of 

 the third day has already been described 

 (page 201), and need not be repeated at this 

 place, so that we shall, at once proceed to the 



FIG. 78. DIAGRAM OF THE VENOUS CIRCULA- 

 TION DURING THE LATER DAYS OF INCUBATION. 

 (After Foster and Balfour.) 



H, heart. V.S.R, right vena cava superior. V.S,L, left 

 vena cava superior. The two venae cavae superiores are the 

 original "ducti Cuvieri " ; they still open into the sinus 

 venosus and not independently into the heart. _/, jugular 

 vein. Su.y, superior vertebral vein. In.V, inferior vertebral 

 vein. W, wing vein. V.C.l, vena cava inferior, which re- 

 ceives most of the blood from the inferior extremities, etc. 

 Z>.F, ductus venosus. P.V, portal vein. M^ vein from intes- 

 tines to portal vein. Of, vitelline vein. U, allantoic vein ; 

 the three last-mentioned veins unite to form the portal vein. 

 /, liver. 



description of the circulation as it is during 

 the latter part of the period of incubation. 



" The heart is now fully formed. The sinus venosus 

 has become absorbed into the right auricle, of which it 

 now forms part: the auricular septum is still incomplete, 

 the large foramen ovale allowing blood to pass freely 

 from the right auricle to the left auricle. The ventricular 



