286 THE SIXTH DAY. [CHAP. 



the ductus venosus from the liver may now be said to join 

 the vena cava instead of being prolonged into the sinus. 

 While this growth of the vena cava is going on, the 

 points at which the ductus Cuvieri enter into the sinus 

 venosus are drawn in towards the heart itself, and finally 

 these trunks fall directly and separately into the auricular 

 cavities, and are henceforward known as the right and 

 left vena cava superior (Fig. 90, V.S.R, V.S.L.). There 



vai- 



f—, 



Diagram of the Venous Circulation during the Later 

 Days of Incubation. 



//. heart. V.S.R. right vena cava superior. V.S.L. left vena cava 

 superior. The two venae cavae superiores are the original 

 ' ductus Cuvieri,' they still open into the sinus venosus and 

 not independently into the heart. J. jugular vein. SU. V. 

 superior vertebral vein. In. V. inferior vertebral vein. W. 

 vein for the wing. V.C.I, vena cava inferior, which receives 

 most of the blood from the inferior extremities, etc. D. V. 

 ductus venosus. P. V. portal vein. M. a vein bringing 

 blood from the intestines into the portal vein. Of. vitelUne 

 vein. U. allantoic vein. The three last mentioned veins 

 unite together to form the portal vein. I. liver. 



The remnants of the inferior cardinal veins are not shewn. 



