228 THE FOURTH DAY. [CHAP. 



coming back again by the vense revehentes, or it may 

 go straight through the ductus venosus without passing 

 at all into the substance of the liver. 



As the alimentary canal by its continued closing in 

 becomes on the fourth day more and more distinct from 

 the yolk-sac, it gradually acquires veins of its own, the 

 mesenteric veins, which first appear as small branches 

 of the vitelline vein, though eventually, owing to the 

 change in the relative size and importance of the yolk- 

 sac and intestine, the latter seems to be a branch of 

 one of the former. 



Corresponding to the increase in the size of the 

 head, the superior cardinal veins (Fig. 74, J.) become 

 larger and more important and are joined by the wing 

 veins (Tf.). As before, they form the ductus Cuvieri 

 (d.c.) by joining with the inferior cardinal veins (c.). 



The latter are now largely developed, they seem to 

 take origin from the Wolffian bodies, and their size and 

 importance is in direct proportion to the prominence of 

 these bodies. They might be called the veins of the 

 Wolffian bodies. 



As the kidneys begin to be formed a new single 

 median vein makes its appearance, running from them 

 forwards, beneath the vertebral column, to fall into the 

 sinus venosus (Fig. 74, V.C.I.). This is the vena cava 

 inferior. 



As the lungs are being formed the pulmonary veins 

 also make their appearance and become connected with 

 the left side of the auricular division of the heart. 



The blood carried to the allantois by the allantoic 

 arteries is brought back by two veins, which very soon 

 after their appearance unite, close to the allantois, into 



