THE VEINS. 311 



appears during the fourth day, and is at first very small, but 

 it becomes larger and more important as the alimentary canal 

 lengthens. 



The mesenteric vein joins the vitelline vein just before this 

 reaches the liver. The blood entering the liver by the meatus 

 venosus is thus derived from three sources : (i) the vitelline vein 

 returns blood, highly charged with nutritious matter, from the 

 yolk-sac ; (ii) the allantoic vein returns blood from the allan- 

 tois, rich in oxygen and freed from carbonic acid ; (iii) the 

 mesenteric vein brings venous blood from the walls of the ali- 

 mentary canal of the embryo. Of these, the vitelline and allantoic 

 veins are, during the period of incubation, much larger than the 

 mesenteric vein ; but inasmuch as the two former vessels return 

 blood from parts outside the embryo, they become obliterated at 

 the time of hatching ; while the mesenteric vein, which alone 

 persists of the three, retains its relations with the liver, and 

 becomes the hepatic portal vein of the adult bird. 



The posterior vena cava appears about the fourth day (cf. 

 Fig. 128, vi). It arises in the mesoblast between the hinder 

 ends of the Wolffian bodies, and runs forwards in the median 

 line, ventral to the aorta ; at its anterior end it joins the meatus 

 venosus, as this emerges from the liver, and just before it reaches 

 the heart. 



As the permanent kidneys become definitely established, the 

 posterior vena cava acquires special relations with them, and 

 increases in size as they develop. At first (Fig. 128), the pos- 

 terior vena cava appears as a comparatively unimportant tribu- 

 tary of the meatus venosus, but with growth of the embryo, and 

 especially with the enlargement of the hind limbs, it steadily 

 increases in size, and towards the close of incubation becomes 

 the larger vessel of the two. 



After the posterior vena cava has attained some size the 

 hepatic efferent veins shift so as to open into it directly, instead 

 of into the meatus venosus, and it is from this time that the 

 vena cava becomes a larger and more important vessel than the 

 meatus venosus. 



5. The Course of the Circulation. 



It will render the description of the development of the 

 blood-vessels easier to follow if a connected account is given of 



