308 THE CHICK. 



4. The Veins. 



The chief peculiarities in the veins of the chick, as distin- 

 guished from those of the tadpole, consist in the large size and 

 great importance of the vitelline and allantoic veins, which return 

 to the embryo the blood from the yolk-sac and the allantois 

 respectively ; the blood of the vitelline veins being laden with 

 food matter absorbed from the yolk, while that of the allantoic 

 veins is charged with oxygen, and freed from its excess of 

 carbonic acid. 



The blood is returned to the heart by three chief veins, as 

 in the tadpole : (i) and (ii) the right and left Cuvierian veins (Fig. 

 128, VD), which return blood from the head and body of the 

 embryo, and which afterwards become the right and left anterior 

 venae cavae : and (iii) the meatus venosus (Fig. 128, VE), a 

 median posterior vein, which is formed in the first instance by 

 the union of the right and left vitelline veins, vv ; is joined a 

 little later by the allantoic veins, VA ; and, later still, receives in 

 addition the posterior vena cava, vi. At the time of hatching 

 of the chick the vitelline and allantoic veins disappear, or 

 become comparatively insignificant vessels, and the posterior vena 

 cava acquires its adult relations. 



a. The System of the Anterior Venae Cavae. 



Towards the end of the second day a pair of longitudinal 

 vessels, the anterior cardinal veins (cf. Fig. 128, VB), are formed 

 in the mesoblast of the sides of the head, slightly ventral to the 

 level of the notochord. They collect the blood from the sides of 

 the head, and carry it backwards. 



A similar pair of vessels, the posterior cardinal veins, vc, 

 appear about the same time in the trunk of the embryo. They 

 return the blood from the hinder part of the body, and more 

 especially from the Wolffian bodies or embryonic kidneys, as soon 

 as these are formed. 



The anterior and posterior cardinal veins of each side unite, 

 opposite the heart, to form a short transverse vessel, the Cuvierian 

 vein (Fig. 128, VD), sometimes called the ductus Cuvieri, which 

 opens into the sinus venosus, RS, or most posterior division of 

 the heart. 



Throughout the earlier stages of development the two 

 cardinal veins return the blood from practically the whole of 



