272 ZOOLOGY. 



following parts, in order from behind forwards: (1) a 

 portal vein receiving the vitelline and allantoic veins, as 

 \\e\l as those from all parts of the rapidly growing 

 digestive region of the mesenteron ; (2) a ductus venosus, 

 the original main line, largely abandoned in favour of 

 the portal circulation through the liver ; (3) a hepatic 

 vein, the portion between the efferent vessels of the liver 

 and the post-caval; and (4) the part nearest the heart, 

 which is now spoken of as part of the postcaval. 



Eventually the ductus venosus is obliterated. 



Later stages of development begin to show the special 

 features of the adult bird's circulation. As these do not 

 concern us, we shall instead indicate the relations between 

 the circulation at this stage (found also in the embryo 

 rabbit) with that of the adult rabbit (fig. 143, c). 



The third pair of aortic arches (first branchial), as in all 

 types, give off the carotids to the head : they lose their 

 connexion with the dorsal aorta, and, as the neck grows, 

 become drawn out straight, forming the posterior part of 

 the carotid arteries. 



The fourth arch disappears on the right side and persists 

 as the aortic arch of the adult on the left. 



The fifth disappears very early. The sixth also disappears 

 on the right, and persists on the left as the pulmonary 

 arch : its original connexion with the dorsal aorta forms 

 the ductus arteriosus, which persists till birth (chap, iii., 

 10). 



The posterior cardinal vein disappears on the left side, 

 persisting on the right as the azygos (chap, iii., 8). 



The postcaval extends back beyond the kidneys so as to 

 intercept the blood from the hind limbs, etc., which origin- 

 ally flowed into the cardinals. 



The development of the heart is indicated in fig. 143, and 

 hardly needs a verbal explanation. 



7. The Skeleton. The cells of the notochord become 

 vacuolated and take on their characteristic appearance in 

 the chick during the fourth day. About the same time the 

 mesoblastic somites give rise to a tube of cells completely 

 surrounding the notochord. (This tube is formed in the 



