270 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the renal portal veins are the posterior part of the posterior cardinal veins, and that the post- 

 caval vein between the kidneys is formed by the subcardinal veins, chiefly the right subcardinal. 

 Similarly in mammals the postcaval vein is formed of the distal ends of the posterior cardinals, 

 of the right subcardinal, of the hepatic veins (vitelline) in the region of and anterior to the 

 liver, and in the region between the kidneys and the hind limbs of other subordinate veins. This 

 will be clearer by reference to Figure 59. The manner of origin of the postcaval from so many 

 different sources explains the numerous variations common to this vein, such as the frequent 

 finding of a double postcaval, and further explains the asymmetrical origin of the veins to the 

 reproductive organs. 



In conclusion it may be stated that the embryology of the mammalian circulatory system 

 furnishes a beautiful and striking example of the repetition of evolutionary stages. In its 

 development the mammalian circulatory system passes successively through each of the stages 

 which we have found to persist as the adult condition in the types we have studied, and the 

 evolution of this system can be determined equally well either by studying its development in 

 the mammal or by studying and comparing its form in the adults of the different classes of 

 vertebrates, which were ancestral to the mammal. 



G. SUMMARY OF THE CIRCULATORY SYSTEM 



1. The entire circulatory system is derived from the mesoderm. 



2. The first blood vessels are the vitelline (omphalomesenteric) veins. These course along 

 the intestine and are continued posteriorly as the subintestinal vein. In forms with yolk 

 sacs they are the veins of the yolk sac. 



3. As the walls of the hypomere fuse on the ventral side of the embryo the two vitelline 

 veins unite to form the heart. The heart lies in the median ventral part of the body inclosed 

 in the ventral mesentery. 



4. The anterior end of the heart continues forward as the ventral aorta. 



5. The ventral aorta forms a series of loops, the aortic arches, around the pharynx. These 

 unite dorsally to form the dorsal aortae, at first double, but subsequently fusing to a single 

 vessel. In typical vertebrates there are six pairs of aortic arches. 



6. The dorsal aorta proceeds posteriorly along the mid-dorsal line of the coelom, supply- 

 ing branches to all parts of the body below the heart. 



7. The chief somatic veins at first are the anterior and posterior cardinal veins uniting at 

 the level of the heart to a common cardinal vein on each side which enters the sinus venosus; 

 the subcardinal veins extending along the kidneys; and the vein of the lateral body wall, the 

 abdominal or umbilical vein, opening into the common cardinal. 



8. Both arteries and veins are provided with paired segmental and unpaired non-segmental 

 branches. The former are of two kinds: the somatic vessels to the products of the epimere, 

 and the lateral visceral branches to the products of the mesomere. The unpaired branches 

 consist of visceral vessels to the digestive tube. The vessels of the appendages subclavian 

 to the anterior appendages, iliac to the posterior appendages consist simply of enlarged 

 somatic vessels. 



9. In all vertebrates the vitelline and subintestinal veins become converted into the hepatic 

 portal system, as explained in the introduction of this section. The proximal portions between 

 the liver and the heart form the hepatic veins; within the liver a network of capillaries is 

 produced; posterior to the liver one of the vitellines with the subintestinal becomes the hepatic 

 portal vein. 



10. The renal portal system develops from the posterior parts of the posterior cardinal 

 veins which detach from the anterior portions and are then known as the renal portal veins. 



They collect from the tail and pour their blood into the kidneys from which the blood passes into 



