270 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



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

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

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

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

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

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

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

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

 reproductive organs. 



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

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

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

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

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

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

 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 all 

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

 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 vitel 

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

 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. Th 

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

 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, supf 

 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 

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

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

 abdominal or umbilical vein, opening into the common cardinal. 



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

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

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

 consist of visceral vessels to the digestive tube. The vessels of the appendages— subcla-v 

 to the anterior appendages, iliac to the posterior appendages— consist simply of enlar 

 somatic vessels. 



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

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

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

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

 portal vein. 



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

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

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



