740 THE CIRCULATORY SYSTEM 



Anteriorly, the two lateral (ventral abdominal) veins lose their connection 

 with the sinus venosus and merge together to form one ventral abdominal 

 vein; the latter acquires a connection with the hepatic portal vein near the 

 liver. A ventral abdominal circulation is established thus between the hepatic 

 portal system and the iliac veins (fig. 335E, F). 



3) Formation of the Inferior Vena Cava. The inferior vena cava is a vessel 

 not found in the venous system of the developing shark. It is a blood vessel 

 associated with and characteristic of lung breathers. As such, the inferior 

 vena cava appears first among the vertebrates in the lungfishes (Dipnoi) and 

 it functions to shunt the blood from the posterior regions of the body over 

 to the right atrial portion of the heart. That is, the inferior vena cava is a 

 vessel correlated with the division of the heart into two parts. One part is 

 devoted to getting the non-oxygenated systemic blood into the lung region, 

 while the other part functions to propel the aerated blood from the lungs into 

 the head region and other parts of the body. This division of labor within 

 the heart is not necessary in strictly gill-breathing fishes, such as the sharks 

 and teleosts, and, in consequence, an inferior vena cava is not developed in 

 these vertebrates. 



The formation of the inferior vena cava in the anuran amphibia is shown 

 in figure 335C-G and need not be explained further. It is to be observed that 

 it forms from four segments: 



(1 ) a right vitelline vein, 



(2) an hepatic segment, 



(3) a segment which extends posteriad from the liver to the fused sub- 

 cardinal vein, and 



(4) the subcardinal vein. (Consult figure 335E.) 



4) Formation of the Renal Portal System. The renal portal system is inau- 

 gurated among the cartilaginous fishes (i.e., the shark group). It does not 

 exist in cyclostomes. As shown in figure 334 relative to the developing shark 

 embryo, it results from the formation of the subcardinal veins, accompanied 

 by the obliteration of the anterior portions of the posterior cardinal veins. 



Fig. 335. Developing venous vessels in the anuran amphibia. (B-G, redrawn and 

 modified from Kampmeier, 1920, Anat. Rec. 19; H, redrawn from Kampmeier, 1925, 

 J. Morph. 41; I, redrawn from Goodrich after Kerr, 1930, Studies on the Structure and 

 Development of Vertebrates, Macmillan, Ltd., London.) (A) Primitive plan of early 

 circulation in frog embryo. The relationship of the primitive venous system shown in 

 B to the rest of the vascular system is evident. (B) Plan of venous system of 4 mm. 

 embryo of the toad, Bufo vulgaris. (C) Plan of venous system of 6 mm. embryo of 

 the toad, Bufo vulgaris. (D) Plan of venous system of 15 mm. embryo of the toad, 

 Bufo lentiginosus. (E) Plan of venous system of 18 mm. embryo of the toad, Bufo 

 lentiginosus. (F) Plan of venous system of young toad of Bufo lentiginosus, immedi- 

 ately after metamorphosis. (G) Plan of venous system of mature Rana pipiens. (H) 

 Left posterior lymph hearts of an adult Rana pipiens. (I) Internal structure of mature 

 frog heart. 



