3. The lateral veins, which we can, as stated, regard for purposes of illustration, without prejudging their genetic significance, as representing the mammalian embryonic umbilical veins, still present the condition corresponding to the early mammalian embryonal stage shown in Fig. 250. They are veins of the body walls, emptying cephalad of the liver, directly into the ducts of Cuvier, and through them into the sinus venosus of the heart. Fig. 262 shows the arrangement of the venous system in a typical selachian diagrammatically. =2. Amphibian.= (_a_) =Urodele.=--The following points are to be noted in comparison with the preceding form: 1. The two ducts of Cuvier entering into the sinus venosus are formed by the anterior cardinal and subclavian veins, which latter, having appeared with the full development of an anterior extremity, receives the posterior cardinal veins, representing the mammalian azygos system. 2. The renal portal circulation persists. The caudal vein is, however, no longer the only afferent vein of this system. With the full development of a posterior extremity an iliac vein returns the blood from the same and gives a large branch (afferent to the portal renal system), while the trunk continues cephalad as an anterior abdominal vein, corresponding to the lateral selachian vein, emptying in the hepatic portal vein. 3. The efferent veins of the renal portal system no longer unite to form the posterior cardinal, as in the Selachian, but empty into a new median vessel, the inferior vena cava, or postcava, which has replaced the distal segments of the posterior cardinal veins. The postcava now carries the blood from the kidneys directly to the heart. The original posterior cardinal veins still persist in their proximal segments, as smaller trunks connecting the distal part of the postcava with the ducts of Cuvier through the subclavian veins. The ducts of Cuvier represent the precavae (venae cavae superiores) of mammalia and the postcardinals the mammalian azygos veins. 4. The hepatic portal system differs in two respects from the Selachian type. (_a_) The blood returned to the liver from the digestive tract by the portal vein becomes mixed before entering the gland with the blood returned from the posterior extremities and abdominal walls by the abdominal vein. This vein, paired below and continuous with the lateral of the two branches into which the iliac vein divides, becomes united into a single trunk above and empties into the portal vein. The abdominal vein represents the lateral vein of the Selachian and corresponds to the umbilical vein of the higher vertebrates. (_b_) The venae hepaticae revehentes do not empty directly into the sinus venosus, but into the proximal portion of the postcava. Hence the adult urodele venous system illustrates, in reference to the mammalian development, these stages: