(_b_) _Hepatic Portal System._--The veins of the digestive tract and appendages unite to form a hepatic portal vein. The blood after traversing the capillary system of the liver is collected by hepatic veins, which form a dilated hepatic sinus emptying into the sinus venosus of the heart. 2. The middle segment of the intestine, presenting a spiral valve in the interior, gives rise to a vein emptying into the portal vein which corresponds to the subintestinal vitelline vein of the mammalian embryo (Fig. 202). 3. The posterior cardinal veins, also greatly dilated and forming the posterior cardinal sinus, join, near the heart, the veins returning blood from the head, the anterior cardinal or jugular, to form a transversely directed trunk, the duct of Cuvier, which empties into the sinus venosus at the auricular extremity of the heart. Into the duct of Cuvier empties on each side a _lateral vein_ returning the blood from the body walls. This vein can be considered, for our present purpose, as representing in general the abdominal vein of amphibians and reptiles, and the umbilical vein of the mammalian embryo. The adult selachian venous system is therefore to be considered as illustrating the following conditions above encountered in our study of the embryology of the mammalian venous system. 1. The heart illustrates excellently the stage in the mammalian development, in which auricular and ventricular segments have differentiated, but before the division of the cavities into a pulmonary and systemic portion by the development of the auricular and ventricular septa and the division of the arterial trunk into pulmonary artery and aorta. The sinus venosus still exists, as an ante-chamber to the auricular cavity proper, receiving on each side the ducts of Cuvier, which represent the fusion product of the systemic veins, anterior and posterior cardinal. 2. The hepatic portal circulation corresponds to the mammalian stage in which the vitelline veins have become omphalo-mesenteric by joining the intestinal veins. The spiral vein remains as a portion of the original vitelline vein corresponding to the subintestinal segment of the mammalian embryo (cf. Figs. 248 and 249). The selachian portal vein represents the united vitelline veins, into which the veins of the digestive tract open. In the liver we find a simple system of venae advehentes, derived from the branching of the portal vein, a hepatic capillary network, and venae revehentes, the proximal remnants of the original vitelline veins which carry the liver blood to the sinus venosus. The condition of the hepatic circulation corresponds therefore to the stage shown in Fig. 250 of the mammalian development. There is as yet no association of the hepatic venous system with the representative of the umbilical vein (the lateral vein of the selachian).