1. The hepatic duct joins the cystic to form the common bile-duct, entering the duodenum by passing obliquely through the intestinal wall (Fig. 242). This form is encountered in man and in most mammals. It is also found in some birds (_Buceros_), many amphibians, and in some fish (_Lophius_). Instead of one hepatic duct two may join the cystic duct separately to form the common bile duct (_Phoca litorea_), or the number of hepatic ducts may be further increased. The separate hepatic ducts then unite successively with the cystic duct. This occurs in many mammals (as _Tarsius_, _Galeopithecus_, monotremes) and in some fishes (_Xiphias_, _Trigla_, _Accipenser_) (Fig. 243). [Illustration: FIG. 242.--Schema of hepatic and cystic ducts. (Nuhn.)] [Illustration: FIG. 243.--Schema of hepatic and cystic ducts. (Nuhn.)] 2. Of two hepatic ducts only one helps to form with the cystic duct the common duct, while the other leads from the liver transversely into the bladder, especially into the neck, forming the hepatico-cystic duct (Fig. 244). This arrangement is found in several mammals (calf, sheep, dog). [Illustration: FIG. 244.--Schema of hepatic and cystic ducts. (Nuhn.)] [Illustration: FIG. 245.--Schema of hepatic and cystic ducts. (Nuhn.)] 3. No common bile-duct is formed. The hepatic and cystic ducts each empty separately into the intestine (hepato-enteric and cysto-enteric ducts), while a hepato-cystic duct carries the bile directly from the liver to the gall-bladder (Fig. 245). _Lutra vulgaris_ among mammalia, the majority of the birds and several reptilia present this type. When the gall-bladder is absent a single large hepato-enteric duct is found, or instead a number of smaller ducts which enter the intestine successively. =I. C. Development of Vascular System of Liver.=--In order to comprehend the peritoneal relations of the adult liver it is absolutely necessary to have a clear understanding of the development of the vascular system in connection with the gland. For our purpose, in the first place, a serial consideration of the successive stages, illustrated by schematic diagrams, will prove most practicable. These diagrams represent the structures in the dorsal view, _i. e._, in the position which they would occupy in the adult liver with the gland resting on its upper or convex surface and with the ventral sharp margin turned toward the beholder (see Fig. 259).