The primitive biliary duct, portal vein and hepatic artery reach the liver between the layers of the lesser omentum. The venae revehentes (hepatic veins) reach the sinus venosus at the attachment of the liver to the septum transversum (primitive diaphragm). The first important change, resulting in a rearrangement of these peritoneal layers, is produced by the connection of the umbilical with the rudimentary portal vein. This junction occupies a relatively wide area on the caudal surface of the liver, and the layers of the lesser omentum are separated somewhat at this point to accommodate the enlarging vascular structures between them. More especially is this the case with the right leaf of the primitive gastro-hepatic omentum. A species of lateral diverticulum is formed by this leaf so as to include the umbilical vein at its junction with the portal. The membrane in the region of this diverticulum turns its surfaces dorsad and ventrad, and its free edge toward the right (Fig. 277). With the gradual increase in the size of the vessels, and with the transverse position which the rotation of the stomach imparts to the opposite border of the lesser omentum attached to the lesser curvature, this transversely disposed portion gradually exceeds in length and size the part of the original omentum enclosing the umbilical vein. This vessel and the investing peritoneum become lodged in a sagittal depression on the caudal surface of the liver (rudimentary umbilical fissure), while the transverse portion, developed as indicated, surrounds the structures connected with the liver at the future transverse or portal fissure. [Illustration: FIG. 277.--Schematic view of embryonic liver, showing influence of vascular connections on the arrangement of the lines of peritoneal reflection.] [Illustration: FIG. 278.--Later stages, showing development of transverse fissure, Spigelian and caudate lobes.] Schematically this rearrangement of the hepatic peritoneal lines of reflection can be shown in Fig. 278. It will be observed that in this way a small part of the caudal surface of the right lobe has become partially marked off from the remainder as a rudimentary Spigelian lobe, bounded ventrally by the transverse fissure and lesser omentum attached to the same; to the left by the two layers of the lesser omentum containing the ductus venosus; while the limit cephalad is afforded by the reflection of peritoneum from liver to diaphragm, forming part of caudal layer of right coronary ligament. To the right this rudimentary Spigelian surface is directly continuous with the rest of the dorsal and caudal surface of the right lobe (Fig. 277). Finally a definite right limit is given to the Spigelian lobe by the increasing size of the postcava and its closer connection with the liver. This vessel now assumes the position of the main venous trunk entering the heart from below.