The liver is constructed according to one fundamental plan in all vertebrates. It is a continuous mass of cells tunnelled by a labyrinth of lacunae, in which the network of sinusoids is suspended. This structure is termed a muralium (‘wallwork’, system of walls). The walls are predominantly two cells thick in lower vertebrates and in some primitive birds. They are only one cell thick in a few other primitive birds and in all song birds and mammals.

Where the liver parenchyma touches the portal and hepatic canals, and the external capsule, it is bounded by the limiting plate, a continuous sheet of liver cells.

The portal veins send short inlet venules through holes in the limiting plate to supply the sinusoids. Individual sinusoids empty into the central veins.

The sinusoids are lined by one kind of cells: potential phagocytes. These cells never bridge the lumen of sinusoids.

The hepatic arteries have direct inlets into the paraportal, intermediate and centrolobular sinusoids.

Meshes of bile canaliculi surround each individual liver cell in mammals. In lower vertebrates they rarely form meshes. They drain into intralobular and periportal ductules.

The liver lobules are ephemeral. They are only expressions of the temporary blood-pressure conditions. Reduction of the portohepatic blood-pressure gradient abolishes hepatic lobules and creates ‘portal’ lobules. This process is reversible.

In man, each portal triad (portal vein branch, hepatic artery branch, hepatic duct tributary) has its own sharply delimited territory. There are normally no anastomoses between territories.

These portal segments are united by the hepatic veins which cross the boundaries of portal segments.

The major branches and roots of these vessels and ducts have been mapped and named. Applications to surgery are suggested.

The uniform structure of adult livers arises from a great diversity of developmental processes. Twelve fundamentally different modes of development have been found among thirty species of twenty-eight genera in sixteen orders out of eight classes of vertebrates. In ten species the mesoderm participates in the formation of the hepatic parenchyma.

The ductal system develops in some cases as an outgrowth from the gut; in more cases it develops from liver cells and joins the intestine secondarily.

Von Baer's first and second law of embryogenesis, as well as the biogenetic law of Müller and Haeckel, are contradicted by these observations. New evidence is presented for the non-specificity of germ layers.