Already in a young embryo (in stage 10, ca. 28 days) is the liver recognizable as an epithelial budding of the endoderm at the transition from the intra- to extraembryonic part of the umbilical vesicle below the cardiac anlage.
From the distal part of this originally endodermal sprout arises the actual liver, the middle bud forms the gall bladder with its passage and from the caudal bud arises the ventral anlage of the pancreas.
Liver development is not simply a sprouting of liver cells into the transverse septum. Instead, complex interactions between the mesoderm and the epithelial cells of the endoderm take place.
It is only with the formation of the vessel system and the development of the portal vein that the definitive organ structure is assembled.