The hepatic plate in embryos of this stage is an endodermal thickening caudoventral to the heart, at the rostral intestinal portal, and is the precursor of the hepatic diverticulum. Early in the stage the hepatic primordium consists of a thickened endodermal plate abutting the septum transversum and which in part contacts the endothelial lining of the atrium.
Angioblastic cords are present in the septum transversum.
By the 17 somite stage e.g. Carnegie Embryo No. 8116, differential growth of the endodermal plate and its contact with the endothelial lining of the sinus venosus together with bilateral folding of the plate results in a definite diverticulum. The endoderm cells also appear in contact with the mesodermal pericardium and a transverse groove in the diverticulum is probably the first sign of a developing cystic diverticulum. The surface of the former is irregular and is believed to be the first stages in the invasion of the septum by liver trabeculae. The cystic portion is smooth surfaced. The epithelial trabeculae that will later form from the biliary bud invade the septum transversum which provides the future liver framework or stroma.