As the caudal flexure develops and the yolk sac stalk moves cephalad, the cloacal membrane and the diverticulum become separated by a layer of mesoderm , the allantois being situated cephalad to the cloacal membrane. Coelomic spaces coalesce forming the future peritoneal cavity. The fourth pharyngeal pouches are present in the 20 somite embryo 2053. The fourth pouch is not present in the 17 somite embryo H951 or the 14-somite embryo 4529. 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 .e.g.
Carnegie Embryo No. 6344. 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.