During the transition from S8 (presomite stage) to S9 and in later stages, the coelomic ring becomes progressively deformed. At S8 (Embryo Heuser) the embryo consists of three germ layers, and grows mainly in two dimensions. Growth in a third dimension is nearly negligible. The heart primordium is still located at the cranial end of the embryo. When neurulation (formation of neural plate, neural groove, neural tube) begins, the central nervous system grows extensively. Due to the elongation of the neural tube, the anlage of the heart and the pericardial cavity rotate ventrally. The rotation of the heart primordium occurs around a transversal axis that virtually passes through the region of the prechordal plate. The rotation of the heart primordium gives rise to the foregut which, for a while, remains separated from the amniotic cavity by the oropharyngeal membrane.