Cranial flexion
At early S9, the lateral plate is found cranially to the prechordal plate and on the lateral margins of the embryonic disc. The intra-embryonic coelom has formed in its upper part. There the coelom is horseshoe-shaped and is closed to the outside. The upper end of the embryonic disc is formed by the medial part of this coelom (part of the pericardial cavity) and the cardiogenic area (cardiogenic plate) which is located ventrally. During S9, the heart primordium rotates approximately 180 degrees in a ventral direction around a virtual transverse axis passing through the prechordal plate. Due to this, the brain and the prechordal plate become the uppermost part of the embryo. The heart prominence narrows the passage towards to the umbilical vesicle. A cavity, the developing foregut, appears which is covered on all sides with entoderm. The transition from surface ectoderm to amnion becomes shifted ventrally to the caudal edge of the heart prominence.