From S10 to S11, the brain curves ventrally as it continues to grow. The oral depression, which is lined with ectoderm, appears between the brain and the heart prominence. This depression is closed against the foregut (pharynx) by the oro-pharyngeal membrane, the endodermal part of which most likely has arisen from the prechordal plate. In the region between the reflection of ectoderm into amnion and the transition towards the umbilical vesicle, there is a large accumulation of condensed mesoderm: the septum transversum. This material represents the transition from parietal to visceral mesoderm in the lateral plate. At S9, it was situated cranially to the pericardial cavity. However, after the cranial flexion it lies caudally to the pericardial cavity.