At this time, the primordium of the brain appears as a thickening in the rostral end of the neural tube. Such thickening causes the brain to project into the amniotic cavity and later to overhang over the oropharyngeal membrane and the heart. The folding of the embryo leads to the internalization of the some part of the endoderm of the yolk sac. The head folding also results to other alterations such that pericardial coelom lies ventral to the heart and cranial to the septum transversum.