Ventral closure of the body wall: the umbilicus
With the growth of internal structures (especially the neural tube and somites), the embryo bulges on its dorsal side into the amniotic cavity. The result is that the embryo expands over the original boundaries of the embryonic disc. In this way, the ectoderm surrounds the umbilicus from both sides. This process resulting in a folding on the lateral edges of the embryonic body is sometimes called lateral folding.