The somites are formed in the mesoderm at the left and right side of the neural walls. In this stage, they are visible as 4 to 5 segmented paired blocks. Afterwards these structures will differentiate in to the vertebrae, the ribs, a part of the skin and the dorsal muscles. Only this head region elevates above the underlying area pellucida. In this preparation, one can see the chorda (notochord) in the region of the differentiating foregut.