Prosencephalon

At S14, the lateral walls of the prosencephalon extend in lateral and cranial directions around the outline of the telencephalon. The hemispheres of the brain develop from these enlargements. The medial part of the telencephalon is called the telencephalon medium or telencephalon impar. The hemispheres of the brain grow very quickly. The telodiencephalic sulcus deepens following lateral elevation of the telencephalon above the diencephalon. The optic primordium is formed in the region of the diencephalon. At S12, it appears initially as an evagination which develops into the optic vesicle (vesicula optica). At S14, the optic vesicle invaginates to form the optic cup, with the spherical lens vesicle (vesicula lentis) embedded inside it.