Surface ectoderm gives rise to the lens and anterior surface of the cornea.
Early in the 4th week optic sulci appear and are transformed into hollow outgrowths of the diencephalon called optic vesicles which approach the surface ectoderm. Continued growth of the vesicle produces an invaginated optic cup which is connected to the diencephalon by the constricted, hollow optic stalk. When optic vesicles come in contact with the overlying ectoderm it forms the lens. At first the ectoderm thickens to form the lens placode, which forms a lens vesicle, and it finally becomes the solid lens.