The development of the eyelids. Part I. External features.

The sequence of developmental events leading to the formation of the eyelids is described in staged human embryos. By the end of the fourth week the optic vesicle lies close to the surface ectoderm. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lense placode (Stage 13). A few days later (about 32 days, Stage 14) the lens placode is indented by the lens pit. A day or two later (about 33 days, Stage 15) the lens pit is closed: however, the lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface. Prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it (stage 16, 37 days). As these grooves deepen, in Stages 17--19, eyelid folds develop, first below, and then above, the eye. In Stages 19--22 the eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19. The inner canthus is established a few days later in Stage 20. Closure of the eyelids is complete in Stage 23.