The eye is derived from three of the primitive embryonic layers: surface ectoderm, including its derivative—the neural crest; neural ectoderm; and mesoderm. Endoderm does not enter into the formation of the eye. Mesenchyme, derived from mesoderm or the neural crest, is the term for embryonic connective tissue. Most of the mesenchyme of the head and neck is derived from the neural crest.The surface ectoderm gives rise to the lens, the lacrimal gland, the epithelium of the cornea, conjunctiva, and adnexal glands, and the epidermis of the eyelids.The neural crest, which arises from the surface ectoderm in the region immediately adjacent to the neural folds of neural ectoderm, is responsible for formation of the corneal keratocytes, the endothelium of the cornea and the trabecular meshwork, the stroma of the iris and choroid, the ciliary muscle, the fibroblasts of the sclera, the vitreous, and the optic nerve meninges. It is also involved in formation of the orbital cartilage and bone, the orbital connective tissues and nerves, the extraocular muscles, and the subepidermal layers of the eyelids.The neural ectoderm gives rise to the optic vesicle and optic cup and is thus responsible for the formation of the retina and retinal pigment epithelium, the pigmented and nonpigmented layers of ciliary epithelium, the posterior epithelium, the dilator and sphincter muscles of the iris, and the optic nerve fibers and glia.The mesoderm contributes to the vitreous, extraocular and lid muscles, and the orbital and ocular vascular endothelium.