Development of retina and optic nerve. Top, At right, fetal neurosensory retina develops from neuroectoderm as ganglion cells migrate from outer primitive zone of closely packed nuclei to inner marginal zone of fibrils. A few axons from ganglion cells grow toward the optic nerve. Retinal pigment epithelium begins melanization in posterior pole. At left, cross section shows fetal optic nerve with center of vacuolating primitive cells through which axons from ganglion cells will grow toward the brain. Neural crest cells as mesenchyme loosely ring the nerve. Hyaloid artery enters vitreous (fifth week). Bottom, At right, migration of nuclei results in three nuclear layers and plexiform layers. Cross section of optic nerve on left shows axons of ganglion cells (black dots) migrating through vacuolating cells, first in periphery of the nerve. Neural crest cells condense to meningeal sheaths of optic nerve (seventh week).