optic stalk, one of a pair of slender embryonic structures that becomes the optic nerve. In the embryo the optic stalk develops during the second week and attaches the optic vesicle to the wall of the brain. The stalk becomes complete during the seventh week of pregnancy when the choroidal fissure closes and is later converted into the optic nerve when retinal nerve fibers fill the cavity of the stalk. A few fibers grow into the stalk from the brain. About the tenth week after birth, the fibers of the optic nerve receive their myelin sheaths. Compare optic cup.
