Early in the 4th week optic vesicles extend from the 3rd ventricle and wall of the forebrain (diencephalon). As the vesicle reaches the surface ectoderm it flattens (a) and progresive invaginates (b) to form the optic cup, which remains attached to the forebrain by the optic stalk (precursor of the optic nerve). Its presence induces the formation of the lens placode from the surface ectoderm.

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