Development of the Eye (2nd of 3) Beginning of 4th week (4 mm, lens placode appears)
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 progressively invaginates (b) to form the optic cup, which remains attached to the forebrain by the optic stalk (precursor of the optic nerve). The asymmetric invagination leaves a groove, the choroid fissure, in the stalk. The adjacent ectoderm thickens to form the lens placode (a & b) which invaginates and (eventually) separates (c) from the ectoderm to form the lens vesicle.
The primary optic vesicle (a) becomes a double-walled optic cup (c). With continued invagination the original lumen of the optic vesicle is reduced to a slit between the 1) inner neural layer and 2) outer pigment layer of the optic cup (c).
Mesenchyme around the optic vesicle will contribute to the fibrous coats of the eye (sclera/cornea) externally and the choroid layer adjacent to the pigment layer. It also forms the hyaloid vessels (c) which pass in the choroid fissure, across the vitreous chamber, to supply the lens.