(A) Scheme showing the orientation of the pictures presented in B–L. (B) Basal neuroretinal surface increases from early to late optic cup stage (dashed yellow lines). (C) Basal neuroretinal surface was measured in 3D (superimposed orange lines), although RPCs undergo basal constriction during optic cup formation, the surface increases 4.7 fold from early to late optic cup stage, (D–L) transition from optic vesicle to optic cup over time, shown at a ventral (D–F), a central (G–I), and a dorsal (J–L) level. The membrane localized GFP is driven by an rx2 promoter (rx2::GFPcaax), which is active in RPCs. The optic vesicle is bi-layered (D, G, J) with a prospective lens-facing (arrows in D and E) and a prospective lens-averted (arrowheads in D, G, J) epithelium, connected to the forebrain by the optic stalk (os in D), at a ventral level both are connected at the distal site (circle in D), at a central level both are connected distally and proximally (circles in G), notably the morphology of the lens-averted epithelium at a dorsal level is different from central and ventral levels (arrowhead in J). Over time at ventral and central levels (D–F and G–I, respectively), the lens-averted epithelium is being integrated into the forming optic cup (arrowheads in D, E, G, H, I and arrow in H). A patch of cells in the lens-averted domain gives rise to the RPE (asterisks in H and arrowhead in J, K), le: developing lens, os: optic stalk, B and D–L were derived from 4D imaging data starting at 16.5 hpf (D, G, J), one focal plane is presented as Video 1, scalebar in B and C = 50 µm.