the optic cup with an outer wall (the retinal epithelium ) and an inner wall comprising layers of retinal cells including photoreceptors, bipolar cells and ganglion cells. ... This resolves a long debate [over] whether the development of the optic cup is driven by internal or external cues." Link: http://www.nature.com/news/biologists-grow-human-eye-precursor-from-stem-cells-1.10835 19