In frogs, the eye begins to develop as the Optic Sulcus at about the 14-somite stage (a). This develops further as an out-pocketing of the fore-brain, as an Optic Vesicle (b). Tissue under the ectoderm begins to bulge inward as the Lens Placode (c), forming first a Lens (d), and then the Sensory layer along with a Pigment layer than becomes the Retina (e). The eye goes on to develop an optic stalk that enervates the Sensory layers from the Brain.

    Spemann in 1901 was able to show that transplantation of the endodermal Lens placode at an appropriate stage allowed induction of the Lens vesicle and complete Lens in other areas of the ectoderm not normally fated to develop as an eye.

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