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 contain the enervation of the
Sensory layers from the Brain.
Spemann in 1901 was able to show that
transplantation of the 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.