In 1901, Hans Spemann published the results of his lens ablation experiment, one of the most significant papers in the history of embryology. With this experiment, Spemann was the first person to provide evidence of embryonic induction. Embryonic induction in when one tissue induces a neighboring tissue to develop into a particular structure.
In his experiment, Spemann cauterized the optic vesicle in a frog embryo during the neurula stage of development. The resulting tadpole had a missing eye and lens on the side where the optic vesicle had been removed, although the other eye developed normally.
Without the inducing characteristics of the optic vesicle, the ectoderm never became placode tissue. In normal development, the placode tissue goes on to induce the formation of the optic cup, which in turn induces the placode tissue to form the cornea and lens of the eye.