l1928- Hans Spemann uses a salamander embryo to demonstrate that the cell nucleus directs cellular division. Ten years later, he proposes replacing the nucleus in an egg cell with the nucleus from another cell. l1952- Robert Briggs and Thomas J. King extract the nucleus from the cell of an advanced frog embryo and insert it into a frog egg that the undergoes division. l1972- John Gurdon transplants the nuclei from frog embryo cells into unfertilized eggs that develop into short-lived tadpoles. He later shows that transplanted nuclei revert to an embryonic state.