How did Robert Briggs and Thomas King clone tadpoles in the 1950s?

In 1952, American scientists Robert Briggs and Thomas King successfully performed nuclear transfer to clone tadpoles, a procedure previously theorized by Hans Spemann but considered technically impossible at the time.12

To achieve this, Briggs and King utilized DNA from the embryonic cells of the frog Rana pipiens. Specifically, they transplanted nuclei from frog blastula cells into egg cells that had been enucleated—meaning the egg's own nucleus had been removed. This experiment was significant because it demonstrated that nuclei from cells that had already begun to lose some of their totipotent properties could still guide embryonic development.21

This work laid the essential foundation for later advancements in cloning, including the development of somatic cell nuclear transfer (SCNT) used in subsequent decades.12