In their experiments, Briggs and King used body cells from frog embryos. From these cells, they were able to produce several tadpoles. The mystery of cell specialization Briggs and King used embryos consisting of only a few thousand cells as the source for body cells and nuclei, because at that stage of development an embryo's cells are still relatively unspecialized. As an embryo develops into a completely formed organism consisting of billions of cells, its cells become increasingly specialized. Some cells become skin cells, for example, while others become blood cells.
Skin cells can normally make only more skin cells.
Likewise, blood cells can normally make only blood cells. By contrast, each of the unspecialized cells of an early embryo is capable of producing an entire body. At the time of Briggs's and King's experiment, researchers were not sure whether specialization occurs because different cells get different assortments of genes or because genes that are not needed in a particular kind of cell become inactive. During the development of an embryo, the embryo's cells divide and become increasingly specialized. Specialization means that cells destined to be part of the organism's skin, for example, can normally give rise only to more cells of that type.