Spemann’s experiments with embryos with developed blastopores (exterior mouth opening of a hollow sphere of undifferentiated cells) led him to believe that, in these, it was the transplanted blastopore lip that influenced the developmental fate of host cells. But, as graft and host cells came from similarly pigmented species, it was hard to determine conclusively whether it was the host or the graft that perpetuated the secondary axis. Taking a page from the American scientist Ross Harrison, who in 1903 had successfully carried out embryonic grafts between different frog species, Rana palustris and Rana sylvatica, Spemann decided to use different pigmented newt species, the white Triton cristasus and the dark Triton taeniatus and Triton alpestris. He assigned the actual experimentation to his PhD student Hilde Proescholdt (later Mangold) as her doctoral dissertation. Of the hundreds of embryos she experimented on only five survived, and in these, histological sections showed clear distinctions between the graft and host cells; the graft cells formed mesodermal elements of a secondary embryo under the gastrula surface and above the host ectoderm cells gave rise to the neural tube of the secondary embryo. Spemann published a paper detailing this experiment in 1924.
In 1928, Spemann carried out the first ever somatic nuclear transfer, a step in the direction of cloning. He transferred the nucleus of a differentiated salamander embryo into an enucleated single-cell salamander embryo. The cell adapted to the nucleus and developed into a normal salamander. Spemann next separated a salamander embryo nucleus from the cytoplasm with a hair knot constriction and allowed the nucleus side to develop into a sixteen-cell embryo. Then, moving the nucleus into the cytoplasm side, he tightened the constriction and let the cells on this side divide to form an embryo. The twin embryos developed into normal salamanders. Spemann detailed these experiments in his 1938 book 'Embryonic Development and Induction'.
This success led Spemann to speculate whether the nuclear transfer method experiment might also work using nuclei from older embryos or even adult organisms. He didn't manage to technically bring this about, but Thomas King and Robert Briggs proved him right in 1952.