I wondered whether the nuclear transfer techniques could be used to introduce purified macro-molecules into an egg, and hence into embryonic cells.

Within six months of starting my Ph.D. work in 1956, I had already obtained feeding tadpoles derived from transplanted nuclei of embryonic cells.

I left my frogs, which I had grown, with my supervisor, who had moved to Geneva, and he and a technician grew them up. So by 1962, they were adults, and one could publish a paper to say that these animals, derived from nuclear transfer, really were absolutely normal. So it took a little time to get through.

The aim of a nuclear-transplant experiment is to insert the nucleus of a specialized cell into an unfertilized egg whose nucleus has been removed.

If you explain to a patient what can be done and what might be the downsides, let the patient choose; don't have ethicists, priests, or doctors say you may or may not have replacement cells.

My first attempts to transplant nuclei in Xenopus were completely unsuccessful, because the Xenopus egg, unlike those of other amphibians, is surrounded by an extremely elastic membrane and jelly layer that make penetration by a micropipette impossible.