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In principle. what is done is to take the nucleus out of a cell with a very fine micro-pipette or needle and introduce it into an egg. That had been done with amphibians a long time ago, and then there was a long pause of many years before people were clever enough to make that work in the sheep.

It struck me what we should be trying to do was pluck the egg from the ovary and fertilise it in the laboratory. We could do this in animals increasingly... this was the way to go in the human species.

I think we can allow the therapeutic uses of nuclear transplant technology, which we call cloning, without running the danger of actually having live human beings born.

As a brand new graduate student starting in October 1956, my supervisor Michail Fischberg, a lecturer in the Department of Zoology at Oxford, suggested that I should try to make somatic cell nuclear transplantation work in the South African frog Xenopus laevis.

Many people thought that, given my knowledge of the egg, I should analyse embryonic mutants.

There is no way now to get around some use of embryos. But my goal is to avoid using them.

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.

Within one year of starting work, I had found that the nucleus of an endoderm cell from an advanced tadpole was able to yield some normal development up to the nuclear transplant tadpole stage.

If you are in support of in vitro fertilization, then you have to recognize that human embryos are being created in excess of what can be used safely to reimplant for a pregnancy. So they're going to end up being discarded.

Nobody knew in advance that in vitro fertilization would be, by and large, safe.