People as early as 1880 had envisioned an experiment where you took the nucleus, with its genes, from one cell and put it into another cell, and asked if the cell that received the new nucleus would then change.  So if you could do this, you would take the nucleus out of, say, a skin cell and put it into a brain cell and see if the brain cell became a skin cell — that sort of concept.

Of course, to do that, you have to take away the nucleus from the brain cell. So you replace. It’s a nuclear replacement experiment.  Technically, that’s difficult to do, so people realized that the best way to do that is to use the egg, which is a large cell, and it’s easier to put a nucleus into the egg or to take the chromosomes from an egg.  So the concept of how to go about this problem was fairly clear.

And the idea was that if you took the nucleus from an adult cell — or specialized cell — and put it into the egg, and took away the nucleus or chromosomes of the egg, you can ask the question whether this incoming nucleus now makes the whole egg form skin or brain, whatever you start with, or whether the egg changes it and makes it go back to the beginning again and behave like the nucleus or chromosomes of the egg.