I had a conversation with Paul and we decided to do a critical experiment. It may not have been our first experiment to prove that this... that there was no virus present, but it was the most important one. At that time, although today it sounds strange, the way... we had... we just learned of a way of distinguishing male from female cells in culture. It was learned at that time by a Canadian investigator by the name of Barr, B-A-R-R, that of the two X chromosomes in a female cell, one of the X chromosomes does not disappear – I use that term advisedly – during the time that the cell is not dividing, what we call interphase between mitosis.

So that chromosome remains intact and is stainable, unlike a male cell which only has one X chromosome, of course, that female X chromosome that's visible is called the inactive X chromosome, inactive genetically. And as I said, stainable, so that when you stain cells, you see Barr bodies or inactive X chromosomes, you know it's a female cell. Well, that was critical because by this time, of course, the embryos that we were culturing came from both sexes and we knew which was male and which was female, so we designed an experiment in which we took, let's say, a million male cells from the 40^th population doubling now knowing that these cultures underwent about 50 population doublings before they stopped dividing. That's critical. We took, let's say, a million female cells... sorry, a million male cells at the 40^th doubling and mixed them with an equal number of female cells at, let's say, the 20^th doubling and then we kept unmixed cultures of each sex as controls, cultured, subcultivated every week like the mixture, side by side in the same incubator.

What we soon learned was that the mix... the cells in the mixture could be cultured as long as the female cells. That is the control female cell culture. While the male cell control culture had stopped dividing many weeks before that event occurred. And when we looked at the mixture at the time that it had stopped dividing, all the cells were female.