The cross currents of events in my laboratory at that time were numerous and so now I'll shift to the work that was being done with the cultured normal human cells, which was happening simultaneously with the mycoplasma events that I described earlier. So that you can get some impression of how active the laboratory was, for that two year period, everything was happening at the same time. And the two year period is 1959-60. The paper on the mycoplasma discovery and the discovery of the human diploid cells strains occurred in 1961, the same year.

At about this time, as I had indicated earlier in describing the technique of growing cells in culture and continuing their propagation for long periods of time, this significant event occurred that I need to describe in some detail. It may be recalled that I mentioned how cells are subcultured from an initial culture made from intact tissue and the cultures upon reaching confluence, or covering the floor of a flask, then stopped dividing and now it's necessary, if you want more cells, to remove them with trypsin from the original culture, divide them into equal halves, put them into bottles and that can be done each week so that the number of bottles can be doubled perhaps twice a week, so that you end up very quickly with more bottles than you can deal with and you can freeze the excess cultures at every... what we call population doubling. And that concept is important as I'll soon describe, so let me define that term now.

Population doubling means precisely what those words say. It does not mean the doubling of a single cell. If you initiate the cultures, as I did, with a million cells, that's the first population. If they double, you now have two million cells and if those two million double, you now have four million cells. We're not talking about the doubling of a single cell. That was a problem – still is a problem – in people's understanding of what we did. It later became a very important fact in respect to some issues that will surface later.