The suggestion that I made, that the finite lifetime of cultured normal cells have anything to do with aging, of course, was disregarded by most people and thought to be a very foolish suggestion, and indeed it was considered... unacceptable by Peyton Rous, who rejected my original submission.

But as the years went by in the '60s, I became more and more interested in the biology of aging, although I... as I said earlier, and I confess to the fact that I was disinterested in the subject entirely until these early events occurred and the laboratory observation was made. It became even more attractive when colleagues, namely George Martin and Samuel Goldstein, published papers a few years after mine, in which they not only confirmed my work, but George Martin went even further and confirmed the work in my second paper, where I showed that there was a difference in population doubling potential in the cells cultured from older adults compared to those cultured from human embryos, which suggested a relationship between the chronological differences in those two populations. Samuel Goldstein also confirmed this and extended it. George also extended the work to some extent. This was very gratifying. I recall vividly reading George Martin's paper – which was the first one to confirm what I had found – and intellectually breathing a sigh of relief that, well, someone out there has confirmed my work. Of course it's been confirmed thousands of times since, worldwide. There's no doubt about the phenomenon. And also, as years went by, the relationship to chronological age became more and more apparent. I believe I mentioned earlier that the preservation of the cells in liquid nitrogen resulted in my understanding that the cells had a memory, which was quite unusual and led me to the conclusion that the cells had some kind of counting mechanism. They must count the number of population doublings that they undergo when they're cultured in vitro, in glassware or plasticware now.

The second thing that supported this belief was that all of the foetal embryonic tissue that I cultured, from various foetuses, showed the occurrence of their finite lifetime happening between 40 and 60 population doublings, which was rather unusual. Why should they all stop replicating during that narrow window? So I, in fact, even later, named this unknown mechanism of cell memory. I called it a 'replicometer', because it was measuring replications. It was not measuring the passage of time, which some people early on confused, but measuring a number of population doublings. Of course I was in total ignorance of what was to occur about ten years later, which I will explain subsequently.