So, this was also happening about this time, that is in '63, '64, and I had also now done more research on these cell populations. And because I had learned that 50 population doublings was the limit for cell cultures derived from human foetuses, and because I'd made this link with aging, it was obvious to do an experiment with tissue derived from human adults. I obtained tissue from human adults, which wasn't terribly difficult at that time, and determined the number of population doublings that they underwent. It turned out that as a group, the adult cell cultures underwent fewer population doublings than the cultures derived from human embryos did as a group. There were variations among individuals in respect to an association with age, but it was clear that there was a difference between the two groups. And, so, that further strengthened the argument that this may... that the phenomenon may be telling us something about aging. And that theme became now more important that interpretation.

The... and that resulted in my second paper. And I should mention that by now the first paper was being, to some extent, recognised, although I should say that it took at least ten years to overcome the dogma. It took ten years for most of the scientific community to agree that normal human cells have a finite capacity to replicate.