Also during this period of time, as I mentioned earlier in my description of how you preserve cells in liquid nitrogen or in super cold conditions... during the period that we were writing our paper, I had described and derived 25 cell populations. 25 different so-called diploid, which means the normal number of chromosomes... the term diploid means, in the case of humans, 46 chromosomes. And I may use that term frequently. That the cells can be preserved in liquid nitrogen.

In the early days during the time the paper was written, we had... liquid nitrogen storage was not known in cell culture and we stored our cultures in a deepfreeze, in a... well, later... later in a CO2 freezer. But when the paper was being written, our cell strains were stored in an electrically-driven deep freeze with a temperature near that of dry ice, -7 degrees Celsius. So, during the writing of the paper, that freezer failed and all of the cultures described in my original paper published in 1961 were dead by the time the paper was published.

During that... when I was informed of the failure, of course, it was very disturbing and I simply started to make another cell strain which, by that time, was an easy proposition. I knew very well how to do it, access to human embryonic tissue was still easy and I'll return to that issue in a moment. But the reason I mention this is that during the time these cells were frozen before we lost them in the freezer failure, we had withdrawn ampoules of cells at various population doublings.

Just to make it simple, if you took an ampoule of cells at the twentieth population doubling that was frozen, reconstituted them in a culture vessel and then continued to subculture them as usual as they divided, you learned that the cells had a memory. They remembered at what population doubling, in this case let's say 20, and they would go on to undergo 30 more doublings. If you reconstituted one at the tenth doubling, it would have 40 left. If you reconstituted one at the fortieth, it would have ten left. Well, that was an amazing observation and the only way I could account for that is to argue that the cells had a memory, they had a mechanism for counting the number of population doublings. That seemed really bizarre but, nonetheless, you had to consider it and I did and this is a substantial part of one of my original papers.