And activities then centred on the chromosomal characterisation of the cells, because it was by this time learned that as the cells approached their... their limited capacity to replicate, a state that I call 'Phase three' – I described the lineage of these cell populations as phase one, which is the period of the primary culture, phase two is the 40 or so period where the cells are actively dividing, and phase three is a period when the cells have stopped dividing.

People generally have called phase three the 'Hayflick Limit'; perhaps I should say a word about the derivation of that term. The derivation occurred in the mid-'60s by a gentleman well-known in the scientific field, at least that time, Sir Macfarlane Burnet, an Australian Nobel Prize laureate who knew about my work and, indeed, made a visit to the Wistar Institute to see Hilary at one time and I was introduced to him then. He went on to write books in several fields outside of the one that he was expert in, which, namely, immunology, and he did a book in which he discussed aging. And in that book, he called what I had observed the 'Hayflick Limit', which is the origin of that term. So, I have Sir Macfarlane Burnet to either thank or not for the use of that term.