Carrel advanced this claim in his 1912 Journal of Experimental Medicine paper (PMID 19867545) and sustained it through three decades of public statements and the 34-year culture maintained in his Rockefeller Institute laboratory. His experimental evidence was a single chick-embryo heart culture designated number 725, started on 17 January 1912, that allegedly continued to grow for over 34 years — well beyond the natural lifespan of a chicken.

The claim is a surrogate-endpoint argument: the in vitro survival of a tissue culture fragment is treated as evidence bearing on the question of human cellular and organismal longevity. No controlled clinical outcome was proposed or measured. Carrel presented the culture’s continued growth as proof of cellular immortality in principle, arguing by extension that proper management of the body’s internal environment — the “fluid” in which cells float — could in principle allow human tissues to avoid the aging that results from environmental deterioration rather than intrinsic cell limits.

Witkowski (1980, PMID 6990125) showed that the culture was almost certainly sustained by inadvertent cellular contamination from the embryonic extract used as nutrient medium. Hayflick and Moorhead (1961, PMID 13905658) and Hayflick (1965, PMID 14315085) provided direct experimental refutation: normal human cells have an intrinsic replicative limit of approximately 50 divisions before senescence, regardless of the quality of the surrounding medium.