This is the longevity claim Carrel drew from his tissue culture experiments and articulated at length in Man the Unknown (Harper & Brothers, 1935). He reasoned that because his chick-heart culture appeared to grow indefinitely when supplied with fresh embryonic extract, the aging of the whole organism must be an artifact of the progressive deterioration of its internal milieu — the blood plasma and tissue fluid that bathe cells in vivo — rather than anything inherent to the cells themselves.

The claim is mechanism-only: it posits a causal pathway (degeneration of the bodily fluid → cell aging → organismal aging) without measuring any hard endpoint in humans or animals. Carrel proposed eugenics as the practical means of maintaining the biological quality of the population and thereby preserving the fluid environment that would sustain cellular and human longevity.

Hayflick (1965, PMID 14315085) directly refuted the mechanism in his mixing experiments: combining cells from old and young donors showed that the proliferative decline of aged cells is intrinsic to those cells, not transferred to younger cells via the shared medium. Witkowski (1980, PMID 6990125) showed the chick-heart culture itself was almost certainly sustained by fresh cell input from the nutrient extract, removing the experimental foundation for the claim.