What the young can give to the old

Paul Bert, a pioneering 19th-century French physiologist, added a gruesome new procedure to laboratory experimentation: grafting mice together rather as a gardener grafts branches onto a tree. Part of the idea was to discover what qualities could be passed from one mouse to the other through the blood they now shared. In the 1950s such “parabiosis” experiments led to the suggestion that one such thing was youth. Older rats, after several weeks grafted to younger rats, showed signs of rejuvenation.