Mammal "seeds" (gametes or zygotes) never evolved to be outside the body for any period of time. In theory you could probably do this with a 2,000-year-old pla

Mammal "seeds" (gametes or zygotes) never evolved to be outside the body for any period of time. In theory you could probably do this with a 2,000-year-old platypus egg that had been preserved in amber. But otherwise mammals just tend to have really short shelf lives for these things.