Hayashi inserted the PGCs into the
testes of mice that were incapable of producing
their own sperm, and waited to see whether
the cells would develop.Saitou thought that it
would work, but fretted. “It seemed like a 50/50
chance,” he says. “We were excited and worried
at the same time.” But, on the third or fourth
mouse, they found testes with thick, dark semi‑
niferous tubules, stuffed with sperm. “It hap‑
pened so properly. I knew they would generate
pups,” says Hayashi. The team injected these
sperm into eggs and inserted the embryos into
female mice. The result was fertile males and
females
6
(see ‘Making babies’).