Thawing slow-frozen embryos is more complicated. And the exact thawing method required varies, depending on how the embryos were preserved and what they were stored in. Recently, she was tasked with retrieving embryos that had been stored inside a glass vial. She flicks through it over a video call and points to the notes she made about the glass vial. “I don’t like it.”The record-breaking embryos had been slow-frozen and stored in a plastic vial, says Atkinson.