According to a study published Thursday in the journal Cell, scientists have developed a chemical treatment that turns lab mice transparent … It takes roughly two days to make a mouse’s organs transparent and one week more to make a mouse fully see-through.
—Newsweek
and then I read this
If you can't be bothered to click through I'll just give you the end bit
The transparent mice are part of a project that would make the entire human brain see-through.