Erica Watson, a research fellow at Cambridge University, shares amazing images of the three-week gestation of a mouse fetus. "The smallest fetus in the video is the thickness of a penny and the largest one is the size of a blueberry," she explains on the YouTube page of the university's Under the Microscope series. 

The ongoing series makes microscopic photos and videos from the university's labs, like this footage of a T-cell destroying a cancer cell, available to the public. 

For more videos from Cambridge University, visit the YouTube channel