For the first time ever, researchers have been able to take a cellular-level look inside a mouse embryo in its early stages of growth.

Researchers with the Howard Hughes Medical Institute's Janelia Research Campus in Virginia released a video on Thursday that shows the gut and heart of a living mouse embryo growing over the course of two days.

The footage, which has been condensed into a 19-second clip, shows the tiny organs beginning to take shape. Heart cells can be seen beating by the end of the video.

It’s the first time such detailed video has been captured of a mammal’s living embryo. 

Researchers say capturing the footage was difficult because a growing embryo is in a constant state of motion. To capture the video, the microscope was designed to track the embryo’s smallest movements millisecond by millisecond.

"I wouldn't say our microscope is smarter than a human," said physicist and biologist Philipp Keller in a statement. "But it's capable of doing things that a human operator would not be able to do."

The microscope features two light sheets to illuminate the embryo and two cameras to record images of it. It uses algorithms to track the embryo’s position and keep it in focus in the cameras’ view.

For this project, the cameras captured nearly one million images over the 48-hour period. Researchers then used a cell-tracking program to trace the cells’ movementsand development.

The research will help scientists understand how organs grow, potentially aiding efforts to solve developmental problems that occur before birth.

The findings were published in the journal Cell, and researchers are making the microscope and computational tools publicly available.