Scientists in the Netherlands who are studying cancer cells have come up with a unique but gruesome way to track how they grow into tumors.

They surgically implanted windows into the bellies of live mice so they can watch the tumors grow inside.

In the past 10 years, scientists stitched tiny glass windows into a mouse's skin to watch how breast and skin cancers develop.

They said that before the windows, researchers had no easy way to look directly into the spleen, kidney, small intestine or liver.

The method allowed scientists to learn more about the way cancer cells spread and how they form into secondary tumors.

For example, they found that after cancer cells migrate to a final destination, they move around in the area before condensing into a secondary tumor. Scientists then used drugs to slow down the movement and in turn, slow down the rate of growth leading to the secondary tumors.

Peter Kuhn with Scripps Research Institute  in La Jolla said the discoveries made by the Dutch team were great, but he warned about drawing medical conclusions too quickly from the experiment.

"It is really important to distinguish between cancer in a mouse and cancer in a human," said Kuhn.

Read more here: Scientific breakthrough a step too far? Window stitched into bellies of mice so scientists can watch tumors grow inside