Embryo Hearts (IMAGE) U.S. National Science Foundation Caption Embryo hearts show evolution of the heart from 3-chambered in frogs to 4-chambered in mammals. Credit Zina Deretsky, National Science Foundation after Benoit Brueau, the Gladstone Institute of Cardiovascular Disease Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.