This is when two separate, primordial heart buds join inside the embryo and begin to beat. Now scientists have located the molecule that jump starts this critical merging. When a Dutch researcher, Fried Zwartkruis, located a gene that triggered S1P to draw the heart halves together, he was working in Boston. His girlfriend, meanwhile, was thousands of miles away in Holland. It's incredibly visible.
Zebra fish are perfect laboratory specimens since they and their eggs are completely transparent.
Zon explains the zebra fish female lays about 300 eggs every week which are then externally fertilized. The eggs are transparent, as are the embryos inside. By placing fertilized eggs under a microscope, scientists have observed as two heart buds inside the zebra fish embryo migrate toward each other and merge about 22 hours into the embryo's life. The heart begins beating soon thereafter. To zero in on the gene and the molecule that cause the two sides of the heart to join, Didier and Zwartkruis analyzed the eggs of zebra fish whose mutations prevented their primordial heart halves from binding.