The heart is behind the eyes, about as far as the distance between the two eyes. It beats 100 times a minute to move blood through the rapidly growing embryo.

This image is at a higher magnification than any previous ones. The frame has a width of 0.4 mm or 400 microns. As a reference, a typical cell is about 10 microns in diameter.

Note this interesting article in Science [v 312, p 751, 5 May 2006]: The Embryonic Vertebrate Heart Tube is a Dynamic Suction Pump, by Arian Forouhar et al. The heart starts beating and pumping blood before valves develop, so it must work in a different way than the adult heart. It had been assumed that the early heart functions as a peristaltic pump, but this work (done in Zebra fish) shows that blood flow is controlled by dynamic (inertial) forces, not the static forces that characterize a peristaltic pump.