Fused Heart Tube
By the 25th day of gestation, a "heart" is already pumping and circulating blood through a network of vessels. These initial heartbeats come from a very different organ than the one seen in an adult. The early heart is really only a simple tube twisted back on itself because there is not enough room to grow. However, by the 5th week, the twisted tube fuses and becomes a two-chambered heart with one atrium and one ventricle.