|| Cardiac Looping
Cardiac looping occurs in humans from late in the fourth week to early in the fifth week. The heart tube is initially straight in an almost rostro-caudal (head-tail) direction) virtually fixed at the upper and lower ends. As the heart tube is grows and elongates it begins to fold or "loop" inside the pericardial cavity.
Links: Quicktime | Quicktime version | Flash version | Tutorial - Cardiac Looping | Cardiovascular System Development