Movie 4.

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Rapid leftward flow and slow counterflow in a mouse nodal pit. The focal plane is first adjusted to 4 µm above the floor of the nodal pit of the mouse (bottom), where the fluid flows rapidly to the left. The focal plane is then lifted by 6 µm (middle), where circulation of the fluid is evident. Beads flow upward near the right edge and go down near the left edge. Beads above the focal plane move toward the right, while beads below the focal plane move toward the left. Finally, the focal plane is lifted twice by 6 µm (top), where the slower rightward return flow is evident. Reproduced from Okada et al. (2005) with permission.