A mouse embryo at 7.5-day in gestation (equivalent to ~23-day in human) has a small patch at the center of its ventral surface, called 'the node'. The node bears hundreds primary cilia, which make vortical motion clockwisely.

DIC image of the node. Image subtraction and contrast enhancement make the cilia make ciliary motion visible.
Since the rotational axis of the node cilia against cell surface are tilted posterior, resultant flow in the node becomes directional flow to the left (nodal flow).
Nodal flow visualized by microbeads: See the movie.
This nodal flow determines left-right specifications in later development. This idea was confirmed by applying artificial nodal flow to culture embryos: when the flow in the node was rightward, the embryo developed with inverted left-right asymmetry including heart looping, embryonic turning, and expression of nodal (left-specific gene).
Articial nodal flow experiment.
Our interest is the mechanism how the nodal flow triggers asymmetric gene expression, for which several models have been proposed but still controversial.
So far we have found that the node cells have intermittent intracellular Ca2+ elevations, and the frequency become greater at the left side of the node than the right when the nodal flow is established. Our and other groups' studies suggest the Ca2+ asymmetry is essential for subsequent left-right asymmetry establishement. We are trying to connect the nodal flow and the asymmetric Ca2+, and the subsequent gene-level left-right asymmetry.
Ca2+ elevation heat map in the node. See the movie.