The essays structure has to include the following:
1.1. Explain left-right asymmetry in bilateria (vertebrates and invertebrates / protostomes and deuterostomes)
1.2. Different possible mechanisms of symmetry breaking have been proposed
1.3. Define nodal flow and lay out the essay plan
2.1. Go through the experimental evidence for nodal flow
2.2. Structure by different symmetry breaking mechanisms that determine the left right axes (two cilia hypothesis, planar cell polarity, cytoskeleton hypothesis, ion flux model)
3.1. Discuss the counter-arguments, gaps in the evidence and avenues for future research.