Fig. 2. (A) Side-view of the experimental set-up for imaging sperm-egg interactions (not drawn to scale). Here, the Taylor-Couette apparatus is mounted vertically for use in fertilization experiments. It consists of two nested cylinders, with an 8 mm wide seawater-filled gap between them. R_in and R_out refer to the radii of the inside (6.1 cm) and outside (6.9 cm) cylinders, respectively. Counter-rotation of the two cylinders produces a steady laminar flow over a wide range of shears (0–10 s^–1) at low Reynolds numbers (Re<150). Further details are provided in the text. (B) Top view of the seawater gap between vertically mounted cylinders with arrows denoting flow velocity vectors. A cross-over point of no translational velocity occurs between the counter-rotating flows. (C) A sequence of overlaid, digitized images (at 0.1 s intervals) of a single sperm, as it slips past an egg at 2 s^–1. The egg is positioned at the cross-over point, and thus remains stationary, while rotating from right to left. Bar, 100 µm.