Fig. 1. (A) Top view of a crawling Ascaris sperm. The lamellipod can be divided into three major regions: (LE), the leading edge where polymerization and gel condensation into macrofibers and ribs takes place. In Ascaris, but not in other nematode sperm, hyper-complexed branched MSP ‘ribs’ are prominent and originate in protuberances called villipodia. (PR) the perinuclear region where the MSP gel solates and generates a contractile stress. (IR) the intermediate region between the LE and PR where the gel density is nearly constant. The proximal-to-distal pH gradient affects the polymerization and depolymerization rates at the LE and in the PR. (B) Schematic diagram showing the Ascaris sperm lamellipod in cross section. The ventral-fiber complexes branch dorsally. The MSP gel forms at the leading edge and is connected mechanically to the substratum through the membrane. As the cell moves forwards, the gel remains stationary with respect to the substratum, eventually entering the perinuclear region where it solates and contracts. (C) The graphs show how the pH, adhesion, gel density and the elastic stress vary with position in the lamellipod.