Model for assembly of the C. elegans eggshell and permeability barrier. Schematic outline of sequential steps in assembly of the eggshell and permeability barrier. The plasma membrane of prefertilization oocytes is coated with an electron-dense vitelline layer. Fertilization activates chitin synthase, which deposits a chitin layer between the vitelline layer and plasma membrane. At anaphase of meiosis I, cortical granules containing CPG-1 and CPG-2 are exocytosed in a wave that proceeds from the anterior (left) to the posterior (right) of the embryo. As cortical granules are exocytosed, the CPG layer is deposited, and a perivitelline space opens up between the plasma membrane and eggshell. Chitin synthase is internalized from the embryo surface after the onset of cortical granule exocytosis. At anaphase of meiosis II, fatty acid derivatives, possibly ascarosides, assemble between the eggshell and the embryo surface to form the permeability barrier. Between the permeability barrier and the embryo surface is the periembryonic space, which receives the contents of vesicles exocytosed after anaphase of meiosis II.