Removal of cortical granules prevents:
Cortical granules lie beneath egg plasma membrane and upon calcium rise fuse releasing contents that transform vitelline membrane into fertilization envelope constituting slow, permanent block to polyspermy. If granules are removed by mild centrifugation that stratifies them away or exocytosis blocked by inhibitors, envelope elevation fails despite normal sperm fusion. Fast electrical block still occurs but membrane repolarizes within minute, allowing supernumerary sperm to fuse causing polyspermy, multipolar mitoses, and lethal aneuploidy. Therefore granule removal prevents polyspermy prevention, while sperm attraction, entry, and cleavage initiation can still initially occur until developmental defects manifest and embryo arrests.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Cortical granule removal prevents polyspermy slow block.