As soon as the spermcell reaches the membrane of the oocyte the acrosomal reaction takes place.
The acrosomal vesicle bursts, releasing the hydrolytic enzymes that digest the pellucidal zone(1)
Some actin filaments of the head of the spermcell polymerise and form the acrosomal process together with the membrane of the burst acrosomal vesicle. (2 and 3)
The acrosomal process fuses with the membrane of the oocyte blocking other spermcells.