when sperm and egg fuse together the sperm get binds to corona radiata layer of ovum..
In this process sperm use his acrosome part (head part) to penetrate ovum's corona radiata..
Lysin release from acrosome ensures that only one sperm fuse with ovum.
After penetrating nucleus of spem and ovum get fuse.. And diploid zygote formed.
Hope this helps you frnd