What keeps multiple sperms from penetrating the egg? Some force must or is supposed to repel them? Will look for an answer tomorrow.
The chemical reaction that is being visualized in this video with a fluorescent dye actually changes the surface of the egg. Prior to fusing with a sperm, the egg has proteins on its surface that match proteins on the sperm surface (like a key and lock). Once the sperm protein fuses to an egg protein, the egg protein changes shape. That shape change causes a chain reaction across the surface of the egg. After that chain reaction, the egg surface proteins no longer match the sperm proteins.
It is possible for two (or more) sperm to fuse with the egg simultaneously. Eggs fused with more than one sperm will not develop any further.
I recall from 8th grade health class that the egg membrane locks down somehow as soon as a single one gets in.