Question

How sperm enter into egg? (In humans)

Answer 1

This penetrance and activation of sperm occurs prior to the actual merging of the two gametes' nuclei during fertilization.

Explanation:

Fertilization happens as the egg and the 'winning' sperm meet somewhere in the upper one-half of the oviduct. This is also called the uterine tube, or Fallopian Tube.

Sperm have enzymes that will digest the outer layers of the egg, and then the actual nuclear contents of the sperm--23 chromosomes--enter into the egg cytoplasm as a 'pro-nucleus'. These will then merge together with the egg's own (maternal) 23 chromosomes which stand as their own pro-nucleus.
23 chromosomes added to another 23 makes a diploid count of 46 chromosomes; all of these now are inside one cell and one nucleus, called the 'zygote'.
This is fertilization.