For a new individual to emerge, gametes merge into pairs, one male and one female, which play different roles in the formation of the offspring. This fusion is fertilization or fertilization.
Both carry the same haploid amount of chromosomes, but only female gametes have nutrients that feed the embryo during its development. In turn, only male gametes are mobile, responsible for the encounter that can happen outside (external fertilization) or inside the female's body (faithinternal foundation). Apart from many arthropods, reptiles, birds and mammals, all other animals have external fertilization, which only happens in the aquatic environment.
When fertilization is external, both males and females produce large gametes to compensate for the loss this environment causes. Many gametes are carried by the waters or feed to other animals. In animals with internal fertilization, females produce only one or a few gametes at a time, and they are protected within the reproductive system.
In addition to the plasma membrane, the egg has another outer coating, the vitelline membrane. When a sperm makes contact with the vitelline membrane, the acrosome membrane fuses with the sperm membrane (acrosome reaction), releasing the enzymes present in the acrosome.
Acrossome enzymes dissolve the vitelline membrane and pave the way for sperm penetration. By fusing the sperm membrane with the egg membrane, the sperm nucleus penetrates the egg. At this moment, the egg membrane undergoes chemical and electrical changes, becoming the fertilization membrane, which prevents the penetration of other sperm.
Inside the egg, the sperm nucleus, now called the male pro-nucleus, fuses with the egg's nucleus, the female pro-nucleus. Each pro-nucleus carries a haploid lot of chromosomes, and the fusion results in a diploid lot, the zygote. In this cell, half of the chromosomes have a paternal origin and half a maternal origin.