The fusion of the gametes takes place in the tuba uterina Before fusing with the female gamete or oocyte the gamete or spermcell has to pass three outer layers. They are called the corona radiata the zona pellucida and the oocyte membrane. The acrosome of the spermcell plays an important role in this process.
After fusion of both cells, the membrane of the oocyte is altered by the cortical reaction This makes it impossible for other spermcells to penetrate the membrane.
Next, the pronuclei of the gametes fuse. Fusion results in a diploid zygote because the individual pronuclei are haploid after meiosis The sex of the zygote is determined by the spermcell because it supplies either the X- or the Y-chromosome.