Fertilization
Human fertilization is the union of a female oocyte (egg) and male sperm, usually occurring in the ampulla of the fallopian tube to form the zygote (single diploid cell). It constitutes the penetration of the oocyte (egg) by a sperm succeeded by fusion of their genetic material. This genetic material consists of the 23 chromosomes contained in the nucleus of the ovum and 23 chromosomes from the nucleus of the sperm. The 46 chromosomes undergo changes prior to the mitotic division which leads to the formation of an embryo having two cells. The fertilized ovum thus begins to divide into several cells, i.e. it starts to undergo cleavage. The two daughter cells are still surrounded by zona pellucida.
Read more about this topic: Human Embryogenesis