During ejaculation about 300 000 000 sperm cells are released. They are up to 1000 times smaller than egg cells. Though a very small amount of sperm make it to the site of fertilisation. Only one sperm cell can fertilise an ovum. Fertilisation occurs in the first third of the fallopian tube. After fertilisation the nuclei of the sperm (23 chromosomes) and egg cell (23 chromosomes) fuse to form a zygote restoring the total number of chromosomes humans have (46 chromosomes). The zygote starts to divide right away and will hopefully nestle itself in the endometrium when it reaches the uterus (implantation).