Pronucleus stage, zygote

Meiosis II is complete about 16 hours after fertilization. At this point, one of the two haploid chromosomal sets, which differ from each other because of crossing-over, is ejected with the second polar body. The oocyte now contains two haploid sets of chromosomes, arranged in a male and a female pronucleus (pronucleus stage). The centriole originates from the spermatozoon.
With the completion of meiosis II and the ejection of the second polar body, genetic uniqueness of the developing human being is established. This is regardless of whether the haploid genomes contained in the male and female pronuclei are still separate, or whether they have merged into one metaphase plate.
During the next 6 hours, the two pronuclei approach each other and duplicate their chromosomes (synthesis- or S-phase). Their nuclear membranes dissolve where they meet but no common nucleus results: the chromatin condenses to form chromosomes, which are arranged in the common metaphase plate or cleavage spindle to produce the zygote. The term zygote is often used to refer to this point in development. However, it could also be used to refer to the pronucleus stage as there is no change in genetic identity from that point.