<P> A pronucleus (plural: pronuclei) is the nucleus of a sperm or an egg cell during the process of fertilization, after the sperm enters the ovum, but before the genetic material of the sperm and egg fuse . Sperm and egg cells are haploid, meaning they carry half the number of chromosomes of somatic cells . The male and female pronuclei don't fuse, although their genetic material does . Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes . Their chromosomes can then combine and become part of a single diploid nucleus in the resulting embryo, containing a full set of chromosomes . </P> <P> The appearance of two pronuclei is the first sign of successful fertilization as observed during in vitro fertilisation, and is usually observed 18 hours after insemination or ICSI . The zygote is then termed a two - pronuclear zygote (2PN). Two - pronuclear zygotes transitioning through 1PN or 3PN states tend to develop into poorer - quality embryos than ones who remain 2PN throughout development, and may be significant in embryo selection in IVF . </P>

Fusion of pronuclei from male and female gametes is known as