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1. One of a pair of nuclei undergoing fusion in karyogamy. 2. In embryology, the nuclear material of the head of the spermatozoon (male p.) or of the ovum (female p.), after the ovum has been penetrated by the spermatozoon; each p. normally carries a haploid set of chromosomes, so that the merging of the pronuclei in fertilization reestablishes diploidy.
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pro·nu·cle·us (')prō-'n(y)ü-klē-əs n, pl -clei -klē-.ī also -cle·us·es the haploid nucleus of a male or female gamete (as an egg or sperm) up to the time of fusion with that of another gamete in fertilization see FEMALE PRONUCLEUS, MALE PRONUCLEUS
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n. (pl. pronuclei)
the nucleus of either the ovum or spermatozoon after fertilization but before the fusion of nuclear material. The pronuclei are larger than the normal nucleus and have a diffuse appearance.
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pro·nu·cle·us (pro-nooґkle-əs) the haploid nucleus occurring after meiosis in the germ cells.Medical dictionary. 2011.