Haploid nucleus formed by either the head of the sperm or the nucleus of the oocyte after fertilization but before their nuclei fuse to form the diploid zygotic nucleus.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 797 (https://doi.org/10.1515/pac-2015-1202)