Glossary of Life (9th ed.) by Sadava et al. (2011)
- The cell created by the union of two gametes, in which the gamete nuclei are also fused. The earliest stage of the diploid generation.
- Sadava, D., Hillis, D. M., Heller, H. C. & Berenbaum, M. R. 2011. Life: The Science of Biology, 9th ed. -xliv + 1266 pp. Sinauer Associates, Inc., Sunderland.