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A number of single-celled organisms multiply by conjugation. In this process, which is analogous to fertilization, two similar unicellular organisms fuse, exchange nuclear materials, and then break apart. Each organism then reproduces by fission; occasionally, after conjugation, the participating organisms do not reproduce, the process in these instances seeming merely to revitalize the organisms. Conjugation is the most primitive method of sexual reproduction by which organisms having genetic characteristics derived from two parents are produced. …

…cell with a head containing the nucleus and a whiplike tail with which it swims. The typical female reproductive cell, which is known as an egg or ovum, is a rounded cell many times larger than the sperm and containing large amounts of cytoplasm surrounding the nucleus. Plant reproductive cells are roughly similar to animal cells, the male cell being known as the sperm or microgamete and the female cell as the ovum or macrogamete.