fertilization, in biology, process in the reproduction of both plants and animals, involving the union of two unlike sex cells (gametes), the sperm and the ovum, followed by the joining of their nuclei. In the flowers of higher plants, the process occurs after pollination has enabled the sperm to contact the egg cell in the plant's ovary. In lower plants and in animals the sperm is actively motile and swims to the egg through an external aqueous medium or through a fluid environment within the reproductive tract of the female. The fundamental principle of fertilization is the same in all organisms. The first sperm to establish successful contact is absorbed by the ovum and the two nuclei unite, thus combining the hereditary material of both parents (see genetics). In higher forms, the sperm contact initiates cell division in the fertilized egg (zygote), and the subsequent embryo develops into a new individual. Cross-fertilization indicates fusion of a sperm of one hermaphroditic plant or animal with an ovum of another, as distinguished from self-fertilization, in which ovum and sperm of the same individual are fused.