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.

Fusion of male and female sex cells (gametes) produced by the same individual. This type of fertilization occurs in bisexual organisms, including most flowering plants, numerous protozoans, and many invertebrates. Many organisms capable of self-fertilization can also reproduce by means of cross-fertilization. As an evolutionary mechanism, self-fertilization allows an isolated individual to create a local population and stabilizes desirable genetic strains, but it fails to provide a significant degree of variability within a population and thereby limits the possibilities for adaptation to environmental change.

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or test-tube conception

Procedure, used to overcome infertility, in which eggs are removed from a woman, fertilized with sperm outside the body, and inserted into the uterus of the same or another woman. The first child thus conceived was born in 1978. IVF includes extraction of eggs, collection of sperm, fertilization in culture, and introduction into the uterus at the eight-cell stage. In a successful procedure, the embryo is implanted in the uterine wall, and pregnancy begins. The most common problem is failed implantation. IVF has been a source of moral, ethical, and religious controversy since its development.

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Fertilization of a human egg. (1) The sperm release enzymes that help disperse the corona radiata elipsis

Reproductive process in which a male sex cell (sperm) unites with a female sex cell (egg). During the process, the chromosomes of the egg and sperm will merge to form a zygote, which will divide to form an embryo. In humans, sperm travel from the vagina through the uterus to a fallopian tube, where they surround an egg released from an ovary usually two or three days earlier. Once one sperm has fused with the egg cell membrane, the outer layer becomes impenetrable to other sperm. Seealso cross-fertilization, self-fertilization.

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Fusion of male and female sex cells from different individuals of the same species. Cross-fertilization is necessary in animal and plant species that have male and female organs on separate individuals. Methods of cross-fertilization are diverse in animals. Among most species that breed in water, the males and females shed their sex cells into the water, where fertilization takes place outside the body. Among land breeders, fertilization is internal, with the sperm being introduced into the body of the female. By recombining genetic material from two parents, cross-fertilization maintains a greater range of variability for natural selection to act on, thereby increasing the capacity of a species to adapt to environmental change. Seealso self-fertilization.

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