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In protozoa, this is done by two whole animals uniting temporarily for nuclear transfer-conjugation.
Gametic union requires two types of gametes. In metazoans,,one of the gametes is large and heavy, usually with incorporated food material (called the egg or ovum) while the other is smaller and mobile usually with a flagellum (called spermatozoa or sperms). The fusion of such dimorphic gametes is called oogamy, which is generally the rule in metazoans. Despite differences in their appearance, the gametes are alike in one important respect: each has in its nucleus a haploid set of chromosomes, the only difference possible being in the sex-chromosomes.
The development of eggs and sperms is carried out in special organs called ovaries and testes respectively (common term gonad). In the great majority of metazoans an individual possesses either testes or ovary only (unisexual) and thus two types of individuals—female and maleare present in metazoans. There are other differences, called secondary sexual characters, also but they have nothing to do with the reproductary system as such.
The union of gametes—sperm with the egg-is called fertilization. It includes both the union of gametic nuclei and the cytoplasm and leads to restoration of deploy and initiation of cell division. In some animals, fertilization is completed only after the sperm penetrates the egg membrane and enters the cytoplasm. In its simplest form common in many aquatic animals—the process of fertilization requires only the synchronized discharge of male and female products into a particular limited are of the surrounding water where union takes place. This is external fertilization and to achieve positive results animals have to produce hundreds of thousands of eggs and millions of sperms.
Considering the slim chances and the wastes involved, species belonging to higher phyla have evolved mechanisms to introduce the sperms directly into the body of the female_internal fertilization. Males possess special structures for depositing the sperms into the female and the process is called copulation or mating. Internal fertilization is unavoidable for all animals which lay eggs on land as they must be
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