the organs that form the sexual products (ova and spermatozoa) in animals and man.

The gonads are either male organs (testes), female organs (ovaries), or hermaphroditic (for example, in Platyhelminthes and Oligochaeta, the leeches, the majority of Gastropoda, and the barnacles). In the embryos of vertebrates the gonads are found in the sides of the dorsal mesentery in the form of sex folds (paired longitudinal folds of epithelium that line the body cavity), into which migrate the primary sex cells (which arise outside the gonads at earlier stages of the body’s development) and into which grow columns of mesenchymal cells from the truncal kidney (the mesonephros), which participate in formation of the gonadal stroma (in particular, their medullary layer). The epithelium of the gonads becomes cylindrical and its cells may be converted into germ cells (so that it is called the germinal epithelium). In fishes the sex folds arise along the entire length of the body cavity and are converted completely to gonads. In amphibians the folds are similarly situated, but only the middle part is converted to gonads. In reptiles, birds, and mammals the sex folds appear only in the region of the lateral surface of the anterior half of the mesonephros, which faces the mesentery. The germ cells multiply, grow, and mature in the gonads. The growth of the germ cells in the gonads takes place under the influence of the gonadotrophic hormones of the pituitary and by means of special nurse cells—Sertoli’s cells (in male gonads) and follicle cells, which in many animals form a follicular membrane around the ova (in female gonads). Both sexes in the process of gonadal development pass through an undifferentiated (bipotential) stage of development, after which their sexual differentiation begins. The ovaries are formed principally from the cortical layer of the gonads, and the testes from the medullary layer. The form of the ovaries differs markedly in different animals (for example, they are tubular in round-worms and insects, racemose in birds, and dense rounded bodies in mammals). The testes in all vertebrates, beginning with the frogs and toads, are constructed of seminiferous tubules, which communicate with the ductus deferens, formed from the tubules of the mesonephros. In higher animals the gonads are also endocrine glands, secreting sex hormones into the blood. The gonads, their conduction paths, and the external sex organs, constitute the genital system.