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the process in animals that precedes fertilization and ensures the union of gametes, that is, ova and spermatozoa.
The success of semination is favored by the simultaneous maturation and expulsion of gametes in males and females. These processes are often associated with a complex set of behavioral reactions and are under the control of certain environmental factors, including season, number of daylight hours, and temperature.
Semination can be external or internal. External semination is characteristic of the majority of animals that live or reproduce in the water; this group comprises many invertebrates, most fishes, and acaudal amphibians. These animals lay their ova and spermatozoa in the water, where fertilization also takes place. The union of gametes is made possible by gamones, which intensify the movement of spermatozoa and prolong the period of their motility, and by substances that promote accumulation of spermatozoa near the ova. Internal semination, or insemination, is characteristic of some aquatic and all terrestrial animals; this group includes the sponges, many coelenterates, worms, arthropods, mollusks, and most vertebrates (elasmobranchs, holo-cephalids, some teleosts, caudate amphibians, reptiles, birds, and mammals). With insemination, sperm is usually introduced into the genital tract of the female.
Spermatozoa are transferred in the form of bundles called encapsulated spermatophores in arthropods, cephalopods, elasmobranchs, holocephalids, and caudate amphibians. In some annelids, in insects, and in teleosts of the family Cyprinodontidae spermatozoa are transported in non-encapsulated spermatophores, in which the spermatozoa adhere to one another. In mammals, the spermatozoa are suspended in semen, introduced into the vagina or uterus, and carried further by muscle contractions of the walls of the genital organs until they have reached the ampulla of the oviduct. Ovulated eggs are brought to the ampulla from the infundibulum of the oviduct by the ciliary movements in the epithelium of the mucous membrane that lines the oviduct. In the concluding stage of insemination, the spermatozoa approach the ovum by active forward movements.
REFERENCESZhizn’zhivotnykh, vols. 1–6. Moscow, 1968–71.
Giliarov, M. S. Zakonomernosti prisposoblenii chlenistonogikh k zhizni na sushe. Moscow, 1970.
Marshall’s Physiology of Reproduction, vol. 1, parts 1–2. Edited by A. S. Parkes. London-New York-Toronto, 1956–60.
A. S. GINZBURG
Artificial insemination. Artificial insemination is a method for effecting the union of sex cells in order to achieve fertilization in animals. In artificial insemination of fish, the roe is mixed with milt and then incubated. In mammals and birds, the sperm is introduced into the genital organs of the female by means of special instruments. The Soviet biologist I. I. Ivanov developed the theoretical foundations and the principles underlying the practical methods for artificially inseminating farm animals. The process relies on the possibility of keeping sperm that is obtained from the male active outside of the animal by means of an artificial vagina and on the ability of female farm animals to ovulate without copulation. The life-span of spermatozoa and ova in the genital organs of the female permit artificial insemination during periods that will ensure fertilization.
Artificial insemination involves five basic procedures: obtaining sperm from the male, evaluating the sperm’s quality, diluting and storing the sperm, and introducing the sperm into the sexual organs of the female. With animals, artificial insemination is conducted in order to make intensive use of purebred sires whose high quality has been verified in their offspring. This permits the mass improvement of breeding qualities and increases the productivity of animals. Artificial insemination also prevents some forms of infertility and the spread of a number of infectious diseases that are transmitted during natural insemination, such as vibriosis, trichomoniasis, and brucellosis.
The full benefits of artificial insemination can only be derived when animals are well cared for and provided with a balanced diet. Qualified artificial insemination specialists are needed, the work area on the farm must be in good order, and the artificial insemination center must be supplied with modern apparatus and equipment for transporting animals.
In the USSR, artificial insemination of animals is conducted according to state plans. Special stations have been established where work is done with purebred strains of farm animals and where artificial insemination programs are organized. Between 1968 and 1973, 72 percent of cows and heifers, 75 percent of ewes, and up to 20 percent of sows were artificially inseminated annually.
Artificial insemination of animals is widely practiced in many countries. In 1962 an international seminar on problems of the exchange of sperm from the best purebred producers was organized in the USSR on the initiative of the Council for Mutual Economic Assistance (COMECON). In 1968 the Sixth International Congress on Animal Reproduction and Artificial Insemination of Animals was held in France, at which prospects for the further development of artificial insemination were studied.
REFERENCEStudentsov, A. P. Veterinarnoe akusherstvo iginekologiia [4th ed.]. Moscow, 1970.
N. A. FLEGMATOV