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In the female, maturation of an ovum is a remarkable process controlled by hormones secreted by the endocrine glands. The menstrual cycle is ordinarily 28 days long, measured from the beginning of one menstrual period to the beginning of the next. During the first 2 weeks of the usual cycle, one of the ova becomes mature enough to be released from the ovary. At the time of ovulation this mature ovum (secondary oocyte) is released and at this point can be fertilized. If fertilization occurs, the fertilized ovum (zygote) is then discharged into the abdominal cavity. Somehow, by mechanisms that are not clear, it moves into a fallopian tube and begins its descent toward the uterus. If the ovum remains unfertilized, menstrual bleeding occurs about 2 weeks later.
In the male there is no sexual cycle comparable to the cyclical activity of ovulation in the female. Mature sperm are constantly being made in the testes of the adult male and stored there in the duct system.
The sperm swim an average of 0.4 to 2.5 cm (0.1 to 1.0 inch) per minute. When one or more vigorous sperm are able to reach the ovum, which is normally in the outer half of the fallopian tube, fertilization occurs. The head end of the sperm plunges through the thick wall of the ovum, leaving its tail outside. The genetic materials, the chromosomes, are injected into the ovum, where they unite with the chromosomes inherited from the mother (see heredity). The sex of the child is determined at this instant; it depends on the sex chromosome carried by the sperm.
If by chance two ova have been released and are fertilized by two sperm, fraternal (dizygotic) twins are formed. Identical (monozygotic) twins are produced by a single fertilized ovum that divides into two early in its development.
As soon as the blastocyst is implanted, its wall begins to change into a structure that eventually develops into the placenta. Through the placenta the fetus secures nourishment from the mother and rids itself of waste products. Essentially the placenta is a filtering mechanism by which the mother's blood is brought close to the fetal blood without the actual mixing of blood cells.
During the early stages of pregnancy, the fetus grows at an extremely rapid rate. The mother's body must undergo profound changes to support this organism. The muscles of the uterus grow, vaginal secretions change, the blood volume expands, the work of the heart increases, the mother gains weight, the breasts prepare for nursing, and other adjustments are made throughout the mother's body.
a·sex·u·al re·pro·duc·tion(ā-sek'shū-ăl rē'prō-dŭk'shŭn)
asexual reproductiona process by which organisms multiply without the formation and fusion of specialized sex cells (GAMETES). Each feature of asexual reproduction has its advantages and disadvantages.
The genetically identical products of asexual reproduction are called CLONES. Asexual reproduction is a feature of lower animals and occurs in all groups of plants, including ANGIOSPERMS, usually in addition to sexual reproduction. Perhaps the fixed location of many plants makes this type of reproduction particularly suitable. Also occurs in MICROORGANISMS. There are several types of asexual reproduction, including:
- fission (see BINARY FISSION where the entire organism splits into two (e.g. bacteria).
- BUDDING: new individuals produced as outgrowths of the parent (e.g. yeast, Hydra).
- FRAGMENTATION: mechanical separation of the plant body into segments (some algae).
- SPORULATION: producing asexual spores (e.g. fungi).
- VEGETATION PROPAGATION: parts of higher plants producing new whole organisms (e.g. potato tubers).
- reproductive cloning: transfer of genetic material of a body cell to an ANUCLEATE OVUM, for implantation into the UTERUS. This is known as somatic cell NUCLEAR TRANSFER and is how DOLLY was produced.