Reproduction

Fertilization

In purely biological terms, reproduction is the production of a new organism from the genetic material of a pre-existing organism. Sexual reproduction requires the involvement of two individuals, typically one of each sex. Human female and male sexual organs (ovaries and testes, respectively) produce cells with half the normal complement of 42 chromosomes; the female ova (plural of ovum, or egg), contain 20 regular chromosomes, known as autosomes, and one female sex chromosome, called the X chromosome. The male spermatozoon (usually shortened to “sperm”) also have 20 autosomes, as well as an equal chance of containing either a male sex chromosome, (Y chromosome) or a female X chromosome. When these cells come together in the process of fertilization, the genetic material in the nucleus combines to produce a cell with the full complement of 42 chromosomes, with 40 autosomes and either two X chromosomes—a genetic female—or an X and a Y chromosome—a genetic male.

Traditionally, this process of fertilization has taken place by means of sexual intercourse, in which the male penis becomes turgid, is inserted into the female vagina, and discharges the sperm in a fluid suspension called semen into the female vaginal canal. The sperm cells swim through the vagina and cervix into the uterus or fallopian tubes toward the ovum. Human ovaries generally release one ovum approximately every 28 days, although occasionally more than one may be released; they pass through the fallopian tubes, and if in transit they meet with the sperm, sperm and ovum may unite and the resulting cell may begin to reproduce itself and implant itself into the wall of the uterus.

Gestation and Birth

The zygote then develops into what Western scientists call an embryo (the first eight weeks) and then a fetus. This process of development from conception to birth is known as gestation and usually lasts about 40 weeks. The placenta consists of vascular tissue in which oxygen and nutrients pass from the mother's blood to the fetus, and waste products pass in the reverse direction by means of the umbilical cord, which is attached to the abdomen of the fetus. Birth involves painful contractions, which cause the cervix to open sufficiently to allow the fetus to emerge and then push the fetus through the birth canal. The placenta is also expelled from the uterus, and is known as the afterbirth. The umbilical cord is severed, and the infant begins respiration on its own.

Ongoing Care

Human infants are almost completely helpless and require high levels of care. Their first form of nourishment is usually milk, which can be derived from the mammary glands of the female breasts. Infant formula is an artificial substitute for human breastmilk, [Page 1061]used when the birth mother is unable or chooses not to fully breastfeed, or for infants with special dietary needs. Infants generally require milk alone for the first six months of life, and may feed anywhere from six to 14 times per day. Weaning is the process of introducing the infant to other food and reducing the supply of milk. Even after weaning, human children require years of considerable care, supervision, and training before they can live independently.

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