As in the case of plants, sexual reproduction in animals begins with fertilization, which is the fusion of the sperm with the ovum are also called the egg cell or egg. In the case of amphibians and fish, the fusion of the gametes takes place outside the body. This is known as external fertilization. The female lays eggs in the surrounding water and the male releases sperms close to the eggs. The sperm and egg fuse in the water to form the zygote. In the case of animals, birds and reptiles, the sperm fuses with the ovum inside the female’s body. This is called internal fertilization. We will only discuss reproduction in human beings in this chapter.
The menstrual cycle
The ovaries usually release an ovum in to the fallopian tube once in 28 days. This process, called ovulation, is followed by the thickening of the uterine wall so that the uterus is prepared to receive and nourish the embryo. The uterine wall receives an increased supply o blood for this purpose.
If the ovum is not fertilized, it is thrown out through the vagina, together with the lining of the uterus and blood. This process is known as menstruation. The whole process of ovulation, thickening of the uterine lining and menstruation is called the menstrual cycle. It is usually a 28 –day cycle. Menstruation stops when woman s around 45-50. This is called menopause.
The male reproduction system
Sperm, the male sex cells, are produced in large numbers in the male reproductive organs, called the tests. There are two tests, which lie outside his abdominal cavity in a saclike body called the scrotum. The sperms mature in the epididymis, a tube like body lying close to the tests. They then travel through a long tube, called the vas deferens, into his urethra, which extends from the urinary bladder to the penis. O mixed with secretions from various glands. This mixture, known as semen, is released through the penis into the opening of the female’s reproductive system. Sperms have ahead and a long tail, which helps them swim towards the ovum for fertilization to take place.
The female reproductive system
Ova are production by a pair of ovaries located in the lower part of the abdominal cavity. Each ovary is attached to a hollow, muscular organ, called the uterus, by an ovarian ligament. Usually, only one ovum is produced every month, by either of the ovaries.
The upper portion of each ovary is surrounded by finger like projection arising from the Fallopian tube, or oviduct. The fallopian tubes open into the uterus and carry the ovum into it after fertilization. The external opening of the uterus is called the vagina, and its lower portion, which joins the vagina, is called the cervix.
Fertilization and pregnancy
The sperms discharged into his vagina during intercourse travel up the vagina and uterus and reach the fallopian tubes. It is ere that fertilization takes place. One of the sperms fuses with the ovum. The zygote thus formed travels down to the uterus and gets attached to its wall. It divides repeatedly to form the embryo. The muscular walls of the uterus have a rich supply of blood vessels, which nourish the growing embryo. Nutrients, oxygen and other substance pass from the blood vessels into the embryo’s blood through a special structure called the placenta, which is linked to the embryo by the umbilical cord. The placenta also carries carbon dioxide and waste materials from the baby’s body into the mother’s blood.
Within the uterus, he baby is surrounded by a protective sac called the amniotic sac, which contains amniotic fluid. This fluid acts like a cushion. The baby grows inside the uterus for about nine months, a period referred to as pregnancy. Then the all of the uterus goes through contractions to let the baby out the amnion bursts and the baby pushes its way out through the vagina.
Sometimes, two ova mature at the same time. They may get fertilized simultaneously, resulting in the birth of wins. Such twins are non identical, or fraternal, twins. They look different from each other. Twins are born also if the fertilized egg divides into two. Since they arise from the same cell, the zygotes develop into identical twins, which are alike.
