Class 8 Science



Reproduction in Animals

Reproduction: The process by which an organism produces its offspring is called reproduction.

Modes of Reproduction

There are two modes of reproduction in animals, viz. sexual and asexual reproduction.

  • Sexual Reproduction: When two parents are involved in reproduction and gamete formation is necessary for reproduction, it is called sexual reproduction. Male gametes are produced by the male parent, and female gametes are produced by the female parent. Fusion of male and female gametes is an important step in this type of reproduction.
  • Asexual Reproduction: When a single parent is involved in reproduction and gamete formation does not take place, it is called asexual reproduction.

Human Reproductive System

Male Reproductive Organs: The male reproductive organs are a pair of testes, two sperm ducts and a penis. Testes produce the male gametes which are called sperms.

Structure of Sperm: A sperm is very small in size, unicellular, and is composed of a head, middle piece and a tail.

Female Reproductive Organs: The female reproductive organs are a pair of ovaries, a pair of oviducts (fallopian tubes) and a uterus. The female gametes, i.e. eggs are produced by ovaries.

Fertilization:

Fusion of a male gamete with a female gamete is called fertilization. During fertilization, fusion of nuclei of sperm and egg takes place. Zygote is formed after fertilization.


Types of Fertilization:

There are two types of fertilization in animals, viz. external fertilization and internal fertilization.

  • External Fertilisation: When fertilization takes place outside the animal’s body, it is called external fertilization. Most of the aquatic animals show external fertilization, e.g. fish and frog. A large number of eggs need to be fertilized in this method because most of the fertilized eggs get washed away by water or are eaten by predators. Large number of eggs ensures survival of some of the offspring.
  • Internal Fertilisation: When fertilization takes place inside the animal’s body, it is called internal fertilization. Internal fertilization is seen in most of the terrestrial animals. Unlike external fertilization, there is no need to produce a large number of eggs in internal fertilization.

Conception and Child Birth:

It takes place in the female reproductive system through following steps:

  • A single egg comes out of the ovary every month, and reaches the fallopian tube.
  • Sperms reach the fallopian tube where a sperm fertilizes the egg. This results in formation of zygote.
  • Zygote undergoes several rounds of cell division to become a ball of cells.
  • This ball of cells moves to the uterus and gets implanted in the wall of the uterus. This process is called conception. Conception marks the beginning of pregnancy.
  • Embryo develops inside the uterus to become a baby.

Gestation: The period from conception to child birth is called gestation period. The gestation period in humans is of 40 weeks.

Embryo: After conception, the ball of cells undergoes further rounds of cell division. During the process, the cells begin to differentiate into different types of tissue. Tissue level stage after conception is called embryo.

Foetus: When the embryo begins to resemble a human being, it is called a foetus. At this stage, various external organs; like legs, head, ear, eyes, etc. become apparent.

IVF (In Vitro Fertilisation): Any biological process carried out in laboratory is called in-vitro. Thus, fertilization carried out in laboratory is called in-vitro fertilization. After in-vitro fertilization, zygote is allowed to develop into a mass of cells and is then transferred into the uterus for subsequent development. Many women may fail to conceive due to certain problems. In-vitro fertilization helps such women to bear a child. A baby which is born with the help of this technique is often called the ‘test tube baby’.