NCERT Class 8 Science Chapter 9 - Reproduction in Animal Notes

Chapter 9 - Reproduction in Animal Notes

1. What is Reproduction?
Reproduction is a process by which living things give birth to a new generation of organism of same kind.

2. Importance of Reproduction
Reproduction is important because without it, a species would not survive over time. Individuals must reproduce to ensure that they will continue on over generations of time.

3. Types of Reproduction
• Asexual Reproduction: The reproduction in which offspring are produced by a single parent, without the union of sex cells, is called asexual reproduction.
• Sexual Reproduction: The reproduction in organisms by the union of male and female sex cells (gametes) is called sexual reproduction.

4. Stages of Sexual Reproduction
• Stage 1: Fertilization
→ The fusion of the genetic material contained in the gametes to form a zygote is known as fertilization.
→ Zygote is the fertilized egg or ovum. It is a single cell, thus we begin our life as a single cell.
→ Types of fertilization
♦ Internal fertilization which takes place inside the body of the female. Example: humans, dogs, cows, birds etc.
♦ External fertilization which takes place outside the body of the female. Example: fish, frogs etc.

• Stage 2: Development of Zygote
→ Zygote divides repeatedly to give rise to a ball of cells. As it passes from the fallopian tube into the uterus it divides further and develops into an embryo.
→ When the embryo gets embedded in the wall of the uterus it is known as implantation. The female is then said to be pregnant or to have conceived.
→ The embryo gradually develops body parts and is called as foetus.
→ When the development of the foetus is complete the mother gives birth to a baby.

• Stage 3: Birth of Young Ones
→ Animals which give birth to young ones directly are called viviparous animals. The embryo obtain nourishment from mother.
→ Animals which lay eggs are called oviparous animals. The embryo obtains nourishment from the yolk and albumin present in the egg itself.
→ The time between fertilization and birth of young one is called gestation period.

• Stage 4: Young Ones to Adults
→ The natural process by which an animal, after birth or hatching, undergoes abrupt changes in its body structure until it reaches an adult stage is called metamorphosis.

5. Asexual Reproduction
• Budding in Hydra
→ In each hydra, there may be one or more bulges. These bulges are the developing new individuals and they are called buds.
→ In hydra, the new individuals develop as outgrowths from a single parent. This type of reproduction in which only a single parent is involved is called asexual reproduction.
→ Since new individuals develop from the buds in hydra, this type of asexual reproduction is called budding.

• Binary Fission in Amoeba
→ Amoeba is a single-celled organism. It begins the process of reproduction by the division of its nucleus into two nuclei.
→ This is followed by division of its body into two, each part receiving a nucleus.
→ Finally, two amoebae are produced from one parent amoeba. This type of asexual reproduction in which an animal reproduces by dividing into two individuals is called binary fission.

6. The male reproductive organs include a pair of testes (singular, testis), two sperm ducts and a penis. The testes produce the male gametes called sperms. Millions of sperms are produced by the testes.

7. Each sperm has a head, middle piece and tail.

8. The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes) and the uterus. The ovary produces female gametes called ova (eggs). In human beings, a single matured egg is released into the oviduct by one of the ovaries every month. Uterus is the part where development of the baby takes place. Like the sperm, an egg is also a single cell.

9. The zygote is the beginning of new individual.

10. Fertilization in Frogs and Toads
During spring or rainy season, frogs and toads move to ponds and slow-flowing streams. When the male and female come together in water, the female lays hundreds of eggs. Frog’s egg is not covered by a shell and it is comparatively very delicate. A layer of jelly holds the eggs together and provides protection to the eggs. As the eggs are laid, the male deposits sperms over them. Each sperm swims randomly in water with the help of its long tail. The sperms come in contact with the eggs. This results in fertilisation.

11. How are the chicks born?
After fertilization, the zygote divides repeatedly and travels down the oviduct. As it travels down, many protective layers are formed around it. The hard shell in hen’s egg is one such protective layers. After the hard shell is formed around the developing embryo, the hen finally lays egg. The embryo takes about 3 weeks to develop into chicks. The development of the chicks takes place inside the egg shell during this period. After the chick is completely developed it bursts open the egg shell.

12. What purpose does the tail in a sperm serve?
The main function of the tail in sperm is to provide motility to the sperm.

13. What are test tube babies?
In some women oviducts are blocked. These women are unable to bear babies because sperms cannot reach the egg for fertilisation. In such cases, doctors collect freshly released egg and sperms and keep them together for a few hours for IVF or in vitro fertilisation (fertilisation outside the body). In case fertilisation occurs, the zygote is allowed to develop for about a week and then it is placed in the mother’s uterus. Complete development takes place in the uterus and the baby is born like any other baby. Babies born through this technique are called test-tube babies.

14. Why do fishes and frogs lay eggs in hundreds whereas a hen lays only one egg at a time?
Though these animals lay hundreds of eggs and release millions of sperms, all the eggs do not get fertilised and develop into new individuals. This is because the eggs and sperms get exposed to water movement, wind and rainfall. Also, there are other animals in the pond which may feed on eggs. Thus, production of large number of eggs and sperms is necessary to ensure fertilisation of at least a few of them.

15. What is karyokinesis and cytokinesis?
→ Cytokinesis is the process by which the cytoplasm of the parent cell divides into two daughter cells.
→ Karyokinesis is a process where the nucleus of the parent cell divides into two daughter nuclei.

16. Which animal has the highest gestation period?
African elephants have the highest gestation period of 22 months.

17. What is mitosis and meiosis?
→ Mitosis is the division of a cell into two daughter cells that are genetically identical to the parent cell.
→ Meiosis is the division of a germ cell into four sex cells (e.g. egg or sperm), each with half the number of chromosomes of the parent cell.


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