Hello students, in this blog you can find important summary notes from the Chapter 9, Science , NCERT Book of Class 8. These notes can be used for preparation of various school Competitive Exams, Olympiads and also developing Fundamentals.
1. Reproduction is essential for the continuation of a species.
Modes of Reproduction :
• Sexual reproduction, and
• Asexual reproduction.
2. Sexual reproduction :
The type of reproduction beginning from the fusion of male and female gametes is called sexual reproduction.
3. The reproductive parts in animals produce gametes that fuse to form a zygote.
4. Male Reproductive Organs :
• a pair of testes
• two sperm ducts
• a penis
5. The testes produce the male gametes called sperms. Millions of sperms are produced by the testes.
6. Though sperms are very small in size, each has a head, a middle piece and a tail.
Each sperm is a single cell with all the usual cell components.
7. Female Reproductive Organs :
• a pair of ovaries
• oviducts (fallopian tubes)
• the uterus
8. 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.
9. Uterus is the part where development of the baby takes place.
10. Like the sperm, an egg is also a single cell.
The first step in the process of reproduction is the fusion of a sperm and an ovum.
11. When sperms come in contact with an egg, one of the sperms may fuse with the egg. Such fusion of the egg and the sperm is called fertilisation.
12. During fertilisation, the nuclei of the sperm and the egg fuse to form a single
nucleus. This results in the formation of a fertilised egg or zygote.
13. The process of fertilisation is the meeting of an egg cell from the mother and a sperm cell from the father. So, the new individual inherits some characteristics from the mother and some from the father.
14. Fertilisation which takes place inside the female body is called internal fertilisation.
Internal fertilisation occurs in many animals including humans, cows, dogs and hens.
15. IVF or in vitro fertilisation :
In case of women who 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.
16. External fertilisation :
In case of frogs, 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 and this results in fertilisation.
17. The type of fertilisation in which the fusion of a male and a female gamete takes place outside the body of the female is called external fertilisation.
• It is very common in aquatic animals such as fish, starfish, etc.
18. Development of Embryo
• Fertilisation results in the formation of zygote which begins to develop into an embryo.
• The zygote divides repeatedly to give rise to a ball of cells.
• The cells then begin to form groups that develop into different tissues and organs of the body. This developing structure is termed an embryo.
• The embryo gets embedded in the wall of the uterus for further development.
The stage of the embryo in which all the body parts can be identified is called a foetus.
19. Viviparous and Oviparous Animals :
• The animals which give birth to young ones are called viviparous animals.
• Those animals which lay eggs are called oviparous animals.
20. Metamorphosis : The transformation of the larva into an adult through drastic changes is called metamorphosis.
• Egg → larva / caterpillar → pupa → adult
• Egg → tadpole (larva) → adult frog
• Humans do not show metamorphosis
21. Asexual Reproduction :
• In each hydra, there may be one or more bulges. These bulges are the developing new individuals and they are called buds.
• In hydra too 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.
22. Since new individuals develop from the buds in hydra, this type of asexual reproduction is called budding.
23. 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.
24. This type of asexual reproduction in which an animal reproduces by dividing into two individuals is called binary fission.