Chapter 6: Reproduction in Animals

1. Introduction to Reproduction in Animals

These notes of  Reproduction in Animals , cover the entire chapter as per latest CBSE Class 8 NCERT textbook (2024-25). Revise diagrams and tables thoroughly for neat answers and good marks!

• Reproduction is the biological process by which living organisms produce new individuals (offspring) of their own kind.
• It ensures the continuity of species.
• Two main modes of reproduction: → Asexual reproduction → Sexual reproduction

2. Asexual Reproduction

• Only one parent is involved.
• Offspring are genetically identical to the parent (clones).
• Common methods in animals:

• Advantages: Fast, no need to find mate, large number of offspring.
• Disadvantages: No genetic variation → less adaptability.

3. Sexual Reproduction

• Involves two parents (male and female).
• Offspring show genetic variation → better survival chances.
• Involves special reproductive cells called gametes. → Male gamete = Sperm → Female gamete = Ovum (egg)

4. Male Reproductive System (in humans and mammals)

• Testes (pair) → produce sperms and male hormone testosterone.
• Scrotum → sac that keeps testes outside body (2–3 °C lower temperature needed for sperm production).
• Sperm ducts (vas deferens) → carry sperms.
• Accessory glands (prostate, seminal vesicles) → secrete seminal fluid → forms semen.
• Penis → deposits semen into female reproductive tract.

5. Female Reproductive System (in humans and mammals)

• Ovaries (pair) → produce ova (eggs) and female hormones (estrogen, progesterone).
• Oviducts (Fallopian tubes) → site of fertilisation.
• Uterus (womb) → place where embryo develops.
• Vagina → birth canal; receives penis during mating.

6. Fertilisation

• Fusion of male gamete (sperm) and female gamete (ovum) → zygote.
• Two types: (i) External fertilisation → Occurs outside the female body (in water). → Seen in frog, fish, starfish. → Large number of eggs and sperms released to increase chances of meeting. (ii) Internal fertilisation → Occurs inside female body. → Seen in humans, cows, dogs, birds, reptiles. → Fewer eggs produced because chances of fertilisation are higher.

7. Development of Embryo

• After fertilisation, zygote divides repeatedly → embryo.
• Two patterns:

8. Special Structures in Some Animals

• Placenta (in viviparous mammals): → Connects embryo with mother’s blood through umbilical cord. → Supplies oxygen, nutrients; removes wastes.
• Metamorphosis: → Drastic change in body form during development. → Frog: Egg → Tadpole (gills, tail) → Adult frog (lungs, legs). → Butterfly: Egg → Larva (caterpillar) → Pupa (chrysalis) → Adult butterfly.

9. Cloning – Dolly the Sheep (1996)

• First mammal cloned from an adult cell (udder cell of sheep).
• Process:
  1. Nucleus removed from an egg cell.
  2. Nucleus from adult udder cell inserted into enucleated egg.
  3. Egg implanted into surrogate mother → Dolly born.
• Dolly was genetically identical to the sheep that donated the udder cell.
• Significance: Showed that specialised cells can be reprogrammed.

10. In Vitro Fertilisation (IVF) – Test-tube Baby

• Fertilisation done outside body in laboratory.
• Used when natural fertilisation fails (blocked oviducts, low sperm count).
• First test-tube baby: Louise Brown (1978, England).

11. Important Terms

• Zygote → Fertilised egg.
• Embryo → Early stage after zygote (up to 8 weeks in humans).
• Foetus → After 8 weeks till birth.
• Gestation period → Duration of pregnancy (≈9 months in humans, 22 months in elephant, 1 month in rabbit).
• Hermaphrodite → Animal having both male and female reproductive organs (e.g., earthworm, leech). Still needs another individual for sexual reproduction.

12. Quick Comparison Table

13. Important Diagrams to Practise

1. Male reproductive system of humans
2. Female reproductive system of humans
3. Life cycle of frog (metamorphosis)
4. Life cycle of butterfly
5. Binary fission in Amoeba
6. Budding in Hydra

Key Points for Exams

• Animals showing external fertilisation are mostly aquatic.
• Viviparous animals give birth to young ones; oviparous lay eggs.
• Human sperm has tail for swimming; ovum is non-motile.
• Testosterone causes secondary sexual characters in boys (beard, voice change).
• Estrogen and progesterone cause secondary sexual characters in girls.
• Cloning produces genetically identical individuals.

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Chapter 7: Reaching the Age of Adolescence

 

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