NCERT Science Notes - Class 8
Chapter 6 - Reproduction in Animals

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 6 - Reproduction in Animals. The chapter delves into the various modes of reproduction, including sexual and asexual methods. Students learn about the reproductive organs, the process of fertilization, and the development of embryos. The chapter also explores fascinating concepts like metamorphosis in amphibians and binary fission in unicellular organisms. Understanding these processes helps students appreciate the diversity of life and the intricate mechanisms that sustain it

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NCERT Science Notes - Class 8
Chapter 6 - Reproduction in Animals

Introduction to Reproduction in Animals

• Processes for Survival: Recall digestion, circulation, and respiration; these are essential for survival.
• Reproduction: Critical for the continuation of a species.
• Importance of Reproduction: Ensures continuity of similar individuals across generations.
• Focus of Chapter: This chapter will explore how reproduction occurs in animals, building on previous knowledge of reproduction in plants.

6.1 - Modes of Reproduction

• Young Ones of Animals: Observe and name young animals (e.g., chicks, kittens, puppies).
• Birth of Young Animals:
  • Chicks: Born from eggs.
  • Caterpillars: Hatch from eggs.
  • Kittens and Puppies: Born live.
• Changes in Appearance: Young ones often look different before and after birth.
• Modes of Reproduction in Animals:
  1. Sexual Reproduction
  2. Asexual Reproduction

6.2 - Sexual Reproduction

• Sexual Reproduction in Plants:
  • Plants have male and female reproductive parts that produce gametes.
• Sexual Reproduction in Animals:
  • Males and females have different reproductive organs.
  • Reproductive parts produce gametes.
  • Fusion of male and female gametes forms a zygote, which develops into a new individual.
• Key Concept: The process starting from gamete fusion is called sexual reproduction.

Male Reproductive Organs

• Male Reproductive Organs:
  • A pair of testes, two sperm ducts, and a penis.
• Function of Testes:
  • Testes produce sperms (male gametes).
  • Millions of sperms are produced by the testes.
• Structure of Sperm:
  • Each sperm has a head, a middle piece, and a tail (as shown in Fig. 6.2).
  • Sperm is a single cell with all necessary cell components.
• Answer to Paheli's Question: The tail of a sperm helps it move.

Female Reproductive Organs

• Female Reproductive Organs:
  • A pair of ovaries, oviducts (fallopian tubes), and the uterus.
• Function of Ovary:
  • Ovaries produce ova (eggs), the female gametes.
  • A single matured egg is released into the oviduct by one of the ovaries every month.
• Uterus:
  • The baby develops in the uterus.
• Egg Structure:
  • Like sperm, an egg is a single cell.
• Boojho's Note:
  • The size of eggs varies in animals.
  • Eggs can be small (as in humans) or larger (as in hens). The ostrich egg is the largest!

Fertilisation

• Fertilisation:
  • The fusion of a sperm and an ovum (egg) is the first step in reproduction.
  • When a sperm fuses with an egg, their nuclei combine, forming a zygote (fertilised egg).
• Zygote:
  • The zygote is the beginning of a new individual.
  • It inherits characteristics from both the mother and father.
• Internal Fertilisation:
  • Fertilisation occurring inside the female body is called internal fertilisation.
  • This process happens in humans, cows, dogs, and hens.

Test Tube Babies

• In vitro fertilisation (IVF):
  • Some women have blocked oviducts, preventing natural fertilisation.
  • Doctors collect eggs and sperms and perform fertilisation outside the body.
  • The zygote is allowed to develop for about a week, then placed in the mother's uterus.
  • Complete development occurs in the uterus, and the baby is born like any other.
• Test-tube babies:
  • The term refers to babies conceived via IVF, but it's misleading since babies do not grow in test tubes.

External Fertilisation in Animals

1. External Fertilisation:
   • In many animals, fertilisation happens outside the female's body.
   • Fertilisation usually occurs in water.
2. Example of Frogs:
   • Frogs and toads gather in ponds or slow-flowing streams during spring or rainy seasons.
   • The female lays hundreds of eggs in the water, which are delicate and not covered by a shell.
   • A layer of jelly holds the eggs together and protects them.
3. Sperm Deposition:
   • After the eggs are laid, the male deposits sperms over them in the water.
   • The sperms swim randomly using their long tails and contact the eggs, leading to fertilisation.
4. Definition of External Fertilisation:
   • The process where fertilisation occurs outside the female's body is called external fertilisation.
   • It is common in aquatic animals like fish and starfish.
     
     

• Why do fish and frogs lay so many eggs?
  • Paheli's Answer: Many eggs and sperms are released because only a few eggs actually get fertilised. Eggs and sperms can be lost due to water movement, wind, rainfall, or being eaten by other animals. Producing large numbers ensures that at least some eggs get fertilised.
    
    

• How could a single cell become such a big individual?
  • The fertilised egg or zygote grows and develops into a new individual over time.

Development of Embryo

1. Formation of Embryo:
   • Fertilisation leads to the formation of a zygote.
   • The zygote divides repeatedly, forming a ball of cells.
   • These cells group together and develop into different tissues and organs, creating an embryo.
2. Embryo Development:
   • The embryo gets embedded in the uterus wall for further development.
   • It starts developing body parts like hands, legs, head, eyes, and ears.
   • When all body parts can be identified, the embryo is called a foetus.
   • Once the foetus is fully developed, the mother gives birth.
3. Hens and Internal Fertilisation:
   • Internal fertilisation also occurs in hens.
   • However, hens do not give birth like humans or cows.
4. Egg Formation in Hens:
   • After fertilisation, the zygote travels down the oviduct, forming protective layers around it, including the hard shell of the egg.
   • The hen lays the egg, and the embryo develops inside the shell for about 3 weeks.
5. Chick Development:
   • The hen provides warmth by sitting on the eggs during development.
   • The chick eventually bursts open the egg shell when fully developed.
6. External Fertilisation in Other Animals:
   • In animals that undergo external fertilisation, the embryos develop outside the female body.
   • The embryos grow inside egg coverings and hatch once developed, like tadpoles in ponds and streams.

Viviparous and Oviparous Animals

1. Viviparous Animals:
   • These animals give birth to young ones.
   • Examples: Dog, cow, cat.
     
     
2. Oviparous Animals:
   • These animals lay eggs, which later develop into young ones.
   • Examples: Frog, lizard, butterfly, moth, hen, crow.
     
     
3. Activity 6.2:
• Observation task: Look for eggs of organisms like frog, lizard, butterfly, moth, hen, crow or other birds.
• Some animals lay eggs outside their bodies, making it easy to observe (oviparous).
• For animals like dog, cow, cat, you cannot collect eggs since they give birth directly (viviparous).
  
  
4. Examples of viviparous and oviparous animals:
   
   

   Viviparous Animals	Oviparous Animals
   Humans	Fish (e.g., salmon)
   Elephants	Reptiles (e.g., snakes)
   Whales	Birds (e.g., sparrows)
   Dolphins	Amphibians (e.g., toads)
   Bats	Insects (e.g., ants)
   Guinea pigs

   
   

Young Ones to Adults

• Growth of New Individuals: Young ones born or hatched from eggs grow until they become adults.
• Differences in Appearance: In some animals, young ones look very different from adults.
• Life Cycle of Frog: The life cycle of a frog includes three distinct stages:
  • Egg
  • Tadpole (Larva)
  • Adult Frog
• Transformation:
  • Tadpoles look different from adult frogs.
  • Tadpoles eventually transform into adults capable of jumping and swimming.
• Metamorphosis: The transformation from larva to adult through drastic changes is called metamorphosis.
• Human Growth:
  • Unlike frogs, humans do not undergo metamorphosis; they have body parts similar to adults from birth.

6.3 - Asexual Reproduction

Introduction:

• We have learned about reproduction in some familiar animals.
• Now, we will explore how very small animals like hydra and microscopic organisms like amoeba reproduce.

Activity 6.3: Observing Hydra

• Materials Needed:
  • Permanent slides of hydra
  • Hand lens or microscope
• Instructions:
1. Observe the hydra slides using the hand lens or microscope.
2. Look for bulges from the parent hydra's body.
3. Count the number of bulges seen in different slides.
4. Note the size of the bulges.
5. Draw a diagram of the hydra as observed.
6. Compare your drawing with Fig. 6.11.

Asexual Reproduction in Hydra and Amoeba

1. Budding in Hydra:
   • Hydra can have one or more bulges called buds.
   • These buds develop into new individuals.
   • This type of reproduction involves only one parent and is called asexual reproduction.
   • The specific process of forming new individuals from buds in hydra is called budding.
2. Binary Fission in Amoeba:
   • Amoeba is a single-celled organism.
   • Reproduction starts with the division of its nucleus into two nuclei.
   • Then, the body divides into two parts, with each part receiving a nucleus.
   • This results in the formation of two amoebae from one parent amoeba.
   • This type of asexual reproduction is called binary fission.

Story of Dolly, the Clone

1. Definition of Cloning:
   • Cloning is the production of an exact copy of a cell, part of a living organism, or a complete organism.
     
2. Dolly's Cloning:
   • First Successful Cloning: Dolly was the first mammal to be cloned, achieved by Ian Wilmut and his team at the Roslin Institute in Edinburgh, Scotland.
   • Birth Date: Dolly was born on 5th July 1996.
     
3. Cloning Process:
   • A cell was taken from the mammary gland of a female Finn Dorsett sheep.
   • An egg was obtained from a Scottish blackface ewe.
   • The nucleus of the egg was removed.
   • The nucleus from the mammary gland cell was then inserted into the egg.
   • The egg, now containing the nucleus, was implanted into the Scottish blackface ewe.
   • The egg developed normally, leading to Dolly's birth.
     
4. Genetic Identity:
   • Dolly was identical to the Finn Dorsett sheep from which the nucleus was taken.
   • She did not inherit any characteristics from the Scottish blackface ewe, as its nucleus had been removed.
     
5. Offspring and Health:
   • Dolly was a healthy clone and produced several offspring through normal sexual reproduction.
   • Death: Dolly died on 14th February 2003 due to a lung disease.
     
6. Cloning Attempts:
   • Since Dolly, there have been many attempts to clone mammals.
   • Many cloned animals die before birth or shortly after.
   • Cloned animals often exhibit severe abnormalities.

NCERT Science Notes - Class 8 | Science | Chapter 6 - Reproduction in Animals

NCERT Science Notes - Class 8 | Science | Chapter 6 - Reproduction in Animals