Chapter 6: Reproduction in Animals

Class 8 Science Study Material

Based on NCERT Textbook 2024–25

Introduction

Do you recall the processes of digestion, circulation and respiration which you have
studied in your previous classes? These processes are essential for the survival of every
individual. You have also learnt about the process of reproduction in plants.
Reproduction is essential for the continuation of a species. Imagine what would have
happened if organisms had not reproduced — species would eventually disappear. Thus,
reproduction ensures the continuation of similar kinds of individuals generation after
generation.
In this chapter, we shall learn how reproduction takes place in animals.
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1. Modes of Reproduction

Have you seen the young ones of different animals? Let us recall some examples by
completing the table below.

S. No. Animal Young One

1 Human Baby
2 Cat Kitten
3 Dog Puppy
4 Butterfly Caterpillar
5 Hen Chick
6 Cow Calf
7 Frog Tadpole

Just as in plants, animals also reproduce by two main methods:

1. Sexual Reproduction

2. Asexual Reproduction

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2. Sexual Reproduction

You have already learnt that in plants, sexual reproduction involves male and female
reproductive parts that produce gametes. In animals too, males and females have
different reproductive organs. These organs produce gametes — the male gamete (sperm)
and the female gamete (ovum or egg).
The fusion of these gametes results in the formation of a zygote, which later develops
into a new individual.
This process of reproduction beginning from the fusion of male and female gametes is
called sexual reproduction.
—

2.1 Male Reproductive Organs

The male reproductive system includes:

• A pair of testes

• Two sperm ducts

• A penis

The testes produce the male gametes called sperms. Millions of sperms are produced
by the testes every day.

[Insert Figure: Male Reproductive Organs in Humans (Fig. 6.1)]

Each sperm is a microscopic, single cell having a head, middle piece, and a tail.

[Insert Figure: Structure of a Human Sperm (Fig. 6.2)]

Did you know? The tail of the sperm helps it to move towards the egg during fertili-
sation.
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2.2 Female Reproductive Organs

The female reproductive system consists of:

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 • A pair of ovaries

• Two oviducts (fallopian tubes)

• The uterus

The ovaries produce female gametes called ova (singular: ovum). The uterus is the part
where development of the embryo takes place after fertilisation.

[Insert Figure: Female Reproductive Organs in Humans (Fig. 6.3)]

Note: The size of eggs varies in different animals. For example, a human egg is very
small, a hen’s egg is much larger, and the ostrich egg is the largest known.
—

3. Fertilisation

The first step in reproduction is the fusion of a sperm and an ovum. When sperms
come in contact with an egg, one of the sperms may fuse with it. This fusion is called
fertilisation.

[Insert Figure: Fertilisation (Fig. 6.5)]

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.

[Insert Figure: Formation of Zygote (Fig. 6.6)]

Important: Fertilisation is the meeting of an egg cell from the mother and a sperm
cell from the father. Hence, the new individual inherits some characteristics from both
parents.
Types of Fertilisation:

• Internal Fertilisation: Takes place inside the body of the female. Examples:
Humans, cows, dogs, hens.

• External Fertilisation: Takes place outside the body (in water). Examples: Fish,
frogs.

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Summary

• Reproduction is essential for the survival and continuity of species.

• Animals reproduce by two main methods — sexual and asexual.

• Sexual reproduction involves the fusion of male and female gametes.

• Male reproductive organs: testes, sperm ducts, penis.

• Female reproductive organs: ovaries, oviducts, uterus.

• Fertilisation is the fusion of sperm and egg forming a zygote.

• Fertilisation can be internal or external depending on the species.

Questions for Practice

1. What is reproduction? Why is it necessary?

2. Name the main organs of the male and female reproductive systems.

3. Define fertilisation. What is a zygote?

4. Differentiate between internal and external fertilisation.

5. Why does the offspring inherit traits from both parents?

Continuation: Reproduction in Animals

Have You Heard of Test Tube Babies?

In some women, the 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 eggs and sperms and keep them together for a few hours for IVF (in
vitro fertilisation) — that is, fertilisation outside the body.
If fertilisation occurs, the zygote is allowed to develop for about a week and then 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.
Note: This term is actually misleading because babies do not grow inside test tubes.

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External Fertilisation

In many animals such as frogs and fish, fertilisation takes place outside the body
of the female. During spring or rainy season, frogs and toads move to ponds or slow-
flowing streams. The female lays hundreds of eggs that are not covered by shells. A
jelly-like layer surrounds and protects these eggs.
As the eggs are laid, the male releases sperms over them. Each sperm swims randomly in
water using its long tail. When the sperm comes into contact with an egg, fertilisation
occurs. This process, where the fusion of male and female gametes takes place outside
the female body, is called external fertilisation.
Example: Fish, frogs, starfish, etc.

In external fertilisation, production of a large number of eggs and sperms ensures that at
least some will survive.

Development of the Embryo

Fertilisation results in the formation of a zygote, which begins to divide repeatedly and
form a ball of cells. These cells then form groups that develop into different tissues and
organs. This developing structure is called an embryo.
The embryo gets embedded in the wall of the uterus for further development. It gradually
develops body parts such as hands, legs, head, eyes, and ears. The stage of the embryo
in which all the body parts can be identified is called a foetus.
When the development of the foetus is complete, the mother gives birth to the baby.

Viviparous and Oviparous Animals

Some animals give birth to young ones while others lay eggs.

• Viviparous animals: Animals that give birth to young ones. Examples: Humans,
cows, dogs, cats.

• Oviparous animals: Animals that lay eggs. Examples: Hens, frogs, lizards, birds.

In hens, internal fertilisation takes place, but the zygote develops a hard protective shell
around itself. The hen lays the egg, and the embryo develops inside for about 3 weeks.
The hen sits on the eggs to keep them warm. When development is complete, the chick
breaks open the shell and comes out.

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Young Ones to Adults

The new individuals that are born or hatched from the eggs continue to grow until they
become adults. In some animals, the young ones look very different from the adults.
Example: Life Cycle of a Frog

Egg → Tadpole (larva) → Adult Frog

The tadpoles gradually transform into adult frogs capable of jumping and swimming. The
transformation of a larva into an adult through drastic changes is called metamorphosis.
In human beings, however, body parts are similar to those of adults from birth, so humans
do not undergo metamorphosis.

Asexual Reproduction

Asexual reproduction involves only one parent. The offspring produced are identical to
the parent.

Budding in Hydra

In hydra, small outgrowths called buds develop on the parent body. These buds grow
and eventually detach to form new individuals.

Budding
Parent Hydra −−−−→ New Hydra

Binary Fission in Amoeba

Amoeba reproduces by dividing into two. First, the nucleus divides into two, followed
by the division of the cytoplasm, resulting in two daughter amoebae. This method of
reproduction is called binary fission.

Parent Amoeba → Two Daughter Amoebae

Story of Dolly, the Clone

Cloning is the production of an exact copy of a cell or organism.

Dolly, a sheep, was the first mammal to be cloned successfully by Ian Wilmut and his
team at the Roslin Institute in Edinburgh, Scotland.

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 • A cell was collected from the mammary gland of a female Finn Dorsett sheep.

• An egg was obtained from a Scottish blackface ewe and its nucleus was removed.

• The nucleus of the Finn Dorsett sheep was inserted into the enucleated egg.

• The egg was implanted into the uterus of the Scottish blackface ewe.

• Dolly was born on 5th July 1996, genetically identical to the Finn Dorsett sheep.

Though Dolly was healthy and reproduced normally, she died in 2003 due to lung disease.
Many other cloned animals have since been created, but cloning still faces challenges, as
many clones suffer from severe abnormalities or die early.

Summary: Reproduction ensures the continuity of life on Earth through both sexual
and asexual methods.