Lesson: Reproduction in Animals
Topic 1: Sexual Reproduction and Internal Fertilisation

What is meant by reproduction?

The process by which organisms produce new organisms similar to themselves is known as reproduction.

Define the term sexual reproduction.

The type of reproduction when two parents are involved and there is a fusion of male and female gametes is called sexual reproduction.

What are the male reproductive parts?

The male reproductive system consists of a penis, a pair of testes and two sperm ducts.

Name the reproductive parts of females.

The female reproductive organs are a pair of ovaries, a pair of oviducts (fallopian tubes) and the uterus.

What is sperm? Describe its structure.

A sperm is a male reproductive cell. It is also called a male gamete. The main parts of a sperm are a head, a middle piece and a tail.

What do you mean by fertilisation?

The fusion process of the male and female gametes resulting in the formation of a zygote is called fertilisation.

What is foetus?

A human embryo from the end of the second month of pregnancy until birth is known as a foetus.

Why does a hen sit on its eggs?

A hen sits on its eggs to keep them warm, so that the fertilised egg can develop into a chick.

What is an embryo? Where does the development of a human embryo take place?

An organism in its early stage of development between the first and eighth week after fertilisation, especially before it has reached a distinctively recognizable form is known as an embryo. The development of human embryo takes place in the mother’s uterus.

A mother gives birth to a baby but the baby has characters of both parents. How is this possible?

The process of fertilisation is the combining of an egg cell from the mother and a sperm cell from the father. So, the baby inherits some characteristics from the mother as well as the father.

Give three differences between a zygote, an embryo and a foetus.

The following are the differences between a zygote, an embryo and a foetus:

Zygote	Embryo	Foetus
A zygote is formed by the fusion of sperm and ovum.	An embryo is formed by the repeated cell division of a zygote.	A foetus is formed by the growth and development of an embryo.
A zygote is the beginning of the formation of a baby.	An embryo is an unborn baby in the uterus in the early stages of development.	A foetus is an unborn baby in the uterus in the later stages of development.
A zygote contains only single cell and therefore the body parts are not distinguishable.	An embryo is multicellular, but the body features of the growing baby in the embryo are not very developed and distinguishable.	A foetus is also multicellular but by this stage all the body parts of the baby have become distinguishable.

How are viviparous and oviparous animals different?

Animals which give birth directly to their young are called viviparous animals. For example $–$ cow, dog, human beings etc.

Animals which lay eggs that later develop into young ones are called oviparous animals. For example$–$ frog, fish, hen etc.

Give reason why it’s possible to see eggs of birds but not those of cats.

This is because it is easy to collect the eggs of oviparous animals like birds because they lay the eggs outside their bodies. However, we cannot collect eggs of viviparous animals such as cat because they do not lay eggs outside their body. They remain inside their bodies and are fertilised internally.

How are chicks born to hens?

After fertilisation takes place inside the body of the hen, the zygote divides repeatedly to form embryo which travels down the oviduct. As it travels down the oviduct, many protective layers are formed around the embryo. The hard shell on the hen’s egg is the outermost protective layer. After the hard egg shell is formed around the developing embryo, the hen finally lays the egg. The embryo takes about three weeks to develop into a complete chick. The development of the chick from egg takes place inside the hen’s egg during this period. After the chick is completely developed, it bursts open the egg shell from within and comes out of it. This is how a chick is born.

What is the difference between the eggs of a hen and of a frog??

A hen’s egg is surrounded by a thick shell. After the shell is completely formed around the developing embryo, the eggs are laid and further embryonic development continues. Unlike a hen’s egg, a frog’s egg is not covered by a hard shell and so is comparatively delicate. A layer of jelly holds the eggs together and provides protection to them.

Explain why reproduction is important in organisms.

Reproduction is very important for all living organisms. The organisms reproduce to produce young ones like themselves. It ensures the continuation of similar kinds of individuals, generation after generation. Without reproduction, all plants and animals would become extinct. Secondly, special characteristics of an organism are carried over to its next generation through reproduction.

Describe the process of fertilisation in human beings.

Human beings reproduce sexually by internal fertilisation. The sperms are injected into the bodies of females. If a matured egg/ovum comes in contact with these sperms, only one of the sperms fuses with it to form the fertilised egg called zygote.

This process in which the ovum and sperm fuse to form zygote is called fertilisation. During this process the nucleus of the sperm fuses with the nucleus of the ovum, thus forming a single nucleus of the zygote.

Explain the development of embryo.

Fusion of sperm and egg results in the formation of a zygote. 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 called an embryo. This embryo gets embedded in the wall of the uterus for further development. The embryo continues to develop in the uterus. It gradually develops body parts such as hands, legs, a head, eyes and ears etc. This stage is called foetal stage. A mother gives birth to a baby when the foetus is developed completely.

Topic 2: Asexual Reproduction and External Fertilisation

What are the modes of reproduction?

The two modes of reproduction are sexual reproduction and asexual reproduction.

What is the similarity between Hydra and Yeast?

Both Hydra and yeast reproduce by budding.

Stages in the lifecycle of an ant are given below. Write them in a correct order.

Pupa, adult ant, egg, larva

Egg, larva, pupa, adult ant

Explain how budding occurs.

A form of asexual reproduction in living organisms in which new individuals form from outgrowths (buds) on the bodies of mature organisms is called budding.

What is binary fission?

A type of asexual reproduction in which a unicellular organism reproduces by dividing into two individuals is called binary fission.

What is asexual reproduction? What are the advantages of this method?

When a single parent is involved in reproduction, it is called asexual reproduction. Its advantages are:

(a)   Only one parent organism is required, with no special organs or cells.

(b)   Offspring produced are identical to the parent and inherit similar characteristics.

(c)   A quick method of reproduction.

What is external fertilisation?

Fertilisation which takes place outside the female’s body is external fertilisation. This type of fertilisation occurs in fish, corals, oysters and some amphibian creatures like the frog.

What is metamorphosis? Outline the life cycles of silkworm and frog.

Metamorphosis is a biological process observed in the life cycle of certain animals, in which the larva or a stage of early development undergoes drastic changes while transforming into the adult form.

Silkworm: - Egg-Larva/ Caterpillar-Pupa/ Cocoon- Adult

Frog: - Egg-Tadpole (Larva)- Froglet- Adult Frog

Do human beings also undergo metamorphosis?

No, human beings and other animals undergo "direct development". Unlike frogs or butterflies, a human baby and adult both have similar body parts, which develop as the child grows up.

Explain how bacteria reproduce.

Bacteria reproduce asexually by binary fission. Single bacteria can reproduce several times an hour to produce thousands of identical bacteria cells.

Hen and frog are both oviparous, but exhibit different types of fertilisation processes. Explain.

Hen and frog both are oviparous animals because they lay eggs. A hen undergoes internal fertilisation of eggs and development of the embryo takes place outside the hen's body whereas frog undergoes external fertilisation and development of the fertilised egg also takes place externally.

How can we say that fish undergo external fertilisation?

A female fish lays thousands of eggs at a time to ensure better chances of fertilisation. A male fish then spreads his sperms all over the eggs to fertilise them. Eggs that get fertilised develop and hatch into new born fish. In such cases, where the eggs are fertilised outside the body of the female, the process is known as external fertilisation. Hence, we can say that fish undergo external fertilisation.

Why do female frogs and fish produce a large number of gametes?

Frogs and fish 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 a large number of eggs and sperms is necessary to ensure fertilisation of at least a few of them.

Differentiate between internal fertilisation and external fertilisation.

Internal fertilisation takes place inside the body of the female while external fertilisation takes place outside her body. In case of internal fertilisation, sperms are released inside the female’s body by the male while in external fertilisation; sperms are discharged in the open.

Describe the two methods of asexual reproduction.

Two methods of asexual reproduction in animals are budding and binary fission.

Budding $–$ This type of reproduction takes place in Hydra and yeast. A part of the organism starts protruding from the body wall. This outgrowth is called a ‘bud’. Slowly the bud detaches and develops into a separate, identical individual.

Binary fission $–$ In this type of reproduction, a single organism gets divided into two. This type of reproduction takes place in amoeba. The nucleus of the amoeba divides into two, followed by division of the cell, with each cell receiving one nucleus and developing into a separate individual.

Explain life cycle of a frog, with a help of labelled diagram.

In Spring or the rainy season, the male and female frogs mate. The female frog lays hundreds of eggs that are covered in a protective layer of jelly. As the eggs are laid, the male frog releases its sperms in the water, which come into contact with the eggs to fertilise them. Tadpole embryos start growing in eggs that have been fertilised. Once the tadpole embryo develops a head, tail and gills, the eggs hatch and the tadpole emerges which then continues to develop further to become more "frog-like". At this stage the tail is still long and powerful and used for movement in water. Very soon the tail disappears and the creature climbs out of the water and onto land. It can now be considered a froglet and continues to grow into a frog.