CBSE Class 8 Science Chapter 6 Reproduction In Animals Notes

Binary Fission vs Budding Difference Explained For Class 8 Modes Of Reproduction

Depending upon the number of parents involved in the process, the phenomenon of reproduction is divided into the following two main types

1. Sexual reproduction
2. Asexual reproduction

Sexual Reproduction

In this type of reproduction, a new individual is produced by the fusion of a male and female gamete. During sexual reproduction, gametes from both parents fuse to form a zygote that develops into a new individual. It mainly occurs in multicellular organisms, such as humans, chickens, dogs, and fish.

Human Male And Female Reproductive System Diagram Notes Class 8 Sexual Reproduction In Humans

In human beings, reproduction involves mainly two sexes, male. male and female. Each sex has a different reproductive system with distinct organs performing different functions

Male Reproductive Organs

In males, the reproductive organs include a pair of testes (singular: testis), two sperm ducts and a penis.

Testes: These are paired, oval-shaped structures outside the abdominal cavity. These produce the male gametes called sperm.

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Sperm (male gamete) is a tiny, motile, single-celled structure with a head, a middle piece, and a tail. The long tail helps it to move through the female tract. Millions of sperm are produced by the testes in a single ejaculation

Sperm ducts (vas deferens) These connect the testes to the urethra.

Penis It is an external male genital organ that transfers sperms into the body of a female.

Female Reproductive Organs

In females, the reproductive organs include a pair of ovaries, oviducts, and a uterus.

Ovaries: These are paired, oval-shaped organs located in the lower part of the abdominal cavity. It produces female gametes called ova (eggs)

Ova/egg (female gamete) It is a microscopic, round single cell with a nucleus, cytoplasm, and a telomere

Oviduct (or Fallopian tube). It is a tube-like structure with a funnel-shaped opening near the ovary. In human beings, a single mature egg is released into the oviduct by one of the ovaries every month. It is also known as the site of fertilisation in human females.

Uterus (womb) It is a hollow, bag-like structure, where the growth and development of fetus takes place.

Most Important Questions From Reproduction In Animals Class 8 CBSE Fertilisation

The process of fusion of the egg and the sperm is called fertilisation. It is the first step of sexual reproduction.

During fertilisation, when the nuclei of the sperm come in contact with an egg, it fuse to form a single nucleus. This results in the formation of a fertilised egg or zygote. The zygote marks the beginning ofa new individual.

The process Involves The meeting of egg cell from the mother and a sprm cell the father. So, the new Individual Inherit’ some characteristics from the mother and some front the father. You must have noticed that some of your siblings characters are similar to those of your mother and some to those of your father.

There are various methods by which fertilisation takes place in different organisms. These are

1. External Fertilisation

Fertilisation that takes place outside the body of a female is called external fertilisation. For example, in aquatic animals such as Fishes, starfish and frogs fertilisation takes place mainly in water.

They lay their eggs and sperm together on the surface of the water. The sperms fuse with the eggs in water to form zygote. The zygote develop into new individuals in the water itself. Animals living in or near ponds and lakes also undergo external fertilisation.

During the spring (or rainy) season, frogs and toads move to the water in ponds and slow-flowing streams.

When the male and female come together in water, the female lays hundreds of eggs. The eggs are delicate and do not have a shell, unlike the hen’s eggs. A layer of jelly-like substance holds the eggs together and protects them.

As the eggs are laid, the male ejects millions of sperm directly on them. Each sperm swims randomly in water with the help of its long tail. The sperms come in contact with the eggs floating in water.

This results in fertilisation. As the gametes are released in water, many of them may be carried away by the movement of water, wind, etc., or may be eaten by other aquatic animals, so a large number is released to compensate for the loss.

2. Internal Fertilisation

Fertilisation that takes place inside the female body is called internal fertilisation. For example, in higher animals such as reptiles, birds, and mammals, examples; humans, cows, dogs, and hens.

In Vitro Fetilisation

Some women are unable to have babies because their oviducts are blocked and sperm cannot reach the egg for fertilisation.

This prevents the ova from being fertilised. The problem can be overcome through a test-tube baby technology or in vitro fertilisation.

In this technique, freshly matured ova arc collected from a woman’s ovaries (by using a special syringe) and sperm trom the man. They are kept together for few hours in an incubator (in vitro).

The sperms fertilise the ova to form a zygote. After a week, the embryo thus formed is then inserted into the woman’s womb, where the embryo develops into a baby in about nine months. The baby thus born is called a test-tube baby.

This technique is called In Vitro Fertilisation (IVF) (Fertilisation outside the body). The success rate is only about 30-40%.

This term is misleading because babies cannot grow in test tubes

Development of Embryo in Humans

Development of the embryo in human beings takes place through the following steps

In human beings, soon after fertilisation, the zygote travels down the oviduct and divides repeatedly to form a ball of cells by cell division.

The cells then begin to form groups that finally develop into different tissues and organs of the body. This developing structure is called the embryo. It gets embedded in the wall of the uterus for further development.

1. Zygote formation and development of an embryo from the zygote (b) Ball of cells (enlarged)
2. Embedding of the embryo in the uterus (enlarged)
3. The embryo continues to develop in the uterus and gradually develops different body parts such as the brain. hands, legs, eyes, ears, etc.
4. The stage of he embryo in which all body parts can be recognised is called a foetus. When the development of the foetus is complete, the mother gives birth to the baby

Handwritten Notes For Class 8 Science Chapter 6 Reproduction Development Of The Embryo In The Hen

Hen is a bird that also undergoes internal fertilisation like humans. But it does not give birth to a young one. You will be wondering how chicks are born. Let’s discuss in the section given below

1. Soon after fertilisation (inside the female hen), the zygote divides repeatedly and travels down the oviduct.
2. Many protective layers are formed around it as it makes its way down. The hard shell that we see in a hen’s egg is one such protective layer (outermost).
3. After the hard shell is formed around the developing embryo, the hen finally lays the egg.
4. The hen then sits on the eggs to provide sufficient warmth for the development of eggs. The embryo takes about 3 weeks to develop into a complete chick. The development of the chick takes place inside the egg shell during this period.
5. After the chick is completely developed, it bursts open the eggshell and comes out of it.

Development of Embryo In other Animals

In animals such as frogs, fish, etc., which undergo external fertilization, development of the embryo takes place outside the female body.

The embryos continue to grow within their egg coverings. After the embryos develop, the eggs hatch, and the young ones come out of them.

During development, the embryo gets its food from the egg yolk and the albumin in egg helps in protection.

Viviparous And Oviparous Animals

The animals that give birth to young ones are known as viviparous animals. Example dogs, cows, cats, etc. The animals that lay eggs are called oviparous animals. Example: birds, fish, etc.

Young Ones To Adults

The new individuals which are born or hatched from the eggs continue to grow till they become adults. In some animals such as amphibians (Example, frog) and certain insects (Example, silkworm), the young ones may look very different from the adults.

This process of change by which a larva is transformed into an adult is called metamorphosis

Life Cycle of the Frog

A frog lays eggs in large numbers (spawning). The baby that hatches out of the frog’s eggs is called a tadpole. The tadpole undergoes several changes before it becomes an adult frog.

egg →tadpole → adult

A tadpole undergoes metamorphosis to become an adult frog. These adult frogs are capable of jumping and swimming.

Differences between tadpole and frog

Exceptions of Metamorphosis

Human beings also exhibit changes in body appearance as they grow, but these changes are not drastic. In human beings, the body parts in adults are similar to those which are present in babies from the time of birth.

So, human beings do not undergo metamorphosis. Some common animals, such as cats, dogs, hens, tigers, lions, deer, horses, cows, etc, also do not undergo metamorphosis during their life cycle

Asexual Reproduction

In this method anew organism is produce from a single parents without the fusion of gamestes. The offspring producing are identical to the parents There are various moderns of the ascual reproduction . some of the are discussed below.

1. Budding

It is a type of asexual reproduction in which a bud(or a bulb develops from the parent body. This bud enlarges and gets detached from the parent. It later develops into a similar identical organism that can live independently, for Example, Hydra, yeast, etc.

Budding in Hydra

It is a multicellular animal that reproduces by budding. In this, one or more small, bulb-like outgrowths called buds are formed on the side of its body by repeated division of cells. The nucleus of the parent body divides, and one goes into the bud.

2. Binary Fission

The process of reproduction in which a parent organism divides (both nucleus and cytoplasm divide), into two daughter cells each of which grow and mature into two separate new individuals is called binary fission, example. Amoeba, Paramecium.

Fission in Amoeba

It is a single-celled organism. It reproduces by this method under favourable conditions. Firstly, the nucleus divides into to it has then, followed by the division of the cytoplasm A constriction develops in the body, which gradually depens to from two daughter amoebae

Besides these two processes, there .ire oilier methods of asexual reproduction (such as fragmentation, spore formation, vegetative propagation). These methods will he studied by you in higher classes.

Differences between sexual and asexual reproduction

Difference Between Zygote And Embryo Class 8 Notes Cloning

Cloning is defined as the production of an exact copy ofa cell or any other living part or a complete organism. Story of Dolly, the Clone

Story of Dolly, the Clone

Cloning ofan animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute

Institute in Edinburgh, Scotland. They successfully cloned a sheep named Dolly. Dolly was born on 5th July, 1996, and was the first mammal to be cloned

Steps Followed in Cloning.

1. During the process of cloning Dolly, a cell was collected from the mammary gland of a female Finn Dorset sheep.
2. Simultaneously, an egg was obtained from a Scottish blackface ewe.
3. The nucleus was removed from the egg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed.
4. The egg thus produced was implanted into the Scottish blackface ewe.
5. Development of this egg followed normally, and finally Dolly was born.

Though Dolly was given birth by a Scottish blackface ewe, it was found to be identical to the Finn Dorset sheep from which the nucleus was taken. Since the nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not show any characteristics of the Scottish blackface ewe.

Dolly was a healthy clone of the Finn Dorsett sheep and produced several offspring of her own through normal sexual means. Unfortunately, Dolly died on 14th February, 2003 due to a certain lung disease.

Since Dolly, several attempts have been made to produce cloned mammals. However, many die before birth or die soon after birth. The cloned animals are often found to be born with severe abnormalities.

Asexual reproduction: The type of reproduction in which only a single parent is involved.

Budding In this, the new individual develops from the bud.

Eggs are Female reproductive cells produced by the ovaries.

External fertilisation The fertilisation that takes place outside the female body is called external fertilisation. Embryo The zygote divides repeatedly to give rise to an embryo. It gets embedded in the wall of the uterus for further development.

Fertilisation: The fusion of ovum and sperm.

Foetus: The stage of the embryo in which all body parts are identifiable.

Internal fertilization: The fertilisation that takes place inside the female body is called internal fertilisation.

Metamorphosis: The transformation of a larva into an adult through drastic change.

Oviparous animals are Animals that lay eggs.

Sexual reproduction: Reproduction resulting from the fusion of male and female gametes.

Sperm: Male reproductive cells produced by the testes. Viviparous animals are Animals that give birth to live young ones.

Zygote Fertilisation results in the formation of a fertilised egg or zygote.