Reproduction in Animals Class 8 Notes Science Chapter 6

Reproduction

Reproduction is the biological process by which living organisms create new individuals of the same kind.

In animals, reproduction is a vital process that helps maintain species. It is crucial for the survival of similar kinds of individuals across generations. This trait is essential for all living things.

In your previous classes, you learned about how plants reproduce. Now, we will explore the interesting world of animal reproduction.

Modes of Reproduction

You may have seen baby animals being born. Have you ever thought about how these young ones look and how they come into the world?

Here’s the list of animals and their young:

Animals reproduce in two main ways:

• Sexual reproduction: where a new life begins from the combination of male and female cells.
• Asexual reproduction: where a new individual is formed from just one parent without combining cells.

Sexual Reproduction

Similar to plants, there are two methods by which animals reproduce:

• Sexual reproduction
• Asexual reproduction

In animals, males and females possess different reproductive organs that create special cells called gametes. The female reproductive organs include ovaries, oviducts, and the uterus, while the male organs consist of testes, sperm ducts, and a penis. When these gametes meet, they combine to form a new cell known as a zygote, which develops into a new individual.

Fertilisation can happen in two ways:

• Internal fertilisation: where fertilisation occurs inside the female body, as in humans, cows, and dogs.
• External fertilisation: where fertilisation takes place outside the female body, as seen in frogs, fish, and starfish.

Animals can also be categorised based on their reproductive methods:

• Viviparous animals: such as humans, cows, and dogs, which give birth to live young.
• Oviparous animals: such as hens, frogs, lizards, and butterflies, which lay eggs.

The process where a larva transforms into an adult through significant changes is called metamorphosis.

A type of reproduction involving only a single parent is termed asexual reproduction. For instance, in hydra, new individuals emerge from buds, a technique known as budding. Amoeba reproduces by splitting itself into two, a process referred to as binary fission.

Sexual Reproduction

The process of creating a new individual by joining male and female gametes is known as sexual reproduction.

This involves the fusion of male gametes with female gametes.

Male gametes fusing with female gametes

Male Reproductive Organs

• The male reproductive organs include a pair of testes (where male gametes called sperms are produced), two sperm ducts, and the penis. Millions of sperms are generated by the testes.
• Although sperms are very small, each consists of a head, a middle piece, and a tail. Each sperm is a single cell with all the standard cell parts.

Male reproductive organs in humans

Structure of Sperm

Structure of Sperm

Female Reproductive Organs

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

• Ovaries: These organs produce female gametes called ova (eggs). Each month, one mature egg is released from an ovary and moves into the oviduct.
• Oviducts (Fallopian Tubes): These tubes transport the egg from the ovary to the uterus.
• Uterus: This is where the baby develops.

Female Reproductive System in Humans

Female Reproductive System in Humans

Like the sperm, an egg is also a single cell. However, the size of eggs can vary greatly among animals. For instance, in humans, the egg is very small. In hens, the egg is much larger, and the ostrich lays the largest eggs of all!

The joining of an ovum and sperm is referred to as fertilisation. The fertilised egg is called a zygote. The zygote divides repeatedly to form an embryo. The embryo gets embedded in the wall of the uterus for further development. The stage of the embryo where all body parts can be identified is called a foetus.

Internal fertilisation occurs when fertilisation takes place inside the female body. This is seen in humans and other animals such as hens, cows, and dogs. External fertilisation takes place outside the female body, as seen in frogs, fish, and starfish.

Female Reproductive System in Humans 

So, the process involves a single egg, which is also a single cell, being released each month, travelling through the oviduct, and potentially meeting a sperm to begin the journey of creating a new life.

Structure of Human Ovum

Structure of Human Ovum

Fertilization

Fertilization is the first step in making a new life. It starts when a sperm from the father meets and merges with an egg from the mother. This fusion is called fertilization.

Here’s how it works:

• Meeting of Sperm and Egg: When sperm come into contact with the egg, one lucky sperm may merge with the egg.
• Fusion of Nuclei: The sperm and egg each have their own nucleus. During fertilization, these two nuclei combine to form a single new nucleus.
• Formation of a Zygote: This combination creates a fertilized egg, also known as a zygote. This zygote is the very start of a new individual!

As a new life begins, the new individual inherits some characteristics from the mother and some from the father. If you look at your siblings, you might notice some features they share with your parents!

Fertilization

Two Types of Fertilization

(a) Internal Fertilization:

• Internal fertilization occurs in many animals including humans, cows, dogs, and hens.

During internal fertilization, a male typically introduces sperm directly into the female's reproductive system. The sperm and egg unite within the female's body, usually in specialized reproductive structures.

In cases where a woman’s oviducts are blocked and sperm can’t meet the egg, doctors collect freshly released egg and sperm and keep them together for a few hours for IVF, or in vitro fertilization (fertilization outside the body).

It is important to note that the size of eggs in animals varies; they may be very small, as in humans, or much larger, as in hens. The ostrich egg is the largest!

Additionally, in many animals, fertilization takes place outside the body of the female.

Here’s what they do:

In some cases, doctors collect a recently released egg and sperm and keep them together for a few hours for IVF or in vitro fertilisation (fertilisation outside the body). If fertilisation occurs, the zygote is allowed to develop for about a week before being placed in the mother's uterus. Complete development occurs in the uterus, and the baby is born like any other baby. This term can be misleading because babies cannot grow in test tubes.

External Fertilisation:

• Fertilisation that occurs inside the female body is called internal fertilisation. In many animals, fertilisation happens outside the female's body, known as external fertilisation. This is very common in aquatic animals such as frogs, fish, and starfish.
• In this type of fertilisation, the joining of a male and a female gamete takes place outside the female's body. Both male and female gametes are released into the external environment, like water. The resulting zygote develops independently in the environment.
• Frogs and Toads: During spring or rainy seasons, frogs and toads head to ponds and streams. The female frog lays hundreds of eggs in the water. Unlike chicken eggs, frog eggs do not have a hard shell. Instead, they are protected by a jelly-like layer that keeps the eggs together. The male frog releases sperm over the eggs in the water. The sperm swim around, trying to find and join with the eggs. When they meet, fertilisation occurs.
• The Numbers Game: To overcome survival challenges, animals lay hundreds or even thousands of eggs and release millions of sperm. This way, they increase the chances that at least some eggs will be fertilised and develop into new individuals.

Development of Embryo

The process of embryo development begins with fertilisation, when the sperm and egg unite to form a single cell called a zygote. This initiates the formation of a cell mass that divides repeatedly. The cells then organise into groups that eventually become different tissues and organs. This growing structure is known as an embryo, which embeds itself in the wall of the uterus for further development.

The embryo continues to develop in the uterus, gradually forming body parts such as hands, legs, head, eyes, and ears. The stage when all body parts can be recognised is called a foetus. When the foetus is fully developed, the mother gives birth to the baby.

The Journey of New Life: From Zygote to Baby

• Starting Point: Fertilisation - When a sperm meets an egg, they join to form a single cell called a zygote. This marks the very beginning of new life!
• Early Development: Becoming an Embryo - The zygote starts dividing into many cells, creating a tiny ball. These cells begin to form groups that develop into various tissues and organs. This structure is called an embryo.
• Settling In: Embedding in the Uterus - The embryo moves to the uterus and embeds itself into the wall, continuing to grow. Over time, it starts to form recognisable body parts like hands, legs, and a head. This identifiable stage is called a foetus.
• The Big Arrival: Birth - Once the foetus has developed completely, the mother gives birth, and a new baby enters the world!

   (a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells (enlarged); (c) Embedding of the embryo in the uterus  

Viviparous and Oviparous Animals

Animals have different ways of bringing new life into the world. The animals which give birth to young ones are called viviparous animals. Examples include:

• Human beings
• Cows
• Dogs

On the other hand, animals that lay eggs are known as oviparous animals, such as:

• Hens
• Frogs
• Lizards
• Butterflies

Some oviparous animals have eggs that can be easily seen because they lay them outside their bodies. However, it is not possible to find the eggs of animals like dogs, cows, or cats, as they do not lay eggs; instead, the mother gives birth to the young ones.

In viviparous animals, the embryo develops inside the wall of the uterus. The stage of the embryo where all the body parts can be recognised is called a foetus.

Young Ones to Adults

When new animals are born or hatch from eggs, they grow into adults.  Sometimes, young animals look very different from their adult forms.

Metamorphosis : The transformation of the larva into an adult through drastic changes is called metamorphosis

 For example: The change from larva to adult, involving major changes, is known as metamorphosis. The tadpoles look quite different from the adults, and they eventually turn into frogs that can jump and swim.

egg → tadpole (larva) → adult.  Life Cycle of Frog 

• Frogs start as eggs.
• They hatch into tadpoles (larvae), which look nothing like adult frogs.
• Over time, tadpoles undergo a significant change called metamorphosis, turning into frogs that can jump and swim.

We see three distinct stages:

• Egg
• Tadpole (larva)
• Adult

Asexual Reproduction

Besides sexual reproduction, there is another type called asexual reproduction, which involves only one parent. For instance:

• In hydra, new individuals grow from buds, a process known as budding.
• Amoeba reproduces by splitting into two, which is referred to as binary fission.

Asexual Reproduction

A type of reproduction that involves only one parent is called asexual reproduction.

1. Budding

In hydra, new individuals form as outgrowths from a single parent. This kind of asexual reproduction is referred to as budding. As the buds grow, they eventually separate and become fully-formed hydras.

  Budding in Hydra 

2. Binary Fission

Another way of asexual reproduction can be seen in the microscopic organism, amoeba. This method, where an organism splits into two individuals, is called binary fission. First, the amoeba’s nucleus divides into two, and then the entire cell splits into two halves, producing two new amoebas from just one parent.

The method, where a single cell turns into two, is known as binary fission. It’s one of the many fascinating ways that tiny organisms can reproduce on their own.

  Binary Fission in Amoeba 

Quick Revision -

ASEXUAL REPRODUCTION -The type of reproduction in which only a single parent is involved is called asexual reproduction. Asexual reproduction is mainly of two types - Budding and Binary Fission .

BINARY FISSION- The method, where a single cell turns into two, is known as binary fission. Amoeba reproduces by Binary Fission.

BUDDING-In hydra, new individuals develop from buds. This method of asexual reproduction is called budding.

EGGS- An egg is a special reproductive cell that female animals produce.
EMBRYO-  An embryo is a tiny, early stage of development in a growing baby. The zygote divides repeatedly to give rise to an embryo. The embryo gets embedded in the wall of the uterus for further development 

EXTERNAL FERTILISATION -Fertilisation that takes place outside the female body is called external fertilisation. This is observed in frogs, fish, starfish, etc.

FERTILISATION -  The fusion of ovum and sperm is called fertilisation. The fertilised egg is called a zygote

FOETUS- The stage of the embryo in which all the body parts are identifiable is called foetus

INTERNAL FERTILISATION- Fertilisation that takes place inside the female body is called internal fertilisation. This is observed in human beings and other animals such as hens, cows and dogs. 

METAMORPHOSIS - The transformation of the larva into adult through drastic changes is called metamorphosis 

OVIPAROUS ANIMALS - Animals such as hen, frog, lizard and butterfly which lay eggs are called oviparous animals

SEXUAL REPRODUCTION - Reproduction resulting from the fusion of male and female gametes is called sexual reproduction 

SPERMS- A sperm  is a special reproductive cell that male animals produce.

VIVIPAROUS ANIMALS - Animals such as human beings, cows and dogs which give birth to young ones are called viviparous animals. 

ZYGOTE- Fusion of male and female gamete produces a single cell called as zygote.

SEXUAL REPRODUCTION

Reproduction that occurs from the joining of male and female gametes is known as sexual reproduction. The reproductive organs in females include:

• Ovaries
• Oviducts
• Uterus

In males, the reproductive organs consist of:

• Testes
• Sperm ducts
• Penis

The ovary produces female gametes called ova, while the testes produce male gametes known as sperms.

The joining of an ovum and sperm is termed fertilisation. The fertilised egg is referred to as a zygote. When fertilisation occurs inside the female body, it is called internal fertilisation, which is seen in humans and other animals like hens, cows, and dogs. Conversely, fertilisation that happens outside the female body is termed external fertilisation, which is common in frogs, fish, and starfish.

Development Process

• The zygote divides multiple times to form an embryo.
• The embryo embeds itself in the wall of the uterus for further growth.
• The stage of the embryo where all body parts can be recognised is called a foetus.

Besides sexual reproduction, there is another type called asexual reproduction, where only one parent is involved. This form does not require the fusion of gametes.

VIVIPAROUS ANIMALS

Animals that give birth to live young, such as humans, cows, and dogs, are known as viviparous animals.

OVIPAROUS ANIMALS

Animals that lay eggs, such as hens, frogs, lizards, and butterflies, are referred to as oviparous animals.