Reproduction in Animals Class 8 Notes Science Chapter 6

Reproduction is the Biological process by which living organisms produce new individuals of the same kind.

Reproduction in animals is a biological process that helps keep species alive. It's like nature's way of starting fresh, making sure that new generations of the same kind keep coming. It's an important characteristic of all living things.

In your previous classes, you explored how plants reproduce. Now, we’re diving into the fascinating world of animal reproduction. Get ready to discover how animals create new life and keep their species going strong!

Modes of Reproduction

You must have seen the young ones of various animals being born. Have you ever wondered what baby animals look like and how are they born ? 

Let’s complete the list for  animals and their young ones:

• Cat: Kitten
• Dog: Puppy
• Butterfly: Caterpillar
• Cow: Calf
• Frog: Tadpole

The new babies are born by the process of reproduction . Animals reproduce  in two main ways:

1. Sexual reproduction: where a new life starts from the combination of male and female cells.
2. Asexual reproduction: where a new individual is created from just one parent without combining cells.

Sexual Reproduction

Just like in plants, animals also have special parts for reproduction. In plants, you might remember the male and female parts like stamens and pistils. Animals have similar, but different parts for making new life.

In animals, males and females have different organs that produce special cells called gametes. These gametes meet and combine to form a new cell called a zygote. This zygote then grows into a new animal.

This process of making a new individual by combining male and female gametes is called sexual reproduction. 

Male gametes fusing with Female gamete

Male Reproductive Organs

• The Male reproductive system includes a pair of  testes (where sperm are made), two sperm ducts, and the penis. The testes produce millions of tiny sperm cells.
• Even though sperm cells are tiny, each one is still a single cell with all the basic parts that a cells need.
• Each sperm has a head, a middle piece, and a tail.
  
  
  
  Male reproductive organs in human
  
  

Structure of Sperm

Female Reproductive Organs

The female reproductive system includes a pair of ovaries, oviducts (also known as fallopian tubes), and the uterus.-

• Ovaries: These are the organs that produce the female eggs, or ova. Every month, one mature egg is released from an ovary and travels into the oviduct.

• Oviducts (Fallopian Tubes): These tubes carry the egg from the ovary to the uterus.

• Uterus: This is where the baby develops. It’s like a cozy home for the baby as it grows.

Female Reproductive System in Humans 

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

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

Structure of Human  Ovum

Fertilization

Fertilisation 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 fertilisation.

Here’s how it works:

1. Meeting of Sperm and Egg: When sperm come into contact with the egg, one lucky sperm may merge with the egg.

2. Fusion of Nuclei: The sperm and egg each have their own nucleus. During fertilisation, these two nuclei combine to form a single new nucleus.

3. Formation of a Zygote: This combination creates a fertilised egg, also known as a zygote. This zygote is the very start of a new individual!

As a new life begins, the zygote carries some traits 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: 

• This happens inside the female’s body. Animals like humans, cows, dogs, and hens use this method.
•  It’s like a secret meeting place inside the  the reproductive tract of the female organism where the sperm and egg join to create a new life. 

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.

What are Test Tube Babies ? 
Have you heard of test-tube babies? If a woman’s oviducts are blocked and sperm can’t meet the egg, doctors use a method called IVF, or in vitro fertilization, meaning "fertilization outside the body." 

Here’s what they do: 

(b) External Fertilization: 

•  Fertilisation that takes place outside the female body is called external fertilisation. . It is very common in aquatic animals such as  frogs , fish, starfish, etc.
• In external fertilization, both male and female gametes are released into the external environment, such as water. The fusion of sperm and egg occurs externally, and 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 don’t have a hard shell. Instead, they’re protected by a jelly-like layer. 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 happens.
  
  
  
  External Fertilization in frogs
  
  Why So Many Eggs and Sperm?
  
  

• Nature’s Challenges: Not every egg will make it. They face tough conditions like water currents, wind, and rain. Plus, other creatures in the pond might eat them.

• The Numbers Game: To overcome these 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 fertilization, where the zygote forms and undergoes repeated cell divisions, forming a cell mass.

The Journey of New Life: From Zygote  to Baby

1. Starting Point: Fertilization -When a sperm meets an egg, they join together to form a single cell called a zygote. This is the very beginning of a new life!

2. Early Development: Becoming an Embryo -The zygote starts dividing into many cells, forming a tiny ball. As these cells continue to divide, they organize into groups that will develop into different parts of the body. This tiny, developing ball is called an embryo.

3. Settling In: Embedding in the Uterus- The embryo travels to the uterus and embeds itself into the wall, where it will continue to grow and develop. Over time, it starts forming recognizable body parts like hands, legs, and a head. At this stage, it’s called a foetus.

4. The Big Arrival: Birth- Once the foetus has developed completely, it’s ready to be born. 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 

Foetus in Uterus

Viviparous and Oviparous Animals

• Animals have different ways of bringing new life into the world. Some animals, like birds and reptiles, lay eggs that hatch into young ones. These animals are called oviparous.
• On the other hand, animals like dogs, cows, and cats don’t lay eggs. Instead, they give birth to live young ones. These animals are called viviparous.
• To see this in action, you can easily find eggs from oviparous animals, like chickens or turtles, because they lay their eggs outside their bodies. But you won't find eggs from viviparous animals like dogs or cats because they give birth to their babies directly.
  
  

Young ones to Adult 

Metamorphosis refers to the drastic changes a larva undergoes to become an adult. 

Life Cycle of Frog

In humans, however, the changes are less dramatic. We don’t go through a complete transformation like tadpoles. Instead, we gradually grow and develop, but our body parts stay similar from birth to adulthood

Asexual Reproduction

This type of reproduction in which only a single parent is involved is called asexual reproduction.

1. Budding 

Hydra can grow new buds—small outgrowths that eventually become new hydras. This is a form of asexual reproduction because only one parent is needed. 

As the buds grow, they eventually break off and become separate, fully-formed hydras. 

Since new individuals develop from the buds in hydra, this type of asexual reproduction is called budding.

Budding in Hydra

2. Binary Fission 

Amoebas have a cool way of reproducing all by themselves. Since they’re tiny single-celled organisms, they use a process called binary fission to make new amoebas.

Here’s how it works: First, the amoeba’s nucleus splits into two. Then, the whole cell divides into two halves, with each half getting one nucleus. In the end, you have 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.