We have previously learned about essential life processes like digestion, circulation, and respiration, which are necessary for individual survival. Another crucial process is reproduction, which is vital for the continuation of a species.

Imagine if organisms stopped reproducing; their kind would eventually disappear. Reproduction ensures that similar individuals continue to exist across generations.

Just as plants reproduce, animals also reproduce to create new individuals. Animals exhibit various ways of reproduction, resulting in the birth or hatching of their young ones.

Examples of young ones of different animals include:

• Human -> Baby
• Cat -> Kitten
• Dog -> Puppy
• Butterfly -> Caterpillar (larva)
• Hen -> Chick
• Cow -> Calf
• Frog -> Tadpole (larva)

While kittens and puppies are born resembling miniature adults, chicks hatch from eggs, and caterpillars hatch from eggs and undergo significant transformation to become adults. This indicates different reproductive strategies in the animal kingdom.

Modes Of Reproduction

Similar to plants, animals reproduce through two primary modes:

1. Sexual Reproduction
2. Asexual Reproduction

Sexual Reproduction

Sexual reproduction involves the participation of two parents, typically a male and a female. Organisms that reproduce sexually have distinct male and female reproductive organs.

These reproductive organs produce specialised cells called gametes. The male reproductive organs produce male gametes (sperms), and the female reproductive organs produce female gametes (ova or eggs).

Sexual reproduction begins with the fusion of a male gamete and a female gamete. This fusion forms a single cell called a zygote. The zygote then develops into a new individual.

This entire process, starting from the fusion of gametes to the development of a new individual, characterises sexual reproduction.

Male Reproductive Organs

In humans, the male reproductive system consists of:

• A pair of testes (singular: testis): These are the primary male reproductive organs.
• Two sperm ducts: Tubes that carry sperms from the testes.
• A penis: The external male reproductive organ.

The testes are responsible for producing the male gametes, called sperms. Millions of sperms are produced by the testes.

A sperm is a microscopic, single cell. It typically has three main parts: a head containing the nucleus, a middle piece containing mitochondria for energy, and a long tail that helps it move towards the egg.

Female Reproductive Organs

In humans, the female reproductive system includes:

• A pair of ovaries: These are the primary female reproductive organs.
• Oviducts (also called fallopian tubes): Tubes that connect the ovaries to the uterus.
• The uterus: A pear-shaped organ where the developing baby grows.

The ovaries produce the female gametes, called ova or eggs. In human females, usually one mature egg is released by one of the ovaries into the oviduct approximately every month.

Like a sperm, an egg is also a single cell. The uterus is the organ where the development of the baby takes place after fertilisation.

Fertilisation

The crucial first step in sexual reproduction is the fusion of a sperm and an ovum (egg). When sperms come into contact with an egg, typically one sperm successfully fuses with the egg.

This fusion of the male gamete (sperm) and the female gamete (egg) is called fertilisation. During fertilisation, the nucleus of the sperm and the nucleus of the egg merge to form a single nucleus.

The cell formed as a result of fertilisation is called a fertilised egg or zygote.

The zygote is the very beginning of a new individual. Since the zygote is formed by the fusion of gametes from both parents, the new individual inherits genetic characteristics from both the mother and the father.

Fertilisation can occur either inside or outside the female body:

• Internal Fertilisation: This is the process where fertilisation takes place inside the body of the female. Internal fertilisation is observed in many animals, including humans, cows, dogs, hens, etc.
• External Fertilisation: This type of fertilisation occurs outside the body of the female, typically in water. It is common in many aquatic animals like fish, frogs, starfish, etc.

In animals with external fertilisation, like frogs and toads, the female lays hundreds of eggs in water. These eggs are not covered by a hard shell and are held together by a layer of jelly for protection. The male then deposits millions of sperms over these eggs in the water. Sperms swim through the water to reach and fertilise the eggs.

Although hundreds of eggs and millions of sperms are involved in external fertilisation, not all eggs get fertilised or develop into new individuals. This is because the gametes are exposed to environmental factors like water currents, wind, and rain, and are also vulnerable to predators. Producing a large number of gametes increases the chances of at least some successful fertilisation and development.

Special cases like 'test tube babies' involve In Vitro Fertilisation (IVF), where fertilisation is carried out outside the body (in vitro) in a laboratory setting. If fertilisation is successful, the early embryo is implanted into the mother's uterus for normal development. This technique is used when conditions like blocked oviducts prevent natural fertilisation.

Development Of Embryo

Following fertilisation, the zygote undergoes repeated cell divisions. This process of division leads to the formation of a ball of cells.

These cells then begin to differentiate and form groups, which develop into the various tissues and organs of the body. This developing structure is called an embryo. The embryo implants or gets embedded in the wall of the uterus, where it continues to grow and develop.

As the embryo develops further in the uterus, it gradually forms distinct body parts such as hands, legs, head, eyes, ears, etc. The stage of development where all the major body parts can be identified is called a foetus.

When the development of the foetus is complete, the mother gives birth to the baby.

In animals with internal fertilisation that lay eggs (like hens), the zygote divides and protective layers, including a hard shell, form around the developing embryo as it travels down the oviduct. The hen then lays the egg. The embryo continues its development inside the egg shell, nourished by the yolk. A hen sitting on the egg provides the necessary warmth for the embryo to develop into a chick, which hatches after about 3 weeks.

In animals with external fertilisation, embryonic development occurs outside the female body, inside the egg coverings. Once development is complete, the eggs hatch, releasing the young ones (e.g., tadpoles from frog eggs).

Viviparous And Oviparous Animals

Animals are classified into two categories based on how they give birth or produce young ones:

• Viviparous Animals: Animals that give birth directly to young ones are called viviparous animals. The development of the embryo into a complete individual takes place inside the mother's body. Examples include humans, cows, dogs, cats, etc.
• Oviparous Animals: Animals that lay eggs are called oviparous animals. The development of the embryo into a young one takes place inside the egg, outside the mother's body (after laying). Examples include hens, frogs, lizards, butterflies, most birds, etc.

Observing the eggs of different animals can help differentiate between these two types. Eggs of oviparous animals are typically laid outside the body, while viviparous animals do not lay eggs, as the young develop internally.

Young Ones To Adults

After being born or hatched, new individuals grow and mature into adults. In some species, the young ones look very different from the adult form, undergoing significant changes during growth.

A prime example is the life cycle of a frog, which has distinct stages:

Egg $\rightarrow$ Tadpole (larva) $\rightarrow$ Adult frog

Tadpoles are aquatic, possess a tail, and respire through gills, looking completely different from adult frogs which are terrestrial, have legs, and breathe through lungs and skin. The dramatic transformation of the larva (tadpole) into an adult frog is called metamorphosis.

In contrast, humans are born with body parts similar to adults and undergo gradual growth and maturation without such drastic transformations, so humans do not undergo metamorphosis.

Asexual Reproduction

In some animals, particularly smaller or microscopic ones, reproduction occurs without the involvement of two parents or the fusion of gametes. This mode of reproduction, involving only a single parent, is called asexual reproduction.

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

• Budding (e.g., Hydra): In organisms like Hydra, new individuals develop from small projections or outgrowths on the parent body called buds. These buds gradually grow into complete new individuals, which then detach from the parent. This process is similar to budding seen in yeast. Since the new organism develops from a bud, this method is called budding.
• Binary Fission (e.g., Amoeba): Amoeba is a single-celled organism. In binary fission, the single parent cell divides into two new, identical individuals. The process begins with the nucleus dividing into two, followed by the division of the cytoplasm and the rest of the cell body. This results in two 'daughter' amoebae from a single 'parent' amoeba.

Other methods of asexual reproduction exist in different organisms, which are typically covered in higher classes.

Story Of Dolly, The Clone

Cloning is a technique that produces an exact genetic copy of a cell, part of an organism, or a complete organism. The first successful cloning of a mammal was achieved in 1996 at the Roslin Institute in Scotland, resulting in a sheep named Dolly.

The cloning process for Dolly involved taking a cell from the mammary gland of one sheep (Finn Dorsett sheep) and an egg cell from another sheep (Scottish blackface ewe). The nucleus was removed from the egg cell. The nucleus from the mammary gland cell of the Finn Dorsett sheep was then inserted into the enucleated egg cell of the Scottish blackface ewe. This reconstructed egg, carrying the genetic information of the Finn Dorsett sheep, was implanted into the uterus of the Scottish blackface ewe, which acted as the surrogate mother.

Dolly was born genetically identical to the sheep from which the mammary gland nucleus was taken, not the surrogate mother. Dolly was a healthy clone and even produced offspring through normal sexual reproduction. However, she died relatively early (at age 6) from a lung disease.

While attempts have been made to clone other mammals since Dolly, cloned animals often face health issues, with many dying before or shortly after birth and frequently exhibiting severe abnormalities.

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