1. Introduction

Do you recall the processes of digestion, circulation and respiration which we have studied? These processes are essential for the survival of every individual. Reproduction is different. It is not essential for the survival of an individual, but it is essential for the continuation of a species.

Imagine what would happen if organisms had not reproduced. You will realize that reproduction is very important as it ensures the continuation of similar kinds of individuals, generation after generation.

2. Modes of Reproduction

Just as in plants, there are two modes by which animals reproduce:

• Sexual Reproduction: Involving the fusion of male and female gametes.
• Asexual Reproduction: Involving only a single parent.

3. Sexual Reproduction

In animals also, males and females have different reproductive parts or organs. Like plants, the reproductive parts in animals also produce gametes that fuse to form a zygote. It is the zygote which develops into a new individual.

A. Male Reproductive Organs

The male reproductive organs include a pair of testes (singular, testis), two sperm ducts and a penis. The testes produce the male gametes called sperms. Millions of sperms are produced by the testes. Though sperms are very small in size, each has a head, a middle piece and a tail. Indeed, each sperm is a single cell with all the usual cell components.

B. Female Reproductive Organs

The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes) and the uterus. The ovary produces female gametes called ova (eggs). In human beings, a single matured egg is released into the oviduct by one of the ovaries every month. Uterus is the part where development of the baby takes place. Like the sperm, an egg is also a single cell.

4. Fertilization

The first step in the process of reproduction is the fusion of a sperm and an ovum. When sperms come in contact with an egg, one of the sperms may fuse with the egg. Such fusion of the egg and the sperm is called fertilization.

During fertilization, the nuclei of the sperm and the egg fuse to form a single nucleus. This results in the formation of a fertilized egg or zygote. The zygote is the beginning of a new individual.

The process of fertilization is the meeting of an egg cell from the mother and a sperm cell from the father. So, the new individual inherits some characteristics from the mother and some from the father.

5. Development of Embryo

Fertilization results in the formation of zygote which begins to develop into an embryo. 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 termed an embryo. The 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, head, eyes, ears etc. The stage of the embryo in which all the body parts can be identified is called a foetus. When the development of the foetus is complete, the mother gives birth to the baby.

6. Viviparous and Oviparous Animals

We have seen that some animals give birth to young ones while some animals lay eggs which later develop into young ones.

• Viviparous Animals: The animals which give birth to young ones. Examples: Humans, dogs, cows, cats.
• Oviparous Animals: The animals which lay eggs. Examples: Hens, frogs, lizards, butterflies.

The eggs of a few animals are easy to observe because their mothers lay them outside their bodies. These are examples of oviparous animals. But you would not be able to collect the eggs of a dog, cow or cat because they do not lay eggs. The mother gives birth to the young ones (viviparous).

7. 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, the young ones may look very different from the adults.

Metamorphosis

In the case of frogs, there are three distinct stages: Egg -> Tadpole (Larva) -> Adult. The tadpole looks very different from the adult frog. It has a tail and fins like a fish and breathes through gills. The adult frog has legs and no tail and breathes through lungs/skin. The transformation of the larva into an adult through drastic changes is called metamorphosis. Insects like silkworms also undergo metamorphosis.

8. Asexual Reproduction

In some very small animals like Hydra and microscopic organisms like Amoeba, reproduction takes place asexually.

A. Budding (in Hydra)

In Hydra, there may be one or more bulges. These bulges are the developing new individuals and they are called buds. The bud develops, matures and detaches from the parent body to become a new individual. This type of asexual reproduction is called budding.

B. Binary Fission (in Amoeba)

Amoeba is a single-celled organism. It begins the process of reproduction by the division of its nucleus into two nuclei. This is followed by division of its body into two, each part receiving a nucleus. Finally, two amoebae are produced from one parent amoeba. This type of asexual reproduction in which an animal reproduces by dividing into two individuals is called binary fission.

The biological process by which new individual organisms are produced from their parents.

Reproduction is essential for the continuation of species and prevents extinction.

Reproduction involving the fusion of male and female gametes.

Reproduction involving only a single parent.

Male reproductive organs that produce sperms and testosterone.

The male gamete. It is a single cell with a head, middle piece, and tail.

Female reproductive organs that produce ova (eggs) and hormones.

The female gamete. It is a single large cell.

Also called fallopian tube. It carries the egg from the ovary to the uterus. Fertilization happens here.

The part of the female reproductive system where the development of the baby takes place.

The fusion of a sperm and an ovum to form a zygote.

The single fertilized cell resulting from the fusion of gametes. It is the first stage of life.

Fertilization that occurs inside the female body (e.g., humans, hens).

Fertilization that occurs outside the female body, usually in water (e.g., frogs, fish).

The developing structure formed by repeated division of the zygote.

The stage of the embryo where all body parts can be identified.

Animals that give birth to young ones (e.g., humans, cows).

Animals that lay eggs (e.g., hens, lizards).

The drastic change which transforms a larva into an adult (e.g., tadpole to frog).

An immature form of an animal that looks very different from the adult (e.g., caterpillar, tadpole).

The larval stage of a frog. It lives in water and breathes through gills.

Asexual reproduction where new individuals develop from buds (e.g., Hydra).

Asexual reproduction where an organism divides into two equal halves (e.g., Amoeba).

In Vitro Fertilization. Fertilization takes place outside the body in a test tube.

A baby conceived through IVF technology.

Producing an exact genetic copy of an organism.

The first mammal (sheep) to be successfully cloned in 1996.

An animal having both male and female reproductive organs (e.g., Earthworm).

The shedding of the uterine lining along with the unfertilized egg and blood.

The process where the embryo attaches itself to the wall of the uterus.

The period of development of the embryo inside the uterus until birth.

Helps the sperm to swim and move towards the egg.

Despite developing into a complex organism, life begins as a single cell - the zygote.

Hen's egg has a hard shell to protect the developing embryo.

The process of keeping eggs warm (by the parent sitting on them) for hatching.

The process by which the zygote multiplies to form a ball of cells (embryo).

The nucleus of sperm and egg carries hereditary information (genes) from parents.

Eggs laid in water are exposed to predators and currents, hence laid in hundreds.

A layer of jelly holds frog eggs together and provides protection.

The two new amoebae produced after binary fission are called daughter cells.

The male parent who contributes the sperm.

The female parent who contributes the egg.

Male sex hormone produced by testes.

Female sex hormone produced by ovaries.

Connects the foetus to the placenta for nutrition and waste removal.

Fluid surrounding the foetus that protects it from shocks.

The age at which reproductive organs become functional.

A type of asexual reproduction (e.g., in Plasmodium) where one cell divides into many.

The bud in Hydra is multicellular, unlike the bud in yeast (which is unicellular).

The series of changes in the life of an organism including reproduction.