1. Introduction to Reproduction in Animals

Every living organism has a limited life span. To ensure the continuation of their species, organisms produce new individuals of their own kind. This biological process by which living organisms give rise to new individuals is called reproduction. In animals, including humans, reproduction is essential not only for survival of the species but also for maintaining balance in nature.

In CBSE Class 8 Science, Chapter 9 – Reproduction in Animals, we mainly study the process of sexual reproduction in animals and some simple forms of asexual reproduction. These revision notes are prepared strictly according to the NCERT textbook so that students can confidently use them for school exams and CBSE Class 8 exam pattern questions.

2. Modes of Reproduction in Animals

Animals show mainly two basic modes of reproduction:

• Sexual reproduction
• Asexual reproduction

2.1 Sexual Reproduction

Sexual reproduction is the process in which two parents (male and female) are involved. It requires the fusion of male and female reproductive cells, called gametes. The offspring produced inherit characteristics from both parents. This mode is common in humans, other mammals, birds, reptiles, amphibians, and many other animals.

2.2 Asexual Reproduction

Asexual reproduction is the process in which only one parent is involved. There is no fusion of male and female gametes. The offspring produced are identical to the parent and are called clones. In this chapter, you will mainly study asexual reproduction in simple organisms like hydra and amoeba.

3. Male and Female Reproductive Systems in Humans

To understand reproduction in animals, it is important to study the human reproductive system. Humans exhibit sexual reproduction, where male and female individuals produce gametes that fuse to form a new individual.

3.1 Male Reproductive System

The main organs of the male reproductive system are:

• Testes (singular: testis) – A pair of oval organs located outside the abdominal cavity in a pouch of skin called the scrotum. They produce male gametes called sperms and also secrete male hormones.
• Sperm ducts – Also called vas deferens. They transport the sperms from the testes to the urethra.
• Urethra – A common tube that carries urine as well as sperms out of the body.
• Penis – External organ through which sperms are passed out from the male body.

The sperms are microscopic, motile cells with a head, middle piece and tail. Each sperm carries genetic material from the father.

3.2 Female Reproductive System

The main organs of the female reproductive system are:

• Ovaries – A pair of small, almond-shaped organs present in the lower abdomen. They produce female gametes called ova (singular: ovum) or eggs, and also secrete female hormones.
• Oviducts (fallopian tubes) – Tubes arising from each ovary and opening into the uterus. The fertilisation of sperm and ovum generally takes place in the oviduct.
• Uterus – A pear-shaped, muscular organ where the embryo develops into a baby.
• Cervix and vagina – They form the lower part of the female reproductive tract and serve as the passage for sperms and for the birth of the baby.

4. Gametes and Fertilisation

Gametes are special reproductive cells produced by the reproductive organs. In humans:

• Male gamete → Sperm
• Female gamete → Ovum (egg)

The process of fusion of a sperm and an ovum is called fertilisation. During fertilisation, the nuclei of the sperm and the ovum combine to form a single nucleus. The cell formed after fertilisation is called a zygote.

4.1 Types of Fertilisation

In internal fertilisation, the male releases sperms inside the body of the female where they meet the ovum and fertilisation takes place. In external fertilisation, large numbers of gametes are released into water to increase the chances of fertilisation.

5. From Zygote to Embryo and Baby

The zygote formed after fertilisation is the first cell of a new individual. It undergoes repeated cell divisions to form a ball of many cells. This developing structure is called an embryo.

In humans, the embryo gets attached to the wall of the uterus. Here it receives nutrients and oxygen from the mother’s blood through a special structure called the placenta. As it continues to grow, the embryo gradually develops body parts such as head, limbs, eyes, etc. At this stage, it is called a foetus.

When the development is complete, the mother gives birth to the baby. This entire process from zygote formation to birth takes about nine months in humans.

5.1 Oviparous and Viviparous Animals

On the basis of the way they give birth to their young ones, animals are classified as:

• Oviparous animals: Animals that lay eggs. Example: hen, frog, lizard, fish, birds, etc.
• Viviparous animals: Animals that give birth to young ones. Example: human, cow, dog, cat, etc.

6. Metamorphosis in Frog

Many animals undergo remarkable changes in their body form during their life cycle. This drastic change in form from larva to adult is called metamorphosis.

In frogs:

1. The adult frogs lay eggs in water.
2. The eggs hatch into young ones called tadpoles.
3. Tadpoles look very different from adult frogs. They have a tail, gills for respiration and are fully aquatic.
4. Gradually, the tadpole undergoes many changes – limbs appear, tail reduces, lungs develop for breathing air.
5. Finally, it transforms into an adult frog which can live both on land and in water.

Thus, metamorphosis in frog is a classic example of change from larval form to adult form during the life cycle.

7. Asexual Reproduction in Animals

In simple animals and microorganisms, asexual reproduction is common. Here, only one parent is involved, and the offspring are exact copies of the parent.

7.1 Binary Fission in Amoeba

Amoeba reproduces by a method called binary fission. The steps are:

• First, the nucleus of the Amoeba divides into two.
• Then, the cytoplasm also divides, forming two daughter cells.
• Each daughter Amoeba receives one nucleus and some cytoplasm.

This method of reproduction allows Amoeba to multiply very quickly under favourable conditions.

7.2 Budding in Hydra

Hydra reproduces by budding. The steps involved are:

• A small outgrowth or bud appears on the body of the parent Hydra.
• The bud grows and develops tentacles and mouth.
• When the bud becomes fully mature, it detaches from the parent and lives as an independent Hydra.

Budding can also be seen in yeast, where small buds grow on the parent cell and separate to form new yeast cells.

8. Important Definitions and Key Terms

• Reproduction: The biological process by which living organisms produce new individuals of their own kind.
• Gametes: Reproductive cells (sperms and ova) that fuse during sexual reproduction.
• Fertilisation: Fusion of male and female gametes to form a zygote.
• Zygote: The single cell formed after fertilisation.
• Embryo: Early stage of development of the baby from the zygote.
• Foetus: Later stage of embryo when body parts are clearly visible.
• Oviparous: Animals that lay eggs.
• Viviparous: Animals that give birth to young ones.
• Metamorphosis: Sudden and drastic change in the body form during the life cycle.
• Binary fission: A type of asexual reproduction where one cell divides into two identical cells.
• Budding: Asexual reproduction where a new individual develops from a small bud on the parent’s body.

9. Quick NCERT-Style Concept Check

1. What is the role of testes in the human male reproductive system?
2. Define fertilisation. How is the zygote formed?
3. Differentiate between internal fertilisation and external fertilisation with one example each.
4. What is the difference between oviparous and viviparous animals?
5. Explain metamorphosis in frog in brief.
6. Describe budding in Hydra as a method of asexual reproduction.