CBSE Class 8 Science Reproduction in Animals Worksheets

Internal and external fertilisation are two methods of sexual reproduction that differ primarily in where the fusion of gametes occurs.

• Internal Fertilisation: The fusion of the male gamete (sperm) and female gamete (egg) occurs inside the female's body. This method is common in terrestrial animals like humans, birds, and reptiles.
• External Fertilisation: The fusion of gametes occurs outside the female's body, typically in a water medium. This is common in aquatic animals like frogs and fish.

Internal fertilisation offers a much higher chance of survival for the offspring because the developing embryo is protected within the mother's body from predators and environmental hazards.

Metamorphosis is the biological process of profound transformation from a larval stage to an adult stage. The life cycle of a frog is a classic example of this:

• Egg: The life cycle begins with eggs laid in water.
• Tadpole (Larva): The eggs hatch into aquatic larvae called tadpoles. A tadpole breathes through gills, has a tail, and swims in water like a fish.
• Froglet: The tadpole gradually develops legs, its tail shortens, and lungs develop to replace the gills.
• Adult Frog: The transformation is complete when the tail disappears and the froglet becomes an adult frog, which can live both on land and in water.

While both are early stages of development, they are distinct. A zygote is the very first stage, formed immediately after the fusion of the male sperm and the female ovum (fertilisation). It is a single cell. The embryo is the next stage, which develops from the zygote through repeated cell division. The embryo is a multicellular structure that gradually develops body parts.

Sexual reproduction is considered crucial for long-term survival because it introduces genetic variation. By combining genes from two different parents, the offspring are genetically unique from their parents and each other. This variation is the raw material for natural selection, allowing a species to adapt to changing environments, resist new diseases, and evolve over time. In contrast, asexual reproduction produces genetically identical offspring (clones), making the entire population vulnerable to the same threats.

Viviparous animals (like mammals) give birth to live young that develop inside the mother's body. This provides a significant survival advantage over oviparous animals (like birds and reptiles) that lay eggs. The key advantages are:

• Protection: The embryo is protected from predators, temperature fluctuations, and harsh environmental conditions inside the mother's womb.
• Nourishment: The developing young receives consistent and direct nourishment from the mother.

These factors lead to a much higher survival rate for the offspring compared to eggs laid in the external environment, which are highly vulnerable.

Both Amoeba and Hydra reproduce asexually, but their methods are different.

Amoeba reproduces through binary fission. In this process, the single-celled parent organism divides into two equal, identical daughter cells. The parent's identity is lost as it becomes two new individuals.

Hydra reproduces through budding. In this method, a small outgrowth or 'bud' forms on the parent's body. This bud grows into a miniature Hydra and eventually detaches to become an independent organism, while the parent organism continues to exist.

For the CBSE Class 8 Science exam (2025-26), some of the most frequently asked and important topics from this chapter include:

• The difference between sexual and asexual reproduction with examples.
• The process of metamorphosis in a frog, often with a request to draw a labelled diagram of its life cycle.
• The difference between internal and external fertilisation.
• Drawing and labelling the male or female reproductive system.
• Defining key terms like zygote, embryo, and fertilisation.