How Animals Bring New Life Into the World
Every animal you see around you — a sparrow on the window, a cat in the lane, even the mosquito you swatted — exists because its parents reproduced. Reproduction is how life continues. Without it, every species would disappear within a single generation.
For Class 8, we study two broad types of reproduction: sexual and asexual. Most animals you know (dogs, frogs, humans, fish) reproduce sexually. A few simpler animals use asexual methods. Understanding the difference — and the biology behind each — is what this chapter is really about.
The deeper idea here is variation. Sexual reproduction shuffles genetic information from two parents, producing offspring that are similar but not identical. That variation is what drives evolution over millions of years. Asexual reproduction produces clones — genetically identical offspring — which is faster but riskier when environments change.
CBSE Class 8 tests your ability to give examples, name organs, describe processes, and fill in the missing step in a life cycle. We’ll cover all of that here.
Key Terms and Definitions
Reproduction — the process by which living organisms produce offspring of their own kind.
Sexual reproduction — reproduction involving two parents, each contributing a gamete (sex cell). Male gametes are sperms; female gametes are eggs (ova).
Fertilisation — the fusion of a sperm and an egg to form a zygote. This is the moment a new individual begins.
Asexual reproduction — reproduction involving only one parent, no gametes, no fertilisation. Offspring are genetically identical to the parent.
Viviparous animals — animals that give birth to live young (e.g., humans, dogs, whales, bats). The embryo develops inside the mother’s uterus.
Oviparous animals — animals that lay eggs (e.g., hens, frogs, lizards, sharks). Development happens outside the mother’s body.
Metamorphosis — the process where an animal’s body changes dramatically in form as it develops (e.g., caterpillar → butterfly, tadpole → frog).
Budding — a form of asexual reproduction where a small bud grows on the parent’s body and eventually detaches to form a new individual (seen in Hydra).
Binary fission — asexual reproduction where a single organism splits into two equal halves (seen in Amoeba).
Clone — offspring genetically identical to the parent, produced by asexual reproduction.
Sexual Reproduction in Animals
The Two Gametes
Male animals produce sperms — tiny cells with a head (containing genetic material) and a tail for swimming. Female animals produce eggs (ova) — larger, non-motile cells containing food reserves for the developing embryo.
Fertilisation can happen in two ways:
- Internal fertilisation — sperm meets egg inside the female’s body. Seen in humans, dogs, hens, most reptiles.
- External fertilisation — sperm meets egg outside the body, usually in water. Seen in frogs and most fish. The female lays eggs in water; the male releases sperms over them.
Remember the pattern: animals that lay eggs in water (frogs, fish) use external fertilisation. Animals that live on land mostly use internal fertilisation — otherwise the gametes would dry out.
From Zygote to New Individual
Once fertilisation occurs, the zygote divides repeatedly to form an embryo. The embryo then develops into a fully formed animal through a series of stages.
In viviparous animals, this entire development happens inside the mother. The embryo gets nutrients directly through a structure called the placenta (in humans and most mammals).
In oviparous animals, all the nutrients the embryo needs are packed inside the egg — the yolk provides food. After sufficient development, the young animal hatches out.
Sperm + Egg → Zygote → Embryo → Young one → Adult
Metamorphosis
Some animals don’t just “grow up” — they transform completely. This is metamorphosis.
Frog life cycle:
- Eggs laid in water (mass of jelly-coated eggs called frog spawn)
- Eggs hatch into tadpoles (aquatic, breathe through gills, have a tail)
- Tadpoles develop legs, lose the tail, lungs replace gills
- Adult frog emerges (terrestrial, breathes through lungs and moist skin)
Butterfly/silkworm life cycle:
- Egg
- Larva (caterpillar in butterflies, silkworm in silk moths) — the feeding stage
- Pupa (cocoon stage) — resting and transformation stage
- Adult (imago) — reproductive stage
CBSE board exams frequently ask you to draw and label the life cycle of a frog or a silkworm. Practice drawing these — a clean labelled diagram fetches full marks. The stages to label: egg → tadpole → adult frog (for frog); egg → larva → pupa → adult (for silkworm/butterfly).
Asexual Reproduction in Animals
Asexual reproduction is common in simpler animals. Only one parent is involved, no gametes are formed, and the offspring is genetically identical to the parent.
Budding (Hydra)
Hydra is a small freshwater animal you’ll find in ponds. When conditions are favourable (plenty of food, warm temperature), a small bud grows on its body wall. The bud gradually develops a mouth and tentacles. Eventually it detaches and lives independently.
Hydra can also reproduce sexually when conditions get harsh. This is a neat survival strategy — use fast asexual reproduction when life is easy, switch to sexual reproduction (which produces varied offspring) when the environment becomes challenging.
Binary Fission (Amoeba)
Amoeba is a single-celled organism. It reproduces by simply splitting into two. The nucleus divides first, then the cytoplasm follows, giving two daughter cells identical to the original parent.
This is extremely fast — under ideal conditions, Amoeba can divide in as little as 20 minutes.
Other Examples Worth Knowing
| Animal | Type of Asexual Reproduction |
|---|---|
| Hydra | Budding |
| Amoeba | Binary fission |
| Planaria (flatworm) | Regeneration |
| Sea anemone | Budding / Fragmentation |
Solved Examples
Example 1 — Easy (CBSE Level)
Q: A hen lays eggs. Is it viviparous or oviparous? Does it use internal or external fertilisation?
Hen is oviparous — it lays eggs.
Fertilisation in hens is internal — the sperm fertilises the egg inside the hen’s body. The fertilised egg is then coated with albumen (egg white) and the hard shell before being laid.
Many students assume that if an animal lays eggs, it must use external fertilisation. Wrong. Hens, most reptiles, and even sharks use internal fertilisation but still lay eggs. The egg you eat from a grocery store is usually unfertilised (no sperm was involved).
Example 2 — Medium (CBSE Level)
Q: Describe the process of fertilisation in frogs.
Frogs use external fertilisation. During the breeding season (usually monsoon), the female frog lays hundreds of eggs in water. The eggs are surrounded by a jelly-like coating that prevents them from sinking and provides some protection.
The male frog, sitting on the female’s back, releases sperms over the eggs simultaneously. The sperms swim through the water to reach the eggs. Each sperm fuses with one egg — this is fertilisation. The resulting zygote develops into a tadpole.
Frogs lay so many eggs because external fertilisation is “risky” — many eggs won’t get fertilised, and many tadpoles will be eaten by predators. Large numbers compensate for high early mortality.
Example 3 — Medium (CBSE Level)
Q: What is the significance of metamorphosis in the life cycle of a frog?
Metamorphosis allows the tadpole and adult frog to occupy different ecological niches — they eat different food and live in different environments. Tadpoles are aquatic and feed on algae; adult frogs are terrestrial carnivores. This means adults and young don’t compete for the same resources, which is a major survival advantage.
Example 4 — Hard (for CBSE scholarship / Olympiad level)
Q: A student says “since Hydra reproduces by budding, it never dies of old age.” Evaluate this claim.
There’s a remarkable truth here. Research has shown that Hydra shows negligible senescence — it doesn’t seem to age. Because it constantly regenerates cells (even when not reproducing), the organism effectively renews itself.
However, the claim “never dies” is an overstatement. Hydra can absolutely be killed — by predation, disease, or environmental changes. The claim is better stated as: Hydra doesn’t show age-related deterioration the way most animals do. This makes it a subject of intense research in aging biology.
For CBSE Class 8, the expected answer is simply: The bud separates and grows into an independent Hydra. The original organism is not destroyed in the process, so in that sense Hydra can potentially live indefinitely under ideal conditions.
Exam-Specific Tips
CBSE Class 8 Board Pattern
This chapter has consistently appeared in CBSE Class 8 science exams (both FA and SA). High-yield question types:
- Define: viviparous, oviparous, fertilisation, metamorphosis, budding
- Give two examples of each: viviparous animals, oviparous animals, animals showing external fertilisation
- Draw labelled diagrams: life cycle of frog, life cycle of silkworm
- Compare: internal vs external fertilisation (usually a 4-mark table question)
- Short note: budding in Hydra, binary fission in Amoeba
The life cycle of a silkworm (Bombyx mori) is specifically important because it connects to the chapter on Fibre to Fabric. CBSE sometimes asks a cross-chapter question: “The silk thread is obtained from which stage of the silkworm’s life cycle?” Answer: the pupa stage (from the cocoon).
Marks Distribution
In a typical 80-mark CBSE Class 8 Science paper, this chapter fetches 6–10 marks across different sections. Diagrams alone can fetch 4 marks, so practise drawing clean labelled life cycles.
Common Mistakes to Avoid
Mistake 1: Confusing oviparous with external fertilisation
Oviparous means egg-laying. It says nothing about where fertilisation happens. Hens are oviparous but use internal fertilisation. Frogs are oviparous and use external fertilisation. Don’t conflate the two.
Mistake 2: Forgetting that humans are viviparous mammals
The question “give an example of a viviparous animal” — students sometimes blank out. Humans, dogs, cats, cows, whales, dolphins, bats — all mammals (except platypus and echidna) are viviparous. Say “humans” or “dogs” confidently.
Mistake 3: Calling the tadpole the “larva” of a frog — then forgetting this for butterflies
In frog life cycles, the intermediate stage is called a tadpole. In insects like butterflies, the intermediate stage is called a larva (or caterpillar). Then comes pupa. Frog life cycles do not have a pupa stage.
Mistake 4: Writing that Amoeba reproduces by budding
Amoeba reproduces by binary fission (splits into two equal halves). Budding is Hydra’s method. These get swapped in exams surprisingly often.
Mistake 5: Saying the zygote IS the embryo
The zygote is the single cell formed by fertilisation. It divides many times to form the embryo. The zygote becomes the embryo — they are sequential stages, not the same thing.
Practice Questions
Q1. What is the difference between viviparous and oviparous animals? Give two examples of each.
Viviparous animals give birth to live young. The embryo develops inside the mother’s body (e.g., humans, dogs, cows, whales).
Oviparous animals lay eggs. The embryo develops inside the egg outside the mother’s body (e.g., hens, frogs, lizards, sharks, butterflies).
Q2. How does fertilisation in frogs differ from fertilisation in humans?
In frogs, fertilisation is external — eggs are laid in water and sperms are released over them. Fusion happens outside the body.
In humans, fertilisation is internal — sperm is deposited inside the female’s body, and the sperm fuses with the egg inside the female reproductive tract (fallopian tube).
Q3. Describe budding in Hydra with the help of a labelled diagram.
In Hydra, a small outgrowth called a bud appears on the body wall. The bud gradually grows larger and develops a mouth and tentacles (miniature Hydra). Eventually, it detaches from the parent body and starts living as an independent organism.
Labelled diagram points: parent Hydra → bud forming → bud with tentacles → detached young Hydra.
Key point: the parent is not harmed — it continues to live and can produce more buds.
Q4. Name the stages in the life cycle of a silkworm (silk moth) in order.
Egg → Larva (silkworm/caterpillar) → Pupa (cocoon) → Adult (silk moth)
Silk is harvested from the cocoon (pupa stage). The cocoon is made of a single continuous silk thread up to 1500 metres long.
Q5. What is metamorphosis? In which animals does it occur?
Metamorphosis is the dramatic change in the body form of an animal as it develops from juvenile to adult.
Complete metamorphosis (4 stages): butterfly, silk moth, housefly, mosquito (egg → larva → pupa → adult)
Incomplete metamorphosis (3 stages): grasshopper, cockroach (egg → nymph → adult — no pupal stage)
Amphibian metamorphosis: frog, toad, salamander (egg → tadpole → adult)
Q6. Why do frogs lay hundreds of eggs at a time, while humans give birth to usually one baby at a time?
Frogs use external fertilisation — not every egg gets fertilised. Additionally, tadpoles face huge predation pressure. Laying hundreds of eggs ensures that enough survive to adulthood.
Humans use internal fertilisation (near-guaranteed fertilisation), and the embryo develops inside the protected uterus with direct nutrition from the mother. The chances of survival are very high. So producing one well-nurtured offspring is far more efficient than producing hundreds.
This trade-off — many eggs with little parental care vs. few offspring with extensive parental care — is a major pattern across the animal kingdom.
Q7. Amoeba is a single-celled organism. How does it reproduce?
Amoeba reproduces by binary fission — a form of asexual reproduction.
The process:
- The nucleus of the Amoeba divides into two (nuclear division / karyokinesis)
- The cytoplasm then divides (cytokinesis)
- Two daughter Amoebae form, each identical to the original parent
Binary fission is extremely fast. Under ideal conditions, an Amoeba can complete one division in about 20 minutes.
Q8. A student says, “Hen lays eggs, so it must use external fertilisation.” Is this correct? Explain.
This is incorrect. Hens use internal fertilisation.
The sperm from the rooster fertilises the egg inside the hen’s body (in the oviduct). After fertilisation, the egg is coated with albumen (egg white), membranes, and the hard calcium carbonate shell. Then the fully formed egg is laid.
The confusion arises because students think oviparous = external fertilisation. That’s a false rule. The correct distinction: oviparous means egg-laying; it says nothing about where fertilisation happens.
Frequently Asked Questions
Q: What is the difference between a zygote and an embryo?
A zygote is the single cell formed immediately after fertilisation (sperm + egg). When the zygote starts dividing — first into 2 cells, then 4, 8, 16, and so on — it becomes an embryo. Think of the zygote as Day 0 and the embryo as the multi-cellular structure that develops from it.
Q: Are all mammals viviparous?
Almost all, but not all. The platypus and echidna (found in Australia) are mammals that lay eggs — they are called monotremes. They are the fascinating exceptions that come up in Olympiad questions. For CBSE Class 8 purposes, you can say “most mammals are viviparous.”
Q: Why are eggs of frogs surrounded by jelly?
The jelly (made of a mucus-like substance) serves multiple purposes: it sticks the eggs together so they don’t scatter, provides some insulation against temperature changes, helps the eggs float near the water surface (where oxygen is more available), and offers limited protection against microorganisms.
Q: Can Hydra reproduce both sexually and asexually?
Yes. Under favourable conditions (abundant food, moderate temperature), Hydra reproduces asexually by budding. When conditions become harsh — food is scarce, temperature drops — Hydra switches to sexual reproduction. Sexual reproduction produces genetically varied offspring that may be better suited to survive the new conditions.
Q: Is binary fission the same as cell division in our body?
They’re similar processes (both involve a cell splitting into two), but they’re not identical. Binary fission refers specifically to asexual reproduction in unicellular organisms — the whole organism becomes two new organisms. Cell division in our body (mitosis) creates new cells for growth and repair, not new organisms. The parent cell “gives rise to” daughter cells that are part of the same body.
Q: What is the exact sequence of development after fertilisation in humans?
For Class 8, the sequence is: Zygote → Embryo (repeated cell division) → Foetus (when organs start forming and the human shape becomes recognisable) → Baby (born after about 9 months). For CBSE, knowing zygote → embryo → foetus → baby is sufficient.
Q: Which animals show incomplete metamorphosis?
Incomplete metamorphosis (also called hemimetabolism) has three stages: egg → nymph → adult. The nymph looks like a miniature wingless adult. Examples: grasshopper, cockroach, dragonfly, termite. There is no pupal stage. This contrasts with complete metamorphosis (egg → larva → pupa → adult) seen in butterflies, moths, flies, and beetles.
Q: Why is reproduction considered a life process?
Life processes are those that are essential for the survival of a species (though not necessarily each individual). Reproduction is what ensures the continuation of a species across generations. Without reproduction, a species would go extinct as existing individuals age and die. It’s classified as a life process because its absence = extinction of the species.