Reproduction in Animals
1. Modes of Reproduction
| Mode | Description | Examples |
|---|---|---|
| ASEXUAL | Single parent. NO gamete fusion. Offspring are GENETICALLY IDENTICAL | Amoeba, Hydra |
| SEXUAL | Two parents. GAMETES fuse. Offspring have GENETIC VARIATION | Most animals |
2. Asexual Reproduction
Binary Fission
The parent cell DIVIDES into TWO equal daughter cells.
Process:
- Nucleus DIVIDES (amitosis)
- Cytoplasm DIVIDES
- TWO daughter cells SEPARATE
Examples:
- Amoeba: Irregular binary fission (any plane)
- Paramecium: Transverse binary fission (across the middle)
- Euglena: Longitudinal binary fission (lengthwise)
Budding
A SMALL outgrowth (bud) develops on the parent body, GROWS, and DETACHES.
Example: Hydra
- A bud forms on the body wall.
- The bud GROWS — develops tentacles and a mouth.
- The bud DETACHES to live independently.
3. Sexual Reproduction
Male Reproductive System (Human)
| Organ | Function |
|---|---|
| TESTES (2) | Produce SPERM (male gametes) and TESTOSTERONE |
| Scrotum | SAC holding testes — maintains LOWER temperature |
| Vas deferens | Tube carrying SPERM from testes |
| Seminal vesicle | Adds FLUID to sperm → SEMEN |
| Prostate gland | Adds NUTRIENT fluid to semen |
| Penis | Delivers SPERM into the female body |
Female Reproductive System (Human)
| Organ | Function |
|---|---|
| OVARIES (2) | Produce EGGS (ova — female gametes) and OESTROGEN |
| Fallopian tubes (Oviducts) | Carry EGG from ovary to uterus. SITE OF FERTILISATION |
| Uterus (Womb) | MUSCULAR organ where EMBRYO develops |
| Cervix | NECK of the uterus |
| Vagina | Birth CANAL |
4. Gametes
| Feature | Male Gamete (SPERM) | Female Gamete (EGG / OVUM) |
|---|---|---|
| Size | VERY SMALL (microscopic) | LARGE (visible) |
| Shape | Has HEAD, MIDDLE PIECE, and TAIL | SPHERICAL |
| Motility | MOTILE (swims using tail) | NON-MOTILE |
| Number produced | MILLIONS at a time | One (or few) at a time |
| Cytoplasm | MINIMAL | LARGE amount (food for embryo) |
5. Fertilisation
The FUSION of SPERM nucleus with EGG nucleus to form a ZYGOTE.
Types of Fertilisation
| Type | Location | Examples |
|---|---|---|
| INTERNAL | Inside the FEMALE body | Humans, dogs, cats, birds, insects |
| EXTERNAL | OUTSIDE the body (in water) | Fish, frogs, toads, amphibians |
Adaptations for external fertilisation:
- LARGE numbers of gametes (many die)
- SIMULTANEOUS release (males and females release together)
- AQUATIC environment (gametes and zygote need water)
6. Metamorphosis
The DRAMATIC change in BODY FORM from an immature stage to an adult.
Frog Metamorphosis
Egg (in water) → Tadpole (lives in water, breathes with GILLS, has a TAIL) → Adult frog (lives on land, breathes with LUNGS and SKIN, no tail)
Silk Moth (Silkworm) Metamorphosis
Egg → LARVA (caterpillar — feeds on mulberry leaves, spins COCOON) → PUPA (inside cocoon) → ADULT (silk moth emerges)
'The silkworm LARVA spins a COCOON made of a SINGLE continuous silk thread 300–900 m long!'
7. Viviparous vs Oviparous
| Type | Description | Examples |
|---|---|---|
| OVIPAROUS | Lay EGGS. Embryo develops OUTSIDE the mother in the egg | Birds, reptiles, fish, frogs, insects |
| VIVIPAROUS | Give birth to LIVE young. Embryo develops INSIDE the mother | Humans, dogs, cats, cows, whales |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Binary fission and budding are the same' | Binary fission: DIVISION into TWO equal parts. Budding: SMALL outgrowth forms and detaches |
| 'All animals reproduce sexually' | MANY reproduce asexually (Amoeba — binary fission, Hydra — budding) |
| 'External fertilisation is safer' | Many GAMETES and ZYGOTES die — it is a LESS safe strategy than internal fertilisation |
| 'Metamorphosis happens only in frogs' | Many insects also undergo metamorphosis (butterfly, silk moth, mosquito, bee) |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define fertilisation, metamorphosis, binary fission
- 3-mark questions: Differentiate between internal and external fertilisation
- 4-mark questions: Human reproductive organs (male or female) with functions
- 6-mark questions: Metamorphosis in frog or silk moth with life cycle diagram
Self-Test
Q1. What is binary fission? Name an organism that reproduces by this method. A1. Binary fission is the DIVISION of a parent cell into TWO EQUAL daughter cells. Example: Amoeba, Paramecium.
Q2. Differentiate between internal and external fertilisation. A2. Internal fertilisation occurs INSIDE the female body (humans, birds). External fertilisation occurs OUTSIDE in water (fish, frogs). Internal is SAFER; external requires MANY gametes.
Q3. What is metamorphosis? Describe the metamorphosis of a frog. A3. Metamorphosis is the TRANSFORMATION from an immature form to an adult. Frog: Egg → Tadpole (gill-breathing, aquatic) → Adult frog (lung-breathing, terrestrial).
Q4. Name the male and female gametes in humans. Which is motile? A4. Male gamete = SPERM (MOTILE — has a tail). Female gamete = EGG/OVUM (NON-MOTILE).
Q5. Why is the scrotum located outside the body cavity? A5. The scrotum maintains a temperature 2–3°C LOWER than body temperature. Sperm production REQUIRES a lower temperature for normal development.
Q6. What is the difference between oviparous and viviparous animals? A6. Oviparous animals LAY EGGS (embryo develops outside). Viviparous animals give BIRTH to live young (embryo develops inside the mother).
