REPRODUCTION IN ANIMALS | Class 8 Science Notes

Fertilisation

Fertilisation is the crucial first step in sexual reproduction where a sperm cell from the male fuses with an egg cell from the female to form a fertilised egg called a zygote. During fertilisation, the nuclei of the sperm and egg merge, restoring the diploid number of chromosomes. This zygote then begins the process of development into a new organism. Fertilisation can occur internally, inside the female's body, as in humans, cows, dogs, and hens, or externally, outside the female's body, usually in water, as seen in frogs and fish. Internal fertilisation provides protection to the fertilised egg and increases the chances of survival. In some cases, such as when a woman's oviducts are blocked, fertilisation outside the body is performed artificially through In Vitro Fertilisation (IVF). In IVF, eggs and sperms are combined in a laboratory, and the resulting zygote is implanted in the uterus. Babies born through this method are called test-tube babies, though they do not actually grow in test tubes. External fertilisation involves the release of large numbers of eggs and sperms into water to increase the chances of fertilisation despite environmental hazards.

📊 Diagram: Fig. 6.5: Fertilisation; Fig. 6.6: Zygote

🧪 Activity: Activity 6.1: Visit ponds or streams during spring or rainy season to observe frog eggs floating in water and note their color and size.

🔗 Connection: This section connects to the next topic on the development of the embryo from the zygote.