Difference Between Internal and External Fertilization

In sexual reproduction, the fusion of a male gamete (sperm) and a female gamete (egg/ovum) to form a zygote is called Fertilization. In the animal kingdom, this crucial meeting of cells can happen either inside the mother's body or out in the open environment.

Question (Click to Flip)

Why is water absolutely necessary for external fertilization?

Answer

Sperm cells need a liquid medium to swim using their tails. Without water, the sperms cannot travel to reach the eggs, and the unprotected eggs would quickly dry up and die in the air.

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Key Facts

In IVF (In Vitro Fertilization) or 'Test Tube Baby' procedures for humans, the fertilization actually happens externally in a glass lab dish, and the embryo is later placed back inside the mother.

Even though birds lay eggs outside, the actual fertilization of the egg happens internally before the hard calcium shell forms around it.

1. What is Internal Fertilization?

Definition: The fusion of the male and female gametes takes place inside the body of the female.
Mechanism: The male physically deposits sperms directly into the female's reproductive tract.
Gamete Count: Because the sperms are safely deposited inside, the female usually produces very few eggs (often just one at a time).
Survival Rate: The chances of successful fertilization and the survival of the offspring are very high because the embryo develops in a highly protected environment.
Examples: Humans, Dogs, Cows, Birds, and Reptiles.

2. What is External Fertilization?

Definition: The fusion of the male and female gametes takes place outside the body of the female, usually in a water environment.
Mechanism: The female releases hundreds of eggs into the water, and the male simultaneously releases millions of sperms over them. They meet and fuse randomly in the water.
Gamete Count: Females must produce thousands of eggs at once.
Survival Rate: The survival rate is extremely low. Many eggs and sperms are washed away by water currents, destroyed by harsh temperatures, or eaten by predators before they even hatch.
Examples: Frogs, Starfish, and most Fish species.

3. Summary of Differences

Feature	Internal Fertilization	External Fertilization
Location	Inside the female body.	Outside (usually in water).
Number of Eggs	Very few (often 1 to 5).	Thousands or millions.
Offspring Protection	High protection.	No protection.
Environment Required	Can happen on dry land.	Requires a wet, aquatic medium.

Questions and Answers

Why is water absolutely necessary for external fertilization?+

Sperm cells need a liquid medium to swim using their tails. Without water, the sperms cannot travel to reach the eggs, and the unprotected eggs would quickly dry up and die in the air.

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