lesson
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A female salmon can lay over 3,000 eggs at once, but an eagle lays only 2 eggs in her nest.
Why do some animals make thousands of eggs while others make just a few? The answer starts with how life begins.
Fertilization is the exact moment a male cell (called sperm) joins with a female cell (called an egg) to start growing a new baby.
πA simple, colorful diagram showing how fertilization works. On the left, a small swimming sperm cell with a tail labeled 'Sperm (Father)'. In the center, a larger round circle labeled 'Egg (Mother)'. An arrow shows the sperm joining the egg. On the right, a glowing circle labeled 'Fertilized Egg -> Grows into baby animal'. Clean card styling, white background, soft rounded corners, dark navy text #1e2945, accent blue and coral colors.
Where does this meeting happen? Let's look at the watery world of fish and frogs first.
External Fertilization in Water
In external fertilization, the sperm joins the egg outside the mother's body, almost always in water.
Fish and amphibians (like frogs) lay soft, jelly-like eggs in ponds or oceans, and the male releases sperm directly into the water so they can swim to the eggs.
πAn animated or step-by-step graphic illustrating external fertilization in a pond. Step 1: Mother frog lays a cluster of soft jelly eggs in pond water. Step 2: Father frog releases sperm into the water. Step 3: Sperm swim through water to fertilize eggs. A small danger tag shows 'Predator fish & water currents', explaining with a badge: 'Strategy: Lay hundreds of eggs so some survive!' Responsive 350px layout, friendly cartoon style, navy text #1e2945.
Because hungry predators or river currents can easily destroy soft eggs, these animals must produce huge numbers of eggs so that at least a few survive.
Water makes external fertilization easy β but what happens when animals live on dry land?
Internal Fertilization on Land
On dry land, eggs would dry up and sperm could not swim, so land animals use internal fertilization.