Egg Cells

The biggest cells around

An egg cell is a special cell that can grow into a whole new animal. When it joins with a cell from a father animal, it starts dividing and grows into a baby.

Egg cells are packed with food for the baby, so they're big:

• In a bird's egg, the yolk is the actual egg cell. One cell!
• The egg white and shell are not cells. They're built around the yolk to protect it.
• A human egg cell is about the size of the period at the end of this sentence. Tiny, but still one of the biggest cells in your body!

What's inside a bird egg?

• Shell: hard and strong, but it has thousands of tiny holes so air can get in.
• Air pocket: a little bubble of air at the wide end.
• Egg white: mostly water and protein. It cushions the yolk like a waterbed.
• Twisty ropes: called chalazae (kuh-LAY-zee). They hold the yolk in the middle like a hammock.
• Yolk: the giant egg cell, full of food.
• White spot: a tiny pale dot on the yolk. That's where the nucleus is, and where a chick would start to grow.

From one cell to a chick

Most eggs from the grocery store will never become chicks. But when a mother hen's egg has joined with a cell from a rooster, something amazing happens:

• The cell at the white spot divides into 2, then 4, then 8, again and again (just like in the animal cell lesson!).
• The cells form a tiny chick, which grows bigger every day.
• The chick uses the yolk as its food.
• After about 21 days, the chick pecks its way out of the shell!

Egg cell quiz