Yolk or the mother's body

The idea

Reptiles and birds lay eggs with enough yolk to nourish the embryo until it hatches into a young one, while in mammals the zygote grows and develops inside the female body.

A hen's egg and a calf

A hen lays an egg, and later a chick comes out of it. A cow gives birth to a calf.

What do you think feeds the young one before it comes out?

• The yolk in the egg, or the mother's body
• The air alone
• The water around it alone

Two ways

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Fed until hatching or birth

The yolk of the egg fed the embryo of the reptile or bird until it hatched. In a mammal the zygote grew inside the mother.

Reptiles and birds lay eggs with enough yolk to nourish the embryo until it hatches into a young one, while in mammals the zygote grows and develops inside the female body.

Notes

Reptiles and birds: eggs with enough yolk until hatching. Mammals: the zygote develops inside the female body.

Check yourself

Reptiles and birds lay eggs. Each egg has…

In mammals, where does the zygote grow and develop?

Which animal's zygote grows inside the female body?

What feeds the embryo in a bird's egg until it hatches?

• only enough yolk to make a larva. That is the case for animals that lay hundreds to thousands of eggs, such as fish.
• enough yolk to nourish the embryo until it hatches — correct. Yes!
• no yolk, since the mother feeds it. In reptiles and birds the yolk nourishes the embryo until it hatches.
• In an egg laid in water. That is not how mammals develop. The zygote grows inside the female body.
• In a nest outside the body. Mammals do not lay eggs for this. The zygote grows inside the female body.
• Inside the female body — correct. Yes!
• A cow — correct. Yes!
• A bird. Birds lay eggs with enough yolk for the embryo.
• A lizard. Lizards, being reptiles, lay eggs with enough yolk for the embryo.
• Milk from the mother. Mammals feed their young ones with breast milk after birth.
• The yolk — correct. Yes!
• Rotten food and manure. That is what the larva of some animals eats after hatching.