Which came first, the chicken or the egg

Asks djtaz11398. This question can have two meanings, so let's deal with each separately.

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Q.� When a chicken is developing, which comes first, the egg or the chick

A.� The egg, or constituents parts which eventually make up the egg.

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The embryo develops within something called the blastoderm, sustained by the yolk, which are surrounded by shock absorbing fluids and the outer shell. The shell of the egg provides for an exchange of gases as well as a means of conserving the food and water supply inside. The shell is composed mainly of calcium carbonate and�takes about 20 hours to form.

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Q.� In evolutionary terms, which came first the chicken or the egg

A.� Again, the answer is the egg. In evolution, animals do not suddenly appear and so the 'first'� chicken did not just happen, it evolved over a very long time.

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Living things evolve through changes in their DNA. In animals, including chickens, a male sperm fertilises a female egg, each contain DNA. The sperm and the egg combine to form a zygote, the first cell of the animal, which then divides over and over until a complete embryo is formed. An animals DNA, therefore, comes from the zygote, and nowhere else.

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Changes in DNA, which occur when it mutates, are responsible for the evolution of chickens from their proto-chicken ancestors. Such mutations only have an effect at the point where a new zygote is created and the zygote is only found in the egg. So without the zygote with the appropriately mutated DNA there would be no chickens.

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Do you have a question about an animal's development Click here to ask.

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by Lisa Cardy