Get cracking with eggs.

Paul Donovan investigates the biology of an egg which is a marvel of nature

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One of the greatest marvels of nature is the egg, for it encapsulates the DNA and genetic coding for the future offspring in a self-contained sphere, which is protected with a hard outer shell of calcium. The egg is the developing embryos life support system.

Once the egg has been laid, the female will then sit on it, keeping it warm and turning it throughout the incubation period. Within this hardened protective shell are all the nutrients the developing chick requires to grow through the duration of the incubation period and, the first few days following hatching.

If the egg has been fertilised, the developing embryo sits on top of the yolk. As the embryo begins to develop it gradually absorbs the yolk and some of the albumen. It will also draw on some of the calcium inside the shell for the formation of its skeletal system. Waste products are collected in a sac called the allantois, and the exchange of carbon dioxide and oxygen occurs through the eggshell. During the final few days, the chick will absorb what remains of the yolk into its abdomen. This will give it sufficient nutrients to tie it over as it begins to learn how to forage, and begins to determine what it can eat. (That's why newly hatched chicks can be in a hatcher or under a broody for at least 24 hours to recover before needing food).

STAGES OF DEVELOPMENT

From the time the oocyte is released from the hen's ovary (known as ovulation), to the point where the egg is laid, takes about 24 hours. There are various stages which the new egg must go through in order for it to emerge in the form as we know it.