Eliza Barrett and Harri Deacon, of Balclutha Primary School, asks :-

Why are there eggs and what are they made of?

Alex Davies, a veterinarian at Massey University, responded.

The egg is the cell produced by the ovary. In some animals, especially birds, we call the large structure that contains food for the chicken developing inside a shell, as well as the developing chicken itself, an egg. All female animals produce eggs in their ovaries. Without eggs, there can be no young animals to replace older ones as they die.

Carrying young inside the body is good in some ways, but there are difficulties, especially in flying animals. So in birds, the egg spends just enough time inside the mother for a food supply and the shell to be made. The chick develops once the egg is laid. It will not develop inside an egg if there has been no mating and fertilisation, or if the egg has not been kept warm. If we collect eggs for eating, there will be no chick inside for either of these reasons, or because the egg has just been laid.

Eggs contain an orange yolk, surrounded by white albumen, all enclosed by a strong membrane and then a shell on the outside. The yolk is made in the hen's ovary. It provides a specially rich food that the chick receives in its blood stream as it develops. Early in development, the chicks must have a beating heart so that the growing body has food delivered from the yolk sac, and air that passes through the shell.

All birds eggs have shells, but many eggs of other animals like fish and frogs do not need a shell. Fertilisation in these animals occurs outside the body, and a shell would prevent this. The shell protects the food supply and the chick inside. The young of fish and frogs grow in water. This is even true for animals developing inside shells or inside their mother's body, because in this case a water-filled bag is provided for the baby to float in. The shell protects both this bag, and the baby inside.