To answer this question appropriately, one must know how an egg is formed.
The eggs form naturally, whether or not it is fertilized. First, the yolk, the yellow nutrients of the egg, is released from an ovary and passed as a tiny fluid membrane ball through the female's reproductive tract. As it develops, the egg whites, or albumen, are secreted around it. Next, the shell develops as layers of calcium and other minerals build up around it. The egg is then passed through the body and exits as a fully formed egg. It is rather like an assembly line. In the birds body, there is a whole line up of undeveloped eggs, each at a more advanced stage than the one behind it, all lined up like small marbles. The eggs continue to develop constantly as long as the animal is alive, but the productivity varies depending on the time of year, whether the bird has chicks or not, and its age.
Different eggs form slightly differently, and with different minerals that their hosts diet dictates, so the construction (and thus colour, shape, etc.) can vary between species.
So, like a human female, eggs are constantly made and expelled. The difference with a chicken is that it has a shell around it because the chicken evolved that way, or that whatever species the chicken came from - genetically proven to be a hybrid between the Red and Grey Junglefowl, and so on and so forth, back through time until you get to the first time the egg shell mutation occured. Much like a human, offspring can only be formed if the ovum is inseminated. Natural selection found it useful for an animal to not have to carry around its young inside it until they reached maturation, so a species that lays its eggs outside and hides them cleverly will be able to survive more easily since it isn't slowed down.
Ovum were around long before the egg, the shell system is only an adaptation to protect it and help the parents survive. The answer is therefor, obviously, the egg, not the chicken.
[edit on 20-8-2008 by OnionCloud]
[edit on 20-8-2008 by OnionCloud]
