Any female sex cell is called an egg, because that is the name for female sex cells. So the egg must come before the off-spring from it. In Darwinian terms, at some stage in the evolution of 'chicken' a proto-type 'chicken' - probably some kind of bird thing which couldn't fly - laid an egg which contained mutated DNA which when hatched led to a new form of 'chicken', and for argument's sake we could call this 'chicken'. Of course, evolution doesn't come in simple units, evolution is a process, a transition, not a fixed unit or step where you can point out a before and after. Thus the idea 'chicken' is only conceived after the long process-event, when something of a species appears different to others of a kind. The word 'chicken' is a type which, say, for biologists, signifies a given set of genetic characteristics which might not be found in any token 'chicken' we happen to point out.