Chickens and eggs are part of the same system. Why does the chicken go in but the eggs don’t?

There is a joke like this: The driver sent the leader to attend a cultural evening party. When the leader entered the venue, the driver was stopped by the security guard. The driver said that the leader and I are in the same system. The security guard said: So are chickens and eggs. In a system, if the chicken X goes in, can the egg go in?

Eggs and chickens are the same system. Hens can lay eggs. After all, neither eggs nor chickens can give birth to other living things. Whether other living things can be created or evolved is unknown for the time being. We can As possible, because no matter what the former is, once the latter appears, the chicken or egg will stay in this cycle. So we might as well think of them as two states of one thing. After all, this is a reproduction system, that is, one state is the way of another state for the purpose of reproduction. It can also be understood that one state is only for the existence of another state, and the latter is only for inheritance. The state of intermediary exists.