The analogy used by Buddha in this statement is very powerful and thought-provoking. He is suggesting that our theories and ideas about the eternal, or what lies beyond our physical realm, are limited by our own experience and understanding. Just as a chick which has not broken its way through its shell cannot fully comprehend the outside world, we too are limited in our understanding of the eternal. We can only form theories and ideas based on what we can observe or experience in our physical realm.
Buddha is reminding us that our understanding of the eternal is limited, and that we must be humble in our theories and ideas about it. We must recognize that our knowledge and understanding of the eternal is finite and that we must be open to learning more as our experience and understanding grows.