Complex sentence: The "egg tooth" is the unusual structure that helps the bird break through the shell, it is only found in emerging chicks and lost soon after hatching that penetrates the hard shell that was once the protector of the embryo.
Split into simpler sentences: The "egg tooth" is the unusual structure that helps the bird break through the shell. It is only found in emerging chicks and lost soon after hatching, after it is used to penetrates the hard shell that once protected the embryo.