Even those parts of a young ox that are edible because of its immaturity, which are not edible in an adult ox, are not permitted. The question is whether you can or cannot register to eat one of those parts of the Korban Pesach. That's a dispute of R. Yochanan and Resh Lakish -- which is revisited by the subsequent Amoraim, and Ravin and R. Abahu finish off the machloket (in favor of Resh Lakish). Also, if you break a bone of the Korban Pesach while it's still pure, that gets lashes, but not if the korban is notar and impure. Why are these cases different, though? 8 different explanations, with each flowing from the previous.