User blog comment:Natsu11/Zeref and Natsu?/@comment-72.191.208.201-20110430042518/@comment-3390396-20110511034957

No one mentioned that Natsu ain't human. He was taught by a Dragon so he has the power to change this body parts into a dragon (Dragon Slayer). Your second part can be true, maybe Zeref really gave Natsu a part of his power.