Board Thread:General Discussion/@comment-4317207-20130313091731/@comment-7313057-20130402150749

Vasto Lorde is the highest level that a hollow can naturally achieve. Arrancars are hollows that Azien modified with the Hōgyoku by removing a section of they mask and sealing they hollow form into a swords. But it hasn't been specified if they're human form were they original form before becoming a hollow.