Oh...that is incredibly disgusting.It seems like the fox is forming inside out as more tails are drawn upon. Looks like the fox's skeleton is forming Naruto now. I originally thought maybe it's the power Itachi left him (Susano as a protective shield), but this makes more sense, and is nastier. I think the reason he might've been able to push this many tails is because of the natural energy since we've never heard of him skipping a tail, and we've never seen him go past 4. The way the transformations are going, I suspect there to be a sort of role reversal when all 9 tails are summoned. Naruto'd be trapped inside while the fox is out.

Also, I hope Naruto willfully summoned the fox at this point. As in, actually set the necklace aside and just went inside himself to summon. His performance in the last fight was brilliant, on par with the best I've seen, yet, it wasn't enough. If there's ever a time to actually rely on the 9 tails' power, THIS is it.