Tobi is part zetsu (new evidence)
Bottom left two panels.
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'Drip Drip'
The arm is dripping/melting apart as a white goo, like Zetsu..
And then this:
First of all, note that he CRUSHES his sleeved arm like clay, NOT cuts it;
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...And then, as his arm falls off, it's driping the same white zetsu goo! (Top right)
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Furthermore, we all know Tobi's arm is inexplicably replaced later on. But how? This explains it.
Madara must have somehow continued his life in a Zetsu clone while his spirit went into the pure world. Separating his physical and spiritual Chakra..
That would be why he says he is "but a shell of his former self"