User blog comment:Xilinoc/Ch. 552 - The Fundamental Virulence/@comment-893506-20131009023437

It seems like the Hollowfication theory played out after all. Again, good foreshadowing on Kubo's part (think Katagiri's concern regarding Masaki).

While it does seem like that's the reason Ichigo's Bankai can't be taken, I'm still a bit confused. From what Urahara and Mayuri said, it would just be poison for them to try to wield it. Assuming this is true though, then the medallions might have been designed to only take pure Shinigami Bankais, which would be the only reason why it would never work on Ichigo.

However, even if I'm wrong on this, we did see Ebern's medallion affect Ichigo. And from what Urahara and Mayuri were discussing, I think only an Arrancar would be able to take Ichigo's Bankai without being poisoned, which may be why Ebern's attempt had affected Ichigo and Quilge's didn't.

Now, as I'm sure some people are asking, if that theory is true, then why did Ebern still fail to take Ichigo's Bankai? Think about what Yhwach said. When he confronted Yamamoto after he beat Royd, he told him that only he was powerful enough to take Zanka no Tachi. This would mean that only an Arrancar of sufficient spiritual power would be able to wield Ichigo's Bankai.

But, regardless of whether or not any of these theories are accurate, Ichigo's Bankai may have changed now that we know everything regarding him and the Zangetsus.

Overall, this was a good chapter, and I'm glad we got what we did with Urahara and Mayuri.