Board Thread:Questions and Answers/@comment-24263585-20131207182614/@comment-8986886-20131208161534

That Straightedge Guy wrote: I wondered how it could have been years since Nel left Las Noches, and didn't think that it could have been that Aizen was involved with Hueco Mundo before he left Soul Society. However, how did he make Arrancar such as Nel without the Hogyoku? I mean, it is stated that Arrancar can be naturally occurring, however, 'Perfect' Arrancar could only be created by Aizen?

There were originally TWO Hogyoku. One was seen when Gin first encountered Aizen, and was spying on him; this is revealed in a flashback, just after Gin is killed by Aizen. This is the Hogyoku Aizen uses to create the Arrancar.

Kisuke had the second one, which he eventually hid within Rukia, after having given up on trying to destroy it. At some point Aizen reveals that he "fed" Kisuke's Hogyoku to his own, in order to fully awaken it (or something along those lines)