Originally posted by PhoenixAsh
A Jenova form appears and there is no Sephiroth left where it was. That part is some of the evidence for the Jenova theory, but only a small part.
Personally, I find it that supports my view quite nicely - after Jenova-DEATH was destroyed, there was nothing substantial left of Jenova's headless body, the lifeless form that broke out of the Shinra building. My reason for saying this...? Birth, Life, and Death - together those form a complete cycle, a whole. Once they were gone, Jenova's body had been expended; all that remained were her head and the few cells liberated from the clones killed by Sephiroth. These combined to form Jenova-SYNTHESIS, which looks like a large entity, but is really very compact - if you happen to see SYSNTHESIS from behind, you'll notice that her ghiant spheroid body is just a hollow shell, trying to look bigger, more menacing, more complete.

...So anyway... once Jenova-DEATH ceased to be, Sephiroth had no more use for his illusory form. All he was doing was 'guiding' the body toward the Reunion; in spite of its strong homing and survival instincts, Jenova's body was helpless without a mind to serve as its guide - just like the 'Clones'. Without Jenova's body to give it substance, Sephiroth's illusory form was nothing more than a 'ghost' - a mere image projected by his mind. Hence why the 'vision' had to use the disguise of Tifa in order to deceive Barret/Nanaki into bringing the Black Materia to Sephiroth's real body. If Jenova's body was still around, Sephiroth would simply have used it to kill the bearer of the Black Materia. Now, I know what you're thinking - why didn't he simply kill Cloud at the Temple, and get the Materia that way, rather than manipulating him? The answer is simple - a test of wills. Cloud's individuality against the power of Jenova's influence; Sephiroth's will against Aeris'.