Board Thread:Warhammer 40k General Discussion/@comment-91.224.27.226-20131031140001/@comment-3258762-20131224183629

We do know that Omegon was not the XI primarch, he was an accidental by-product of Alpharius. Had he been no. XI, he would have had his own gestation pod to grow in, and would have been scattered from Alpharius when the pods were whisked away.

You do have a point in that it is very unlikely that the legionnaires would just accept it, however we must assume that those who were integrated were those deemed fit to live (AKA Loyal to a fault), for something very bad must have happened for a Primarch to deserve censure (Not even Magnus was meant to be censured at first, and he royally f*cked over the entire species of Mans future). What could also be the case is that the legionnaires were mind-wiped, perhaps by the Emperor himself through psychic means.

Despite all this, I highly doubt that any legionnaires had been taken up by the Ultramarines. I think the UM simply got a big growth spurt (The Ultramarines had 500 worlds dedicated to them, it isn't that hard to imagine them growing in number that quickly) and the II legionnaires were executed. Perhaps through resistance, we do not know, but it would certainly explain why the Space Wolves were such a small legion during most of the Heresy (Fighting a legion head on is bound to cause massive casualties)