Board Thread:Warhammer 40k General Discussion/@comment-91.224.27.226-20131031140001/@comment-173.189.231.50-20140105062550

I've always told myself that the two missing sons were found on worlds that had fallen to chaos, and had mutated, or sold their souls to the Chaos Gods, which (at the time) was a straight up affront to the mission of the emperor and the legions during the Great Crusade. This would certainly explain why the two Lost Sons are not mentioned, and the initial legions were made with leftover material from the Primarch Project, the Astartes already in existence would be free from the corruption of Chaos. So they were folded into the Ultramarines, thus saving those warriors from the shame of bearing the title of a Leaderless legion.

An issue I have with the GW decision to leave the 2 legions blank is that the fracturing of the legions in accordance with the Codex Astartes creates an almost infinite number of descendent chapters to paint however you want. Though the current totals of Space Marines in existence seems very low compared to what you would think.