Board Thread:Warhammer 40k General Discussion/@comment-7699236-20150618150629/@comment-8881468-20150621111041

@Sanguinius: that is in "Scars" in one of the early chapters where Malcador and Dorn debate the allegiance of Yagathai Khan...I only have trhe audiobook available but I listened to exactly that part the other day.

In deliverance Lost, we already know that the primarchs were created at the same time (in the scene where after they obtained the gene sample, and are back in deliverance, corax helps the techpriest and the apothecary in streamlining the material and research.)