: A recruit (aspirant) undergoes a grueling transformation involving the implantation of 19 or more specialized organs, such as a second heart and a third lung.
: The foundation of their transformation is gene-seed , genetic material derived from the Primarchs —the 20 "sons" of the Emperor. Warhammer 40000: Space Marine
: Most follow the Codex Astartes , which dictates a standard Chapter size of 1,000 battle-brothers to prevent the mass rebellions seen during the Horus Heresy . Creating a Homebrew Chapter : A recruit (aspirant) undergoes a grueling transformation