Creating Gene-Seed Implants
The gene-seed of a Space Marine Chapter is passed down through the
Progenoid Glands, the 18th implant that collects and stores genetic data. There are two of these glands, one in the throat, and one in the reinforced chest cavity.
This is done mainly for safekeeping, but it also provides an opportunity for a Chapter that has suffered massive losses to regenerate itself over a long period of time.
The germ cells within a Progenoid Gland mature over a period of five to ten Terran years, and can be harvested at any time after reaching maturity, up until a short time after death. Each gland contains the seed for one of each of the specialised organs necessary to create a Space Marine.
After removal, samples are taken and cultivated to grow new organs and allow a Chapter to increase in number (although those Chapters that strictly adhere to the dictates of the
Codex Astartes will generally avoid doing this because the
Codex dictates that a Chapter must have only 1,000 Space Marines at maximum).
Thus, an Astartes' death means another will be added to the ranks, and the fallen hero lives on in some small way in each new warrior that follows in his footsteps using the gene-seed replicated by and harvested from his body.
In the event of a Chapter suffering disastrous losses of its gene-seed, it may become necessary for the Apothecaries of the Chapter to use test-slaves to breed enough new gene-seed to bring the Chapter rapidly back up to full strength.
These test-slaves are always Human males, usually the condemned Imperial criminals that would otherwise be turned into
servitors. They are kept inside an artificial environment within the Chapter's
fortress-monastery alive and partly conscious, but unable to leave their transparent
plasteel gestation chamber for the full 5-10 year maturation process.
Needless to say, this solution is far from ideal, and is regarded as a last option to keep a Chapter alive. This method is also used by the
Adeptus Mechanicus to create the large amount of gene-seed necessary for the
Founding of a new Chapter since no existing Chapter could spare 1,000 copies of its gene-seed.
However, as each Chapter is required to tithe 5% of its gene-seed away to
Mars to be checked for genetic purity, it is possible that the Imperium could build up a collection of gene-seed, and found new Chapters that way
en masse.