Space Marines - Gene-Seed: The gene-seed of an Astartes is the foreign genetic material originally engineered using one of the primarchs' genomes as a foundation. The gene-seed develops into the special organs that are then implanted into a potential Space Marine's body. These gene-seed organs are responsible for most of a Space Marine's physical enhancements over baseline Human capability.All Space Marine gene-seed was originally cultivated by the Emperor Himself from the DNA of the Emperor's 20 genetically-engineered sons (each son being the primarch of one of the 20 Space Marine Legions of the First Founding), and is a rare and precious resource for the Space Marines of the Imperium, even in death.The biotechnology necessary to create new gene-seed was long forgotten or lost to Humanity in the millennia before the creation of the Primaris Space Marines; therefore, it had to be cultivated after being retrieved from dead or dying Astartes warriors and returned to the Chapter's Apothecaries, who oversaw the creation of new Astartes from the Chapter's raw recruits.The gene-seed is the very essence of a Space Marine Chapter and it carries each of the characteristics that are particularly unique to a given Chapter, be they mental, physical, spiritual, or martial.Unfortunately, Space Marine gene-seed is vulnerable to mutations over time, which can phenotypically manifest in various ways. In addition, there are various genetic flaws that have developed in the gene-seed, the majority of which derive from the particularities of each primarch's genetic code.It is these mutations that led to the emergence of the Flaws in the Blood Angels' gene-seed (specifically their susceptibility to the conditions known as the Black Rage and the Red Thirst), the "Mark of the Wulfen" for the Space Wolves or the rapidly increasing rate of mutation that afflicted the Renegade Soul Drinkers Chapter before it was destroyed and reconstituted by Primaris Marines.It must also be noted that the presently existing Space Marine Chapters are more numerous than the original 20 Space Marine Legions, excluding those two Legions that were removed from Imperial records. Only the original 9 Legions that remained loyal to the Emperor during the Horus Heresy produced the numerous Space Marine Chapters of the Second Founding who share traits with their founder Chapter, which is itself the remnant of one of the Loyalist Legions.During the recruitment and enhancement process, some aspirants may not survive the rigours of training and the later medical treatments one must undergo to become a full-fledged battle-brother of the Chapter.First and foremost, a potential Space Marine recruit must be male, as the gene-seed and the developing Space Marine organs are compatible only with the Human male's hormonal balance and genetics. Trying to implant a woman with Space Marine gene-seed would result only in a painful, agonising death. The three following requirements also apply:Space Marine aspirants must be adolescents or very young adults, as the implants must be able to coordinate with a Human male's natural growth hormones during adolescence to stimulate the growth and development of the various unique physiological features of a Space Marine. In specific terms, the recruit must be about 10-16 Terran years of age, although the process has been documented to still work in recruits as old as 20 as long as they have not yet reached their full adult growth.Much like a blood transfusion or organ transplant, there must be immunogenetic compatibility between the recruit and the implants; otherwise organ failure may result, causing the recruit to die or simply degenerate into a state of madness as their own tissues come under autoimmune attack.The mental state of a potential Space Marine must also be susceptible to the various training and psycho-conditioning regimes of the Chapter and cannot already be tainted by the Ruinous Powers of Chaos.These three main criteria bar all except a minuscule percentage of Human males within the Imperium of Man from becoming Space Marines.If all the tests prove successful, the Space Marine recruit transforms from a neophyte into a Scout Marine or even a full initiate depending on the Chapter's individual organisation.The recruit is then taken to live at the Chapter's fortress-monastery where he is instructed in the ways of battle and taught the values and history of the Chapter. At this stage, organ implantation, psycho-conditioning, and physical training begin.Each step in this stage has its own dangers, ensuring that only the truly worthy initiates become Space Marines. After several standard years of training, conditioning, and implantation surgeries the initiate becomes a true Astartes, undergoes his Rites of Fire in his first combat action, and becomes a true battle-brother of his Chapter.