In the Warhammer 40,000 fictional universe, Gene-seed is the material that allows for the creation of the superhuman Space Marines. There are 19 types of Gene-seed, each used to create one of the 19 specialized organs implanted into the body of a human warrior to create a Space Marine. As many of these implants are neural in nature, it is important to note in many of these implantations the subject must be awake, without painkillers.
Gene-Seed Implants
Since the first founding, Space Marines chapters have lost the ability to make new implants, or otherwise do not have all 19 implants available to their chapter. All chapters are different in this respect, but the "purest" chapters in this respect are the Grey Knights and Ultramarines. However, all chapters have the Black Carapace, as this is necessary for a marine to use his armour.
The Implantation of new gene-seed organs has over the centuries turned into a near-religious ritual, and therefore some of the organs have lost their meaning to the implanter, and because of this and the implantation turning into a ritual instead of an operation, some Marines will need surgery after the implant to fix imbalance as a result of poor surgery.
Certain implants are used as plot devices, such as the use of the Sus-an Membrane in one of the Warhammer 40,000 novels by the publishing arm of Games Workshop. Used by a main character to enter suspended animation, it allowed him to survive in deep space for thousands of years, thereby tying the past to the present in the novel. Similarly, the Omophagea is used in another novel, concerning the Soul Drinkers, when a main character absorbs some of the memories of a fallen combatant, after consuming his brain matter.
Secondary Heart
This is the first and least difficult implant to install. The Secondary Heart increases blood supply and pumping capacity, is capable of taking over entirely should the primary heart fail, and enables Marines to survive low-oxygen conditions and traumatic injury.
Ossmudula
This implant uses specially created hormones, combined with a diet high in ceramic-based chemicals, to considerably strengthen the skeleton of a Space Marine. Two years after the implantation, the Marine's skeleton will be exponentially stronger; and the rib cage will be fused into a solid mass of interlaced bone plates.
Biscopea
Implanted into the chest cavity, this small, spherical implant bolsters muscle growth throughout the Marine's body.
Haemastamen
Implanted into a main blood vessel, the Haemastamen alters the Marine's blood composition to carry oxygen and nutrients more efficiently. It also acts as a growth control for many of the later implants.
Larraman's Organ
This organ manufactures Larraman Cells. These serve the purpose of platelets, but act faster and more effectively. When a Marine is wounded, Larraman Cells are released, attached to leukocytes. At the site of the injury, they form a skin substitute of near-instant scar tissue, effectively preventing massive blood loss and infection.
Catalepsean Node
Implanted into the back of the brain, this implant allows a marine to function in situations where sleep is impossible to safely attain. Instead, when deprived of sleep, the Catalepsean Node, 'cuts in', allowing the Marine to 'switch off' sections of the brain sequentially, while remaining awake and alert (although prolonged use can be hazardous). The longest any Space Marine has ever been on active combat duty without rest is 328.7 hours, achieved by a squad of the Crimson Fists. This implant bears a resemblance to a theory as to how dolphins such as the Bottlenose "sleep": shutting off sections of their brain while remaining awake and alert under the functions of the other sections.
Preomnor
The Preomnor is a decontamination chamber inside the chest cavity, effectively a 'pre-stomach'. It chemically analyses ingested materials and neutralizes toxins. The Preomnor enables the Marine to eat normally inedible substances and resist poisons.
Omophagea
Implanted into the spinal cord, this organ is designed to absorb DNA related to experience or memory. This enables the Marine to gain information, in a survival or tactical sense, simply by eating an animal indigenous to an alien world or situation. The Marine can actually absorb vivid memories and information from a human being by eating their brains. This organ may also have led to the various flesh-eating or blood drinking rituals performed by some chapters, notably the Blood Angels and their successors.
Multi-lung
The multi-lung is a 'third' lung, able to absorb oxygen from environments poor in oxygen. Breathing is accomplished through a sphincter in the trachea. A similar muscle closes the primary lungs off in toxic environments, while oxygen is absorbed by filtering out the poisonous elements in the multi-lung.
Occulobe
Essentially, this organ allows technicians to enhance a Marine's eyesight, granting him exceptional vision and the ability to see normally in low-light environment.
Lyman's Ear
This implant renders a Marine immune to dizziness and nausea (see Cochlea), and enables a Marine to consciously filter out "white noise".
Sus-an Membrane
This implant allows a Marine to enter a catatonic or "suspended animation" state. It can allow a mortally wounded Space Marine to survive his injuries, and bring the metabolism to a standstill. However, specific stimulations from an external source are required to bring a suspended Space Marine back to full life. The longest recorded period of this state was Brother Silas Err of the Dark Angels for 567 years. It should be noted that Talonmaster Zso Sahaal of the Night Lords legion slept through 100 centuries after his cruiser was entombed in the Warp by the Eldar shortly after the Horus Heresy, only to be awoken late in the 41st millennium.
Melanochrome
Linked to pigment cells in skin, this allows the Marine's skin to shield him from dangerous levels of radiation and extreme temperatures along with the Mucranoid. Because of its link to pigment some chapters have no variation in skin color.
Oolitic Kidney
This organ works in conjunction with the Preomnor, filtering blood to rapidly remove toxins.
Neuroglottis
This organ allows a Marine to assess a wide variety of things simply by taste. From poisons to chemicals to animals, a Marine can even track his quarry by taste alone.
Mucranoid
Altering sweat glands, this organ secretes an oily substance that coats the skin, protecting it from extreme temperatures and to some extent vacuum environments.
Betcher's Gland
Implanted into multiple locations inside a Marine's mouth, these glands transform a Marine's saliva into corrosive, blinding acid, strong enough to work through iron bars. It is generally accepted that the Marines have a coating of some kind inside their mouth, to prevent their own tissue from being burnt by this acid. The Imperial Fists and its successor Chapters are deficient in this implant.
Progenoid Glands
Implanted into both the neck and chest cavity, these serve to collect and cultivate the gene-seed from a Space Marine's body, and to safeguard it for the continuity of a Chapter. The neck gland matures after 5 years, while the chest gland matures after 10 years and both glands can be removed either after maturation or immediately after a marine has died in battle. The Progenoid Gland is one of the most important organs for a chapter (alongside the Black Carapace), for if a chapter were to lose the Progenoid there would be no more Space Marines.
The Black Carapace
The last and possibly one of the most important of all implants, this neuroreactive material is implanted directly under the skin. After a few hours, the material hardens and interlinks with the Marine's own nervous system. Points are then cut into the Carapace which allows a Marine to directly interface with his Power Armour.