The various implants cause vital changes in a Marine's physique and mental state. Many of these changes are controlled by natural hormonal secretions and growth patterns. Implants may not prove effective, or may not become fully functional, if they are carried out once the recipient has reached certain stages of natural development. It is therefore inevitable that recruits must be reasonably young. Tissue compatibility is also essential, otherwise organs may fail to develop properly.
The third consideration is mental suitability. The catalepsean node, occulobe, and sus-an membrane will only develop to a useable condition under the stimulus of hypnotic-suggestion. A recruit must therefore be susceptible to this particular treatment.
These considerations mean that only a small proportion of people can become Space Marines. They must be male because zygotes are keyed to male hormones and tissue types, hence the need for tissue compatibility tests and psychological screening. If these tests prove successful, a candidate becomes a neophyte. With the completion of organ implantation and attendant chemical and hypnotic training, the subject becomes an initiate. An initiate receives training before joining the ranks as a full brother. A Marine usually joins the ranks between the ages of 16-18, but such are the hormonal changes induced by the process of creating a Space Marine that recruits are physically fully grown before then. Pressures during wartime may accelerate the process.
THE RISKS
Although the Chapters are careful to select only the most suitable candidates, not all neophytes survive to become initiates. This is due in part to the degeneration of knowledge amongst the individual Chapters that makes screening procedures less effective than they once were. Nor are operational methods entirely satisfactory in some cases. In many Chapters implant surgery is heavily ritualised, and is often accompanied by scarring, incantation, periods of prayer, fasting and all sorts of mystical practices which compromise medical efficiency.