You find an Embryo attached to your left hand. An Embryo observes your characteristics. Once the analysis is complete, the Embryo takes form and hatches. Past the zeroth form, there are seven forms that an Embryo can take. The seventh is known as a Superior Embryo. An Embryo's evolution is entirely dependent on the characteristics and effort of the user. While up to the sixth form can be attained by anyone with enough time, evolution to the seventh form has no relation to those factors, being noticeably more difficult to achieve. There is also a final evolution known as an Infinite Embryo.
An Embryo can take on different types based on the nature of its form or ability. The nature of the Master is what determines the type of Embryo they receive.