The Moral Status of the Human Embryo.

Moral status ascribes equal obligations and rights to individuals on the basis of membership in a protected group. Substance change is an event that results in the origin or cessation of individuals who may be members of groups with equal moral status. In this paper, two substance changes that affect the moral status of human embryos are identified. The first substance change begins with fertilization and ends with the formation of the blastocyst, a biological individual with moral status comparable to that ascribed to human organs. The second substance change begins at implantation and ends late in embryological development with the formation of the human body, an organism with moral status as a human being. The bioethical implications of each substance change are explored. The Two Substance Change theory is contrasted with continuity theories, which recognize no substance change in embryological development and with fertilization-only substance change theories.