The term conception usually refers to fertilization, but is sometimes defined as implantation or even the point at which human life begins, and is thus a subject of semantic arguments about the beginning of pregnancy, within the abortion premeditated. Gastrulating is the point in development when the rooted blast cyst develops three germ layers, the endoderm, the exoderm and the mesoderm. It is at this point that the inherited code of the father becomes fully engaged in the development of the embryo. Until this point in development, twinning is feasible. Additionally, interspecies hybrids which have no chance of growth stay alive until gastrulation. However this stance is not completely necessary since human developmental biology literature refers to the concepts and the medical literature refers to the products of commencement as the post-implantation embryo and its surrounding membranes. The term "conception" is not normally used in scientific literature because of its variable definition and suggestion.