The term "conception" commonly 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 deliberate. Gastrulating is the point in development when the implanted 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 occupied in the development of the embryo. Until this point in development, twinning is probable. Additionally, interspecies hybrids which have no chance of growth survive until gastrulation. However this stance is not entirely necessary since human developmental biology literature refers to the "concepts" and the medical literature refers to the "products of conception" as the post-implantation embryo and its surrounding membranes. The term "conception" is not generally used in scientific literature because of its variable definition and suggestion.