National Review has an article that discusses this issue. It begins with this:
For people who advocate the killing of embryonic human beings in the cause of biomedical research, the Holy Grail is an argument that would definitively establish that the human embryo, at least early in its development, is not a living human organism and therefore not a human being at all. The problem for these advocates is that all the scientific evidence points in precisely the opposite direction. Modern human embryology and developmental biology have shown that fertilization produces a new and distinct organism: a living individual of the human species in the embryonic stage of his or her development.
Read the whole thing.