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I. Introduction
The question as to when a human being begins is strictly a scientific question, and should be answered by human embryologists-got by philosophers, bioethicists, theologians, politicians, x-ray technicians, movie stars, or obstetricians and gynecologists. The question as to when a human person begins is a philosophical question. Current discussions on abortion, human embryo research (including cloning, stem cell research, and the formation of mixed-species chimeras), and the use of abortifacients involve specific claims as to when the life of every human being begins. If the "science" used to ground these various discussions is incorrect, then any conclusions will be rendered groundless and invalid. The purpose of this article is to focus primarily on a sampling of the "scientific" myths, and on the objective scientific facts that ought to ground these discussions. At least it will clarify what the actual international consensus of human embryologists is with regard to this relatively simple scientific question. In the final section, I will also address some "scientific" myths that have caused much confusion within the philosophical discussions on "personhood."
II. When does a human being begin?
Getting a handle on just a few basic human embryological terms accurately can considerably clarify the drastic difference between the "scientific" myths that are currently circulating, and the actual objective scientific facts. This would include such basic terms as: "gametogenesis," "oogenesis," "sperm ato genesis," "fertilization," "zygote," "embryo," and "blastocyst." Only brief scientific descriptions will be given here for these terms. Further, more complicated, details can be obtained by investigating any well-established human embryology textbook in the library, such as some of those referenced below. Please note that the scientific facts presented here are not simply a matter of my own opinion. They are direct quotes and references from some of the most highly respected human embryology textbooks, and represent a consensus of human embryologists internationally.
A. Basic human embryological facts
To begin with, scientifically something very radical occurs between the processes of gametogenesis and fertilization-the change from a simple part of one human being (i.e., a sperm) and a simple part of another human being (i.e., an oocyte-usually referred to as an "ovum" or "egg"), which simply possess "human life", to a new, genetically unique, newly existing, individual, whole living human...