The following references illustrate the fact that a new human embryo, the starting point for a human life, comes into existence with the formation of the one-celled zygote:
"Development of the embryo begins at Stage 1 when a sperm fertilizes an oocyte and together they form a zygote."
[England, Marjorie A. Life Before Birth. 2nd ed. England: Mosby-Wolfe, 1996, p.31]
"Human development begins after the union of male and female gametes or germ cells during a process known as fertilization (conception).
"Fertilization is a sequence of events that begins with the contact of a sperm (spermatozoon) with a secondary oocyte (ovum) and ends with the fusion of their pronuclei (the haploid nuclei of the sperm and ovum) and the mingling of their chromosomes to form a new cell. This fertilized ovum, known as a zygote, is a large diploid cell that is the beginning, or primordium, of a human being."
[Moore, Keith L. Essentials of Human Embryology. Toronto: B.C. Decker Inc, 1988, p.2]
"Embryo: the developing organism from the time of fertilization until significant differentiation has occurred, when the organism becomes known as a fetus."
[Cloning Human Beings. Report and Recommendations of the National Bioethics Advisory Commission. Rockville, MD: GPO, 1997, Appendix-2.]
"Embryo: An organism in the earliest stage of development; in a man, from the time of conception to the end of the second month in the uterus."
[Dox, Ida G. et al. The Harper Collins Illustrated Medical Dictionary. New York: Harper Perennial, 1993, p. 146]
"Embryo: The early developing fertilized egg that is growing into another individual of the species. In man the term 'embryo' is usually restricted to the period of development from fertilization until the end of the eighth week of pregnancy."
[Walters, William and Singer, Peter (eds.). Test-Tube Babies. Melbourne: Oxford University Press, 1982, p. 160]
"The development of a human being begins with fertilization, a process by which two highly specialized cells, the spermatozoon from the male and the oocyte from the female, unite to give rise to a new organism, the zygote."
[Langman, Jan. Medical Embryology. 3rd edition. Baltimore: Williams and Wilkins, 1975, p. 3]
"Embryo: The developing individual between the union of the germ cells and the completion of the organs which characterize its body when it becomes a separate organism.... At the moment the sperm cell of the human male meets the ovum of the female and the union results in a fertilized ovum (zygote), a new life has begun.... The term embryo covers the several stages of early development from conception to the ninth or tenth week of life."
[Considine, Douglas (ed.). Van Nostrand's Scientific Encyclopedia. 5th edition. New York: Van Nostrand Reinhold Company, 1976, p. 943]
"I would say that among most scientists, the word 'embryo' includes the time from after fertilization..."
[Dr. John Eppig, Senior Staff Scientist, Jackson Laboratory (Bar Harbor, Maine) and Member of the NIH Human Embryo Research Panel -- Panel Transcript, February 2, 1994, p. 31]
"The development of a human begins with fertilization, a process by which the spermatozoon from the male and the oocyte from the female unite to give rise to a new organism, the zygote."
[Sadler, T.W. Langman's Medical Embryology. 7th edition. Baltimore: Williams & Wilkins 1995, p. 3]
"The question came up of what is an embryo, when does an embryo exist, when does it occur. I think, as you know, that in development, life is a continuum.... But I think one of the useful definitions that has come out, especially from Germany, has been the stage at which these two nuclei [from sperm and egg] come together and the membranes between the two break down."
[Jonathan Van Blerkom of University of Colorado, expert witness on human embryology before the NIH Human Embryo Research Panel -- Panel Transcript, February 2, 1994, p. 63]
"Zygote. This cell, formed by the union of an ovum and a sperm (Gr. zyg tos, yoked together), represents the beginning of a human being. The common expression 'fertilized ovum' refers to the zygote."
[Moore, Keith L. and Persaud, T.V.N. Before We Are Born: Essentials of Embryology and Birth Defects. 4th edition. Philadelphia: W.B. Saunders Company, 1993, p. 1]
"The chromosomes of the oocyte and sperm are...respectively enclosed within female and male pronuclei. These pronuclei fuse with each other to produce the single, diploid, 2N nucleus of the fertilized zygote. This moment of zygote formation may be taken as the beginning or zero time point of embryonic development."
[Larsen, William J. Human Embryology. 2nd edition. New York: Churchill Livingstone, 1997, p. 17]
"Although life is a continuous process, fertilization is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is thereby formed.... The combination of 23 chromosomes present in each pronucleus results in 46 chromosomes in the zygote. Thus the diploid number is restored and the embryonic genome is formed. The embryo now exists as a genetic unity."
[O'Rahilly, Ronan and Müller, Fabiola. Human Embryology & Teratology. 2nd edition. New York: Wiley-Liss, 1996, pp. 8, 29. This textbook lists "pre-embryo" among "discarded and replaced terms" in modern embryology, describing it as "ill-defined and inaccurate" (p. 12}]
"Almost all higher animals start their lives from a single cell, the fertilized ovum (zygote)... The time of fertilization represents the starting point in the life history, or ontogeny, of the individual."
[Carlson, Bruce M. Patten's Foundations of Embryology. 6th edition. New York: McGraw-Hill, 1996, p. 3]
"[A]nimal biologists use the term embryo to describe the single cell stage, the two-cell stage, and all subsequent stages up until a time when recognizable humanlike limbs and facial features begin to appear between six to eight weeks after fertilization....
"[A] number of specialists working in the field of human reproduction have suggested that we stop using the word embryo to describe the developing entity that exists for the first two weeks after fertilization. In its place, they proposed the term pre-embryo....
"I'll let you in on a secret. The term pre-embryo has been embraced wholeheartedly by IVF practitioners for reasons that are political, not scientific. The new term is used to provide the illusion that there is something profoundly different between what we nonmedical biologists still call a six-day-old embryo and what we and everyone else call a sixteen-day-old embryo.
"The term pre-embryo is useful in the political arena -- where decisions are made about whether to allow early embryo (now called pre-embryo) experimentation -- as well as in the confines of a doctor's office, where it can be used to allay moral concerns that might be expressed by IVF patients. 'Don't worry,' a doctor might say, 'it's only pre-embryos that we're manipulating or freezing. They won't turn into real human embryos until after we've put them back into your body.'"
[Silver, Lee M. Remaking Eden: Cloning and Beyond in a Brave New World. New York: Avon Books, 1997, p. 39]