In The Party of Death, National Review’s Ramesh Ponnuru writes:

We have developed ways of talking that enable us to pretend that the point can be blinked away. In the case of abortion and embryo research, the main technique is to suggest that there is some great mystery about “when life begins,” and that this alleged question is a religious or philosophical one. Yet science has since solved the mystery. From conception onward, what exists is a distinct organism of the human species. The philosophical question is what we make of that fact. To jumble these issues together—the essentially scientific question of categorizing an embryo as human and living, and the moral question of whether it follows from that categorization that it has a right to life—is a logical error. Justice Blackmun, of course, proceeded in just this erroneous fashion in Roe. And if we are not careful, talking in terms of “meaningful life,” or, as [author Ronald] Dworkin does, of “life in earnest,” can lead us into this error as well.

All of us who read this page were once human embryos. The history of our bodies began with the formation of an embryo. We were those embryos, just as we were once fetuses, infants, children, and adolescents. But we were never a sperm cell and an egg cell. (Those cells were genetically and functionally parts of other human beings.) The formation of the embryo marks the beginning of a new human life: a new and complete organism that belongs to the human species. Embryology textbooks say so, with no glimmer of uncertainty or ambiguity.

That new organism is alive rather than dead or inanimate. It is human rather than a member of some other species. It is an organism distinct from all others. It is not a functional part of a larger organism (the way a kidney is part of a larger organism). It maintains its own organic unity over time. It directs its own development, according to its genetic template, through the embryonic, fetal, and subsequent stages. Such terms as “blastocyst,” “newborn,” and “adolescent” denote different stages of development in a being of the same type, not different types of beings. At each of our earlier stages of life, we have been, as we are now, whole living members of the species Homo sapiens. (hardcover, p. 77-78)

And medical textbooks do indeed say so:

Human Embryology, 3rd Edition by William Larsen, Lawrence Sherman, S. Steven Potter, & William Scott: “In this text, we begin our description of the developing human with the formation and differentiation of the male and female sex cells or gametes, which will unite at fertilization to initiate the embryonic development of a new individual.” (p. 1)

The Developing Human: Clinically Oriented Embryology, 7th Edition by Keith Moore & TVN. Persaud: “Human development begins at fertilization, the process during which a male gamete or sperm (spermatozoon) unites with a female gamete or oocyte (ovum) to form a single cell called a zygote. This highly specialized, totipotent cell marked the beginning of each of us as a unique individual.” (p. 18)

Human Embryology & Teratology, 3rd Edition by Ronan O’Rahilly & Fabiola Muller: “Although life is a continuous process, fertilization (which, incidentally, is not a ‘moment’) is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte.” (p. 8 )

Developmental Biology, 6th Edition by Scott Gilbert: “Fertilization is the process whereby two sex cells (gametes) fuse together to create a new individual with genetic potentials derived from both parents.” (p. 185)

Van Nostrand’s Scientific Encyclopedia, 7th Edition by Douglas Considine: “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.” (p. 943)

Langman’s Medical Embryology, 7th Edition by TW Sadler: “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.” (p. 3)

Patten’s Foundations of Embryology, 6th Edition by Bruce Carlson: “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.” (p. 3)

UPDATE: Pro-Life on Campus has more:

“When fertilization is complete, a unique genetic human entity exists.” (C. Christopher Hook, MD, Mayo Clinic, as quoted by Richard Ostling in an AP news story, 9/24/99)

Testimony before a US Senate subcommittee (Subcommittee on Separation of Powers to Senate Judiciary Committee S-158, Report, 97th Congress, 1st Session, 1981)

“It is incorrect to say that biological data cannot be decisive…It is scientifically correct to say that an individual human life begins at conception.” (Professor Micheline Matthews-Roth, Harvard University Medical School)

“I have learned from my earliest medical education that human life begins at the time of conception.” (Dr. Alfred M. Bongioanni, Professor of Pediatrics and Obstetrics, University of Pennsylvania)

“After fertilization has taken place a new human being has come into being. [It] is no longer a matter of taste or opinion…it is plain experimental evidence. Each individual has a very neat beginning, at conception.” (Dr. Jerome LeJeune, Professor of Genetics, University of Descartes)

“By all the criteria of modern molecular biology, life is present from the moment of conception.” (Professor Hymie Gordon, Mayo Clinic)

“The beginning of a single human life is from a biological point of view a simple and straightforward matter—the beginning is conception.” (Dr. Watson A. Bowes, University of Colorado Medical School)

The official Senate report

“Physicians, biologists, and other scientists agree that conception marks the beginning of the life of a human being—a being that is alive and is a member of the human species. There is overwhelming agreement on this point in countless medical, biological, and scientific writings.” (Subcommittee on Separation of Powers to Senate Judiciary Committee S-158, Report, 97th Congress, 1st Session, 1981)

Quotes from leading abortion advocates

“Perhaps the most straightforward relation between you and me on the one hand and every human fetus from conception onward on the other is this: All are living members of the same species, homo sapiens. A human fetus, after all, is simply a human being at a very early stage in his or her development.” (David Boonin, A Defense of Abortion, New York: Cambridge University Press, 2002, p. 20)

“Whether a being is a member of a given species is something that can be determined scientifically, by an examination of the nature of the chromosomes in the cells of living organisms. In this sense there is no doubt that from the first moments of its existence an embryo conceived from human sperm and eggs is a human being.” (Peter Singer, Practical Ethics, 2nd Edition, Cambridge: Cambridge University Press, 1993, pp. 85-86)

“A human fetus is not a nonhuman animal; it is a stage of human being.” (Wayne Sumner, Abortion and Moral Theory, Princeton: Princeton University Press, 1981, p. 10)

“Life begins with fertilization and abortion is legalized destruction of life.” (Dr. Arthur Morris, Jr., Abortionist, as reported in the Asheville Citizen-Times, April 4, 1976)

“We tell her exactly like it is … when they abort, they’ll be aborting a small baby.” (Dr. Arthur Morris, Jr., Abortionist, as reported in the Asheville Citizen-Times, April 4, 1976)

UPDATE 5/17/11: Even more here.

Comprehensive week-by-week fetal development information can be found here.