An individual human life begins at conception when a sperm cell from the father fuses with an egg cell from the mother, to form a new cell, the zygote, the first embryonic stage.
The zygote grows and divides into two daughter cells, each of which grows and divides into two grand-daughter cells, and this cell growth/division process continues on, over and over again. The zygote is the start of a biological continuum that automatically grows and develops, passing gradually and sequentially through the stages we call fetus, baby, child, adult, old person and ending eventually in death. The full genetic instructions to guide the development of the continuum, in interaction with its environment, are present in the zygote. Every stage along the continuum is biologically human and each point along the continuum has the full human properties appropriate to that point."
Dr. William Reville, University College Cork, Ireland. Quote from a letter to the Irish Independent.
"After fertilization has taken place a new human being has come into being...[this] is no longer a matter of taste or opinion, it is not a metaphysical contention, it is plain experimental evidence...." -Dr Jerome LeJeune, Professor of Genetics at the University of Descartes, Paris, discoverer of the chromosome pattern of Down's Syndrome, and Nobel Prize Winner
"Development begins at fertilization when a sperm fuses with an ovum to form a zygote; this cell is the beginning of a new human being."Moore, Keith L., The Developing Human: Clinically Oriented Embryology, page 12, W.B. Saunders Co.,
"In that fraction of a second when the chromosomes form pairs, the sex of the new child will be determined, hereditary characteristics received from each parent will be set, and a new life will have begun."Kaluger, G., and Kaluger, M., Human Development: The Span of Life, page 28-29, The C.V. Mosby Co.
"A new individual is created when the elements of a potent sperm merge with those of a fertile ovum."Encyclopedia Britannica, "Pregnancy," page 968, 15th Edition, Chicago 1974
"Each individual has a very neat beginning, at conception."Prof Micheline Matthews-Roth Harvard University Medical School
"The beginning of a single human life is from a biological point of view a simple and straightforward matter - the beginning is conception"Dr Landrum Shettles, discoverer of male and female producing sperm
"By all the criteria of modern molecular biology, life is present from the moment of conception"Dr. Hymie Gordon, Chairman of the Department of Genetics at the Mayo Clinic
"The exact moment of the beginning of personhood and of the human body is at the moment of conception"Dr. McCarthy de Mere, a medical doctor and Law Professor at the University of Tennessee