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Six week old human embryo. Photo:Getty

Chris, a commenter in the embryonic stem cell post passes along these great quotes from medical texts. Many thanks Chris!

“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.”
– Before We Are Born: Essentials of Embryology and Birth Defects. 4th ed. 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.”
– Human Embryology. 2nd edition. 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.”
Human Embryology & Teratology. 2nd edition. 1996, pp. 8, 29.

“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. … Fertilization takes place in the oviduct … resulting in the formation of a zygote containing a single diploid nucleus. Embryonic development is considered to begin at this point… This moment of zygote formation may be taken as the beginning or zero time point of embryonic development.”
Essentials of Human Embryology 1998 1-17.

“[The Zygote] results from the union of an oocyte and a sperm. A zygote is the beginning of a new human being. Human development begins at fertilization, the process during which a male gamete or sperm … unites with a female gamete or oocyte … to form a single cell called a zygote. This highly specialized, totipotent cell marks the beginning of each of us as a unique individual.”
The Developing Human: Clinically Oriented Embryology, 6th ed. 1998, pg. 2-18.

“Fertilization is an important landmark because, under ordinary circumstances, a new, genetically distinct human organism is thereby formed… Fertilization is the procession of events that begins when a spermatozoon makes contact with a secondary oocyte or its investments… The zygote … is a unicellular embryo…”
Human Embryology & Teratology 1996 pg. 5-55.

To these I add this one:

Developmental Biology by Scott Gilbert is arguably the leading text in the field. Gilbert is on faculty at Swarthmore College.

“Traditional ways of classifying catalog animals according to their adult structure. But, as J. T. Bonner (1965) pointed out, this is a very artificial method, because what we consider an individual is usually just a brief slice of its life cycle. When we consider a dog, for instance, we usually picture an adult. But the dog is a “dog” from the moment of fertilization of a dog egg by a dog sperm. It remains a dog even as a senescent dying hound. Therefore, the dog is actually the entire life cycle of the animal, from fertilization through death.”

If that can be said with such certainty of one vertebrate, it can be said of all vertebrates.

Hope these are helpful. We’ll be building on them in the future.