The Developing Human Being
By Keith Moore, and T.V.N. Persaud
7th edition, 2003
From an introductory definition section:
“Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. Cell division, cell migration, programmed cell death, differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell – a zygote – into a multicellular human being. Although most developmental changes occur during the embryonic and fetal periods, important changes occur during later periods of development: infancy, childhood, adolescence, and early adulthood. Development does not stop at birth. Important changes, in addition to growth, occur after birth (e.g., development of teeth and female breasts). The brain triples in weight between birth and 16 years; most developmental changes are completed by the age of 25. Although it is customary to divide human development into prenatal (before birth) and postnatal (after birth) periods, birth is merely a dramatic event during development resulting in a change in environment.” (p. 2)
“Zygote. This cell results from the union of an oocyte and a sperm during fertilization. A zygote is the beginning of a new human being (i.e., an embryo).” (p. 2)
“Embryo. The developing human during its early stages of development. The embryonic period extends to the end of the eighth week (56 days), by which time the beginnings of all major structures are present.” (p. 3)
From chapter 2: “The Beginning of Human Development: First Week”
First sentence of the Chapter: “Human development begins at fertilization when a male gamete or sperm (spermatozoon) unites with a female gamete or oocyte (ovum) to form a single cell – a zygote. This highly specialized, totipotent cell marked the beginning of each of us as a unique individual.” (p. 16)
“Studies on early stages of development indicate that human oocytes are usually fertilized with 12 hours after ovulation. In vitro observations have shown that the oocyte cannot be fertilized after 24 hours and this it degenerates shortly thereafter.” [This would buttress our argument that sperm and ovum by themselves are parts of the parents and not entire beings. That there is a substantial change between gametes and zygotes.] (p. 31)
“The zygote is genetically unique because half of its chromosomes come from the mother and half from the father. The zygote contains a new combination of chromosomes that is different from that in the cells of either of the parents.” (p. 33)
“Cleavage consists of repeated mitotic divisions of the zygote, resulting in a rapid increase in the number of cells. The embryonic cells – blastomeres – become smaller with each cleavage division. First the zygote divides into two blastomores, which then divide into four blastomores, either blastomeres, and so on.” (p. 36-37) [We can use the cleavage discussion to show that now the embryo is operating on its own and developing.]
William J. Larsen
3rd edition, 2001
“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)
“After the oocyte finishes meiosis, the paternal and maternal chromosomes come together, resulting in the formation of a zygote containing a single diploid nucleus. Embryonic development is considered to begin at this point. The newly formed embryo undergoes a series of cells divisions called cleavage as it travels down the oviduct toward the uterus. The cleavage divisions subdivide the zygote first into two cells, then into four, then into eight, and so on.” (p. 1-3)
Langman’s Medical Embryology
T. W. Sadler
9th edition, 2004
“Development begins with fertilization, the process by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote.” (p. 3)
“The results of fertilization are (a) restoration of the diploid number of chromosomes, (b) determination of chromosomal sex, and (c) initiation of cleavage.” (p. 48)
Remaking Eden: Cloning and Beyond in a Brave New World
Lee M. Silver
New York: Avon Books, 1997, p. 39
"[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.'"