Human embryology is the scientific study of the material aspect of the developing human embryo and fetus with focus on the embryonic period, from the beginning of fertilization [or "when the matter is appropriately organized"] through 8 weeks (O'Rahilly and Müller 2001, p. 7). It has been systematically documented in the Carnegie Stages of Early Human Embryonic Development (CSEHED) since 1942. Human embryos can be reproduced both sexually (by fertilization - both natural and artificial, such as in IVF in vitro fertilization and other artificial reproductive technologies) and asexually (as in naturally occurring human monozygotic identical twinning in vivo as well as in many different kinds of cloning and genetic engineering techniques in vitro). The immediate products of both human reproductive processes are new genetically unique individual living human beings, who immediately produce specifically human proteins and enzymes, and continuously form specifically human cells, tissues, and organs throughout development. Such objective scientific facts should be the starting point for any discussions, debates, policies, or legislation on these issues. Unfortunately, they often are not.
Since the fine details of human embryology have been corrupted for political purposes (Biggers 1990, pp. 1-6), it is important to identify and address some of those details, especially those occurring just before and in the first few weeks after the initiation of embryonic development.
The genome of a member of a species is determined by the total amount of DNA in both the nuclear and cytoplasmic chromosomes (Lewin 2000, pp. 4, 81; Strachan and Read 1999, pp. 10, 18, 139). However, the species to which an individual belongs is determined only by the number of nuclear chromosomes per cell and the specific kinds and sequences of base pairs of amino acids in the genes comprising them. For humans, that species number is 46 (plus or minus). When a cell in the human body contains 46 chromosomes, it is called diploid. If it contains only 23 chromosomes, it is called haploid. There are two basic categories of diploid cells: somatic (body) cells, and germ line cells (future sex gametes). Before fertilization can take place, the number of chromosomes in each diploid germ-line cell must be cut in half through the process known as gametogenesis. The final effect of gametogenesis is the production of haploid sex gametes, the sperm and the oocyte, which have only 23 chromosomes in each cell. [The terms "egg" and "ovum" are rejected as unscientific (O'Rahilly and Müller 2001, p. 12).] The oocyte must first be fertilized before its chromosomes are halved (Carlson 1999, p. 2; Emery 1983, pp. 52-53, 60, 91; Larsen 1998, p. 4; Moore and Persaud 1998, p. 18; O'Rahilly and Müller 2001, pp. 12, 19, 25; Strachan and Read 1999, p. 30).
It has been known empirically for over a hundred years that fertilization is the beginning of sexually reproduced human beings (Wilhelm His 1880-1885). However, it is obviously not the beginning of asexually reproduced human beings. During the process of fertilization, the sperm and the oocyte fuse. [The terms "fertilized oocyte" and "fertilized egg" are rejected as unscientific (O'Rahilly and Müller 2001, p. 12).] The diploid number of chromosomes is restored, and a new single-cell genetically unique living human being is reproduced. This is also the beginning of: the human embryo, the human organism, the human individual, the genetic sex of the embryo, the embryonic period, and normal pregnancy, which begins at fertilization in the fallopian tube, or ovaduct, of the mother, not at implantation in her womb (Carlson 1999, pp. 2, 23, 27, 32, 444; Larsen 1998, pp. 1, 17; Moore and Persaud 1998, pp. 2, 12, 18, 34, 37; O'Rahilly 2001, pp. 31-33). All cell constituents, including the nuclear chromosomes and chromosomes in the mitochondria that are outside the nucleus in the cytoplasm of the cell, now belong properly to the new embryo.
This single-cell embryo is totipotent, that is, capable of forming all the cells, tissues, and organs of the later embryo, fetus, and adult. The cells (blastomeres) of the early developing human embryo will also exhibit a range of totipotency, that is, if separated from the developing embryo, these totipotent cells are capable of forming new human organisms (as in natural and artificial monozygotic identical "twinning"). This totipotent capacity also applies to the cells of the developing embryo from 2 cells (about 1½-3 days) until the first formation of the free floating blastocystic cavity (about 4 days), to the cells of the inner cell mass of the implanting blastocyst (about 5-7 days), and to the diploid primitive germ-line cells (future haploid sex gametes) (as early as 2½ weeks) of the later blastocyst. Thus to refer to these cells as being all "pluripotent" rather than "totipotent" is scientifically erroneous, and done in order to mis-frame the debates (American Medical Association 1994, pp. 1-9; American Society for Reproductive Medicine 2004, pp. S256-257; Carlson 1999, pp. 43-45, 73; Denker 2008, pp. 1656-1657; German National Ethics Council 2004, p. 14; Institute of Medicine and National Research Council 1989, pp. 25, 102ff; Irving 2005, pp. 1-36; Lewin 2000, p. 605; A. Liu 2005, pp. 369-378; O'Rahilly and Müller 2001, pp. 23, 24, 37, 39, 136-137, 139; Strachan and Read 1999, pp. 508-509; Schieve et al 2004, pp. 1154-1163). This new single-cell human being immediately directs his/her own further continuous human growth and development by producing specifically human proteins and enzymes (Emery 1983, p. 93; Hao et al. 2006, p. S513; Holtzer et al. 1985, pp. 3-11; Illmensee et al. 2006a, pp. 1112-1120; Irving 1993a, pp. 18-46; Irving 1999, pp. 22-47; Kollias et al. 1987, pp. 5739-5747; H. Liu et al. 2005a, p. S368; H. Liu et al. 2005b, p. S370; Moore and Persaud 1998, p. 12) that will cascade (will be produced on demand) throughout development (Emery 1983, p. 91; Lewin 2000, pp. 63, 914, 950). This embryonic development is most accurately documented in the Carnegie Stages of Early Human Embryonic Development (CSEHED).
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