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265 Glossary Glossary amnion / a thin membrane forming a closed sac about the embryos and fetuses of reptiles, birds, and mammals and containing the amniotic fluid—called also amniotic sac. androgenote / development in which the embryo contains only paternal chromosomes due to failure of the egg nucleus to participate in fertilization. blastocyst / the modified blastula of a placental mammal; an early metazoan [i.e., an animal that is multicellular as an adult, excluding sponges] embryo typically having the form of a hollow fluid-filled rounded cavity bounded by a single layer of cells. chorion / the highly vascular outer embryonic membrane that is associated with the allantois [a transient embryonic gut structure] in the formation of the placenta. chromatin / a complex of nucleic acid with basic proteins (as histone) in eukaryotic cells that is usually dispersed in the interphase nucleus and condensed into chromosomes in mitosis and meiosis development / the process of growth and differentiation by which the potentialities of a zygote, spore, or embryo are realized; we have defined the ordered formation of the first two distinct cell types as the minimum requirement for development. diploid / having the basic (haploid) chromosome number doubled. Diploid is the normal state for somatic (i.e., body) cells. dizygotic twins / twins derived from two ova; fraternal or non-identical twins. DNA / any of various nucleic acids that are usually the molecular basis of heredity, that are constructed of a double helix held together by hydrogen bonds between puAll definitions are taken from the NIH-administered medical dictionary (http://www, with minor modifications for clarity, as indicated by italics. 266 Glossary rine and pyrimidine bases, which project inward from two chains containing alternate links of deoxyribose and phosphate, and that in eukaryotes are localized chiefly in cell nuclei—also called deoxyribonucleic acid. embryo / an animal in the early stages of growth and differentiation that is characterized by cleavage, the laying down of fundamental tissues, and the formation of primitive organs and organ systems; especially the developing human individual from the time of fertilization to the end of the eighth week after conception (cleavage commences immediately after fertilization to produce the two-cell embryo). epigenetic / relating to, being, or involving a modification in gene expression that is independent of the DNA sequence of a gene. enucleated egg / an egg or oocyte lacking a nucleus. eukaryotic / any of a domain (Eukarya) or a higher taxonomic group (Eukaryota) above the kingdom that includes organisms composed of one or more cells containing visibly evident nuclei and organelles. fertilization / the process of union of two gametes whereby the somatic chromosome number is restored and the development of a new individual is initiated. gamete / a mature male or female germ cell (sperm or egg) usually possessing a haploid chromosome set and capable of initiating formation of a new diploid individual by fusion with a gamete of the opposite sex—also called sex cell. gene / a specific sequence of nucleotides in DNA that is located usually on a chromosome and that is the functional unit of inheritance controlling the transmission and expression of one or more traits by specifying the structure of a particular polypeptide and especially a protein or controlling the function of other genetic material —also called determinant, determiner, factor. genome / one haploid set of chromosomes with the genes they contain. gastrulation / the process of forming the gastrula, or an early metazoan embryo in which the ectoderm, mesoderm, and endoderm are established. At gastrulation, the body plan of the adult is established, and twinning is no longer possible. haploid / having the gametic number of chromosomes or half the number characteristic of somatic cells. hydatidiform mole / a mass in the uterus that consists of enlarged edematous degenerated chorionic villi [fluid-filled structures similar to those found in the placenta] growing in clusters resembling grapes, that typically develops following fertilization of an enucleate egg, and that may or may not contain fetal tissue. Glossary 267 inner cell mass / (ICM) the portion of the blastocyst of a mammalian embryo that is destined to become the structures of the postnatal body. implantation / in placental mammals : the process of attachment of the embryo to the maternal uterine wall—called also nidation. membrane / a semipermeable limiting layer of cell protoplasm consisting of a fluid phospholipid bilayer with intercalated proteins—called also cell membrane, plasmalemma . meiosis / the cellular process that results in the number of chromosomes in gameteproducing cells being reduced to one half and that involves a reduction division in...


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