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Unformatted text preview: February 1, 2011 Lecture Three Changing ideas over time embryology: the study of prenatal development aristotle --> epigenesis - emergences of new structures and function during devel- opment Conception results from the union of two gametes (germ cells)-- the egg and sperm meiosis -- specialized cell division all cells --> 46 chromosomes (23 pairs - one member of each chromosome pair is inherited from each parent) sex cells (egg and sperm) -- 23 chromosomes Sex differences begin at conception the sperm bearing the Y chromosome are lighter and swim faster to the gg than those bearing an X chromosome -- more males are conceived BUT male embryos are miscarried at higher rates boys are more vulnerable to developmental disorders and illness The Zygote the fertilized egg has full complement of human genetic material, half from each parent Four developmental processes transform a zygote --> embryo -->fetus 1. cell division results in the proliferation of cells 2. cell migration is the movement of cells from heir point of origin to some- where else in the embryo 3. cell differentiation transforms the embryos unspecialized embryonic (stem) cells into roughly 350 different types of cells a. what determines a cells function? switching on of genes in the cell or the cells location 4. the selective death of certain cells, or apoptosis , also enables prenatal de- velopment Periods of prenatal development germinal (conception to 2 wks) embryonic (3rd to 8th wks) fetal (9th to birth) The Roles of Hormones on Development hormones play a crucial role in sexual differentiation all human fetuses can develop either male or female genitalia, depending on the presence or absence of testosterone By 4th day after conception the zygote arranges itself into a hollow sphere of cells with a bulge of cells (inner cell...
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- Spring '11