Preformation vs. epigenesis

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Chapter 47
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Preformation vs. epigenesis

• Preformation embryo development is miniature
  versions of adult like Russian nesting dolls
• Epigenesis relative formless egg develops into
  adult

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Fertilization in mammals

• Acrosomal Reaction
• Sperm reaches egg acrosome releases hydrolytic
  enzymes by exocytosis.
• Enzymes activate cytoskeletal elements that that
  elongate the end structure and penetrate the
  jelly coat of the egg.
• Tip of acrosome has proteins that bind to
  receptors on the vitelline layer just exterior to
  the plamsa membrane. In some animals, there is a
  lock and key recognition to assure that they
  are only fertilized by the proper species.
• Egg and sperm plasma membranes fuse.
• This causes an electrical response in the egg
  plamsa membrane.
• This depolarization prevents other sperm from
  penetrating the egg (fast block to polyspermy)
• Fusion also causes vessicles inside egg to
  exocytosis contents which catalyze a hardening of
  the extracellular matrix of the cell thus acting
  as the slow block to polyspermy)
• In mammals, fusion of egg and sperm nuclei does
  not occur until after the first mitotic division.

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• Apoptosis programmed cell death necessary for
  proper development
• accounts for the lack of webbing between our
  fingers and toes (which is their in embryos)
• totipotent a cell that has retained the
  potential to form all parts of the animal
• determination progressive restriction of a
  cells developmental potential.
• Induction the ability of one group of cells to
  influence the development of an adjacent groups
  of cells
• Tissue-specific proteins proteins found only in
  a certain type of cell
• Genomic equivalence all cells contain the same
  genes
• Maternal effect genes phenotypes expressed by
  these genes are exclusively the product of the
  mothers contribution usually mutations in genes
  that are necessary for the egg polarity to be set
  up properly.
• Homeotic genes genes that determine the
  anatomical identify of different segments of an
  individual.