Section 344 Fertilization and Development

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Section 34-4 Fertilization and Development

• Fertilization and Early Development
• Gastrulation
• Patterns of Development

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Fertilization and Early Development Gametes

• Sperm male. Usually specialized for movement
  (flagellum). Small and streamlined. Head contains
  chromosomes
• Egg female. Typically large. Contains
  chromosomes, large store of cytoplasm, and yolk.
  Size of a species egg depends on how long the
  food supply in the yolk must last

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Fertilization

• Union of male and female reproductive cells.
• Combination of haploid (1n) sets of chromosomes
  from 2 individuals into a single diploid (2n)
  cell ? the zygote.

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Cleavage and Blastula Formation

• Cleavage divisions of the zygote immediately
  following fertilization.
• Mitotic divisions rapidly increase the number of
  cells but the cells do not increase in size.
• Cleavage increases the surface area/volume ratio
  of each cell enhancing gas exchange and
  environmental interactions.

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Cleavage and Blastula Formationcont.

• Blastula mass of dividing cells (16-64). Shaped
  like a hollow ball. Central cavity called a
  blastocoel.

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Gastrulation

• Blastopore an area of the blastula that begins
  to collapse inward.
• Gastrulation the process in which the blastula
  transforms into a multilayered embryo or
  gastrula.
• Differs for different phyla

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Parts of a Gastrula

• Archenteron deep cavity in the gastrula that
  will function as the gut.
• Ectoderm
• Outer germ layer.
• Gives rise to skin, hair, nails, nervous system

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Parts of a Gastrula

• Endoderm
• Inner germ layer.
• Gives rise to throat passage, gills, lungs, the
  gut and associated organs (pancreas, liver, etc.)
  
• Mesoderm
• Third middle layer that forms in most phyla.
• Lines the interior of the outer body wall and
  surrounds the gut.
• Gives rise to skeleton, muscles, inner skin,
  circulatory system, lining of body cavity.

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Blastopore Fate and Cleavage

• Coelom a body cavity completely lined by
  mesoderm.

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Protostomes

• first mouth
• Blastopore develops into a mouth and another
  opening eventually arises into an anus.
• Undergoes spiral cleavage
• Determinate cleavage path of each cell
  determined early in development. If cells are
  separated at 4 cell stage, each will die.
• Mollusks, arthropods, and annelids

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Deuterostomes

• second mouth
• Blastopore develops into an anus and a second
  opening becomes the mouth.
• Undergoes radial cleavage
• Indeterminate cleavage if cells of most 4 cell
  stage embryos are separated, each will become a
  separate organism. (identical twins)
• Ex. Chordates and echinoderms

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Coelom Formation

• Schizocoely split body cavity In protosomes,
  mesoderm formation occurs at the rim of the
  cup-shaped embryo at the junction of the endoderm
  and ectoderm

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Coelom Formation

• Enterocoely gut body cavity In deuterostomes,
  Mesoderm forms when cells lining the dorsal (top)
  part of the archenteron begin dividing rapidly
  and roll outward

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Coelom Formation

• In both, mesodermal cells spread out to
  completely line the coelom and the blastocoel
  disappears.

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Acoelomates

• Body cavity absent.
• Ex. Flatworms

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Pseudocoelomates

• Mesoderm lines the interior of the coelom but
  does not surround the endodermic gut.
• false body cavity
• Ex. Roundworms, rotifers

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Coelomates

• True coelom.
• Mesoderm lines body cavity and surrounds and
  supports the gut.
• Also forms tissue attachments for organs.
• Ex. Mollusks, annelids, arthropods, chordates,
  echinoderms.