fertilization in echinoderms

• Vitelline membrane


  Yolk membrane surrounding the unfertilized egg
• Isolecithal egg


  Egg with evenly distributed yolk material
• Unfertilized egg


  • Surrounded by vitelline membrane
  • Yolk material evenly distributed in cytoplasm
• Fertilization membrane


  Membrane that arises immediately after sperm penetration to prevent further fertilization
• Peri-vitelline area


  Space between fertilization membrane and yolk
• Fertilized egg


  • Presence of fertilization membrane
  • Peri-vitelline area present
• Cleavage


  1. Division of yolk into two equal blastomeres (holoblastic cleavage)

  2. First and second cleavages are meridional and perpendicular

  3. Third cleavage is equatorial and perpendicular to first two

  4. Subsequent cleavages become more asymmetrical, forming micro-, macro- and mesomeres
• Morula


  Pluricellular stage where cytoplasm is completely divided over blastomeres
• Blastula


  Stage where blastomeres spread to form a monolayer around a central blastocoel cavity
• Gastrulation


  1. Invagination of blastomeres at vegetal pole to form ectoderm and endoderm

  2. Ingression of primary mesenchyme cells

  3. Separation of second group of mesoderm cells from archenteron
• Pluteus larva


  Final larval stage with bilateral symmetry, radial symmetry develops later
• Stages of early embryology of sea urchin

  • Unfertilized egg
  • Fertilized egg
  • Two-cell
  • Four-cell
  • Eight-cell
  • Sixteen-cell
  • Thirty-two cell
  • Morula
  • Blastula
  • Early gastrula
  • Late gastrula
  • Pluteus larva