Development

• Morula


  A ball of 12-16 blastomeres
• Morula turns into

  1. Blastocoel, then

  2. Blastula
• Blastula develops into

  1. Trophoblast

  2. ICM
• Trophoblast

  • Responsible for extraembryonic membrane formation, uterine implantation, and HCG production
• ICM

  • Develops into the blastocyst
• Within the blastocyst

  The bilaminar disk forms, which has two layers: the hypoblast and the epiblast
• Hypoblast

  Develops into the yolk sac
• Epiblast

  Develops into the main embryo
• Epiblast forms

  the main embryo and the primitive streak, which is crucial for gastrulation
• Blastopore

  Entrance to the archenteron, which forms the digestive tract
• Blastopore

  Becomes either the mouth in protostomes or the anus in deuterostomes
• Deuterostomes

  Undergo radial cleavage
• Protostomes

  Undergo spiral cleavage
• Indeterminate cleavage

  Gives rise to identical twins
• Holoblastic cleavage

  Occurs through the entire embryo, when there is minimal yolk
• Meroblastic cleavage

  Occurs through part of the embryo, when there is substantial yolk
• Zona hatching


  When the blastocyst replaces the zona pellucida with trophoblast cells
• Gastrulation

  Makes a trilaminar embryo
• Parts of the embryo from inner to outer

  • Amnion
  • Allantois
  • Chorion
• Amnion

  Functions as a cushion
• Allantois

  Functions as waste storage, and develops into the umbilical cord
• Chorion

  Eventually forms the placenta
• Tissues formed from the three germ layers

  • Ectoderm: CNS, PNS, sensory, epidermis, glands, jaws, teeth, adrenal medulla
  • Mesoderm: Skeletal bones, muscles, cardiovascular system, gonads, adrenal cortex, spleen, notochord
  • Endoderm: Tract linings and PLTT
• Notochord from mesoderm


  Induces formation of neural tube from ectodermal cells
• Neural crest cells


  Become teeth, bones, and pigment
• Mesoderm cells


  Become somites, which become the vertebrae
• The primitive streak becomes the blastopore after invagination