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The most significant event of one's life is not graduation from college, marriage or buying a home but rather gastrulation. At the end of gastrulation, the bilaminar embryo is converted to a trilaminar embryo consisting of three primary germ layers:

  • Ectoderm
  • Mesoderm
  • Endoderm
These three germ layers will subsequently develop into all of the organ systems and tissues of the body. To fail gastrulation is to fail life.

  • Primitive Streak
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    The process begins with the appearance of the primitive streak in the epiblast of the bilaminar embryonic disc. The primitive node is at the rostral end of the primitive streak. Cells of the epiblast begin to migrate into the primitive streak and to sink below the surface. In doing so, some enter the hypoblast to become endoderm and some migrate between the epiblast and endoderm to become mesoderm. At the end of this migration, the cells remaining in the epiblast become the ectoderm. This view looks down on the bilaminar embryonic disc. The buccopharyngeal membrane marks the location where the mouth will eventually be located. The arrows are indicating the migratory paths taken by cells as they pass through the primitive streak and below the surface.

  • Trilaminar Embryonic Disc

    Cells from the epiblast migrate into the space between epiblast and hypoblast to form a new layer of cells called the mesoderm. Some of the cells from the epiblast insinuate themselves into the hypoblast layer to create the endoderm. At the end of gastrulation, the remaining epiblast cells become the ectoderm. Thus the epiblast, through the process of gastrulation produces all three primary germ layers of the trilaminar embryo. This is illustrated in the animation