The second week of development is often called the week of “twos”
  • Trophoblast forms two layers
    • Cytotrophoblast as said previously is the inner layer of cuboidal, epithelial like cells.
    • Syncytiotrophoblast is an amorphous mass of protoplasm that begins to invade the uterine endometrium.
      • It is derived from the fusion of cells in the cytotrophoblast
  • Embryoblast forms a two layered disc (bilaminar germ disc)
    • The more dorsal mass is the epiblast
      • It will give rise to all three germ layers of the trilaminar embryonic disc in the third week.
    • The hypoblast is the layer of cells located ventral to the epiblast and which will contribute to the formation of the primary yolk sac.
  • Two cavities form during the second week of development
    • The amniotic cavity develops dorsal to the epiblast
    • The primary yolk sac develops ventral to the hypoblast