2nd week of pregnancy

Also known as the week of two’s:


1. 2 embryoblast layers form:

  • Endoderm (Hypoblast)
  • Ectoderm (Epiblast)

Therefore forming a bilaminar germ disc

2. 2 trophoblast layers form:

  • Cytotrophoblast
  • Syncytiotrophoblast – has lacunae Image result for lacunae in syncytiotrophoblast

3. 2 cavities form:

  • Amniotic cavity – the cytotrophoblast cells surrounding the amniotic cavity become amnioblast cells
  • Primary yolk sac – endodermal cells grow down, line cytotrophoblast forming Heuser’s membrane (exocoelomic membrane)

Image result for heuser's membrane

4. 2 extraembryonic mesodermal layers form:

In the cytotrophoblast, extraembryonic mesoderm forms, in which lacunae appear and fuse to form extraembryonic coelom. Therefore dividing the extraembryonic mesoderm into 2 layers.

  • somatopleuric mesoderm – lines cytotrophoblast and amniotic cavity
  • splanchnopleuric mesoderm – lines primary yolk sac

Image result for splanchnopleuric mesoderm



  • Secondary yolk sac also forms, when a large part of primary yolk sac separates and disappears leaving the smaller part

Image result for secondary yolk sac

  • Primary chorionic villi forms – cytotrophoblast projects into syncytiotrophoblast (more in the next chapter)



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