Also known as the week of two’s:

1. Two embryoblast layers form:

• Endoderm (Hypoblast)
• Ectoderm (Epiblast)

Therefore forming a bilaminar germ disc

2. Two trophoblast layers form:

• Cytotrophoblast
• Syncytiotrophoblast – has lacunae 

3. Two 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)

Heuser’s membrane/exocoelomic membrane – Short lived combination of hypoblast cells and extracellular matrix

4. Two 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

NB: 

• Secondary yolk sac also forms, when a large part of primary yolk sac separates and disappears leaving the smaller part. It is completely lined with endoderm.

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

Syncytiotrophoblast – Placental barrier between maternal and fetal blood