Endodermal cells from the hypoblast migrate to the primary
yolk sac
As the extraembryonic coelom grows, the large primary yolk
sac pinches off, leaving behind a smaller, secondary yolk sac
which is "permanent"
Note: The extraembryonic fluid is actually surrounding
the entire amnion and yolk sac, except at the connecting stalk
The extra-embryonic coelom (future chorionic cavity) separates
the extra-embryonic somatic mesoderm (lining the trophoblast
and amnion) from the extra-embryonic splanchnic mesoderm (lining
the yolk sac only)
The embryo, surrounded by two cavities, floats in a large
"bubble" (future chorionic cavity)
