Chorionic villi
 
Primary villi - 11-13 days
Cytotrophoblast covered by syncytiotrophoblast



Secondary villi - 16 days
Core of extraembryonic mesenchyme



Tertiary villi - 21 days Embryonic blood vessels
Chorionic cavity
Anchoring villus
Stem villus
Normal implantation:  Larsen, WI, Human Embryology, 3rd ed., Churchill Livingston, NY (2001), p. 45
Mesoderm in villi form blood cells and small blood vessels.  These make contact with capillaries in chorionic plate and connecting stalk.
Embryo primitive streak - gastrulation beginning at 16 days
Note cytotrophoblastic shell.