ABSTRACT
TROPHOBLAST TO CHORION In the 3rd week, blood islands appear in the body stalk mesoderm coating the allantoic diverticulum. They coa lesce to form umbilical (chorio-allantoic) capillaries and veins. Umbilical arterial twigs from the dorsal aortas complete the chorionic circulation. The chorion is vas cularized trophoblast. Capillary loops extend into the surface villi, forming the chorion frondosum (4 weeks). Thereafter, the villi deep to the decidua capsularis are lost, leaving the smooth chorion laeve (Figure 1).
