From about day 40, the chorionic sac can be divided into five zones:  A __1__ zone occupying the central half to two thirds of the chorion. This zone is characterized by a rich capillary network and many folds, villi and areolae.  Two narrower __2__ zones lying on either side of the __1__ zone. These areas have fewer folds, villi and areolae, and the blood vessels arranged more longitudinally.  Two ____ ____ lying peripheral to the __2__ zones; here there are no folds, villi, areolae or blood vessels.