When the expanding exocoelom reaches the choriovitelline placenta on about day ___, most of the mesoderm between the yolk sac and chorion is also split into ____ and ____. This loosens the yolk sac from the chorion, except for its most ____ and most ____ points, which remain attached to the chorion for the duration of the pregnancy. The loosened yolk sac and chorion are then physically separated by the expanding ____, which pushes between them on about day ___.