By the end of the first third of gestation, adjacent chorionic sacs lie with their necrotic tips close together. With continued expansion, the sacs invaginate each other until the ____ zones make contact (by the end of the second third of gestation) and, eventually, the borders of the placental areas lie in apposition. Neighbouring sacs then fuse to each other, usually without vascular anastomoses developing between the placentae (fusion of adjacent allantoic circulations with the development of freemartins is extremely rare).