Proliferation of cells in a ring around the feather germ creates a cylindrical depression, the follicle, at its base. 环绕羽芽有一圈细胞不断增生，在基部形成圆柱状凹陷，称为毛囊。
That is largely because greater Santiago is built in a trough-like depression between two mountain ranges, which traps polluted air over the city and prevents its dispersal. 这主要原因是圣地亚哥是建立在槽状凹陷的两条山脉间的城市，这是一座陷入空气污染并要防止其扩散的城市。
